Asbestos Air Sampling Techniques







Air sampling and analysis is critical in determining exposure to airborne asbestos fibers for both the worker and the unprotected public in the area surrounding an abatement project.

Due to the microscopic size of asbestos fibers, air sampling is the only definitive way of determining these exposures as well as to determine if an abatement site can be released for re-occupation. Collection of reliable data requires a thorough knowledge of the techniques and equipment used in air sampling. This section presents a basic introduction to the topic.




  1. 1.    Personal Air Sampling:  Purpose: OSHA Compliance/worker protection


Air samples are collected in the breathing zone of the worker using a portable, battery operated sampling pump. Sampling requirements include both full shift, 8-hour Time Weighted Average (TWA) samples, and 30 minute, Excursion Limit (EL) samples.  Personal air samples are also referred to as “breathing zone samples”.


  1. 2.    Area Air Sampling:  Purpose: Code Rule 56 and/or AHERA compliance


Area air samples are collected in pre-selected locations using either low volume battery operated or high volume electric sampling pumps. New York State Code Rule 56 specifies four distinct categories of asbestos abatement project area air monitoring:


  • Background
  • Pre-Abatement
  • Daily (work-in-progress)
  • Final Clearance (aggressive)


In addition, the United States Environmental Protection Agency (USEPA) AHERA Standard requires clearance air sampling for school abatement projects.












Personal air sampling is performed to comply with OSHA requirements for monitoring and documenting workplace exposures to airborne hazardous substances. In addition to fulfilling the OSHA requirements, these samples serve the following functions:

  • To determine the level of respiratory protection needed.
  • To determine the quality of work practices.
  • In order to provide sufficient data for a Negative Exposure Assessment.

General Considerations

To perform air sampling in the breathing zone of the worker, we cannot use stationary (120 VAC) sampling equipment. Asbestos abatement workers are very mobile and do not have a permanent workstation. Therefore, portable battery powered sampling pumps are used.

The portable pump is placed on the worker, typically suspended from a waste belt made from duct tape. Positioning the pump at the back minimizes the interference of the pump with the work being performed. The air tubing is strung from the pump over the shoulder of the worker and is securely taped there. The air sample cassette must be placed within the worker’s breathing zone (within a 1 foot hemisphere centered around the employees mouth), typically at the shoulder or lapel area. The cassette should be angled downward to prevent water sprays and/or debris from falling directly into the filter.


The exposures of representative workers assigned to each distinct task must be evaluated. For example, if there are two people scraping, 2 people bagging and 2 people in the waste decon station, a minimum of 3 personal full shift (TWA) samples must be run and 3 EL samples must be run.


Personal air sampling has to be performed initially to determine worker exposures at each site. If measured levels are statistically reliable and consistently demonstrated to be below the PEL of 0.1 fiber per cubic centimeter (f/cc) or if a Negative Exposure Assessment has been obtained, the personal air sampling can be discontinued for the employees for whom the air sampling was representative.


Personal air sampling is performed for the duration of the shift and is calculated as an eight hour Time Weighted Average (TWA). Excursion samples (EL – 30 min.) are performed during peak exposure periods. The fiber concentrations in the work area will depend, in large degree, on the type of work practices being employed. It is obvious that the fiber concentrations will not stay constant during the shift. If one cassette is used for the duration of the sampling, no special calculation is necessary because all of the fibers will be collected on the same filter and fiber concentrations will be averaged by the nature of the sampling method. In some situations, however, the fiber and dust concentrations will be too high to permit continuous air sampling on the one filter  for the full shift.


In this case, the filter will become overloaded with fibers and/or dust and will become unreadable. When the expected fiber or dust levels are high, the sampling period can be

split into shorter periods, with a new cassette being used for each period. In any case,

the sampling must continue throughout the full work shift.


The TWA for this type of sampling will be calculated in the following manner:

Sequential Samples____________ Fibers/cc__________ Hours of Sampling

P-l                                         0.1                                                  2

P-2                                        0.2                                                  2

P-3                                         0.3                                                  2

P-4                                        0.1                                                  2

P-5                                        0.1                                                  2

0.1 x2 + 0.2 x2 + 0.3 x2 + 0.1 x 2 + 0.1 x2

TWA    =                                                                                       = 0.2 f/cc



Note that in this case, the shift lasted ten hours; however, the average is based on an eight-hour exposure. The reason for this is that the PEL is established as a TWA over an eight-hour period. In other words, you cannot be exposed to the PEL for more than 8 hours per day. Exposure to 0.1 fibers/cc for more than 8 hours/day will result in exceeding the PEL.


Calibration of Air Pumps


OSHA recommends a flow rate of from 0.5 to 2.5 liters per minute (lpm) for personal air sampling. The calibration procedure for personal pumps is identical to the calibration of stationary high-volume pumps. The calibration should be repeated at the end of the sampling period and both the starting and ending flow rates should be recorded on the chain-of-custody form (data sheet).


Some pump models have built-in rotometers. These rotometers should be calibrated at least once a month as they are often subject to contamination and abuse under field conditions. It is good practice however, to use a precision rotometer for all daily pump calibrations even when the pump is equipped with a built-in rotometer. Pumps that are calibrated incorrectly will result in an inaccurate volume, thereby providing the worker (and employer) with incorrect estimates of the actual exposure. In most situations, incorrect information is worse than no information at all.



The Sampling Procedure


Personal air sampling, as per OSHA regulations, is the responsibility of the employer. In practice, the project air-sampling technician may be called upon to answer questions regarding the techniques of personal air sampling because the knowledge of contractor field personnel in this area may be limited.


Personal sampling must be started simultaneously with the start of exposure, typically when the worker enters the containment. The sampling is stopped when the exposure is discontinued (lunch or other breaks). If the pump is left running inside the containment, the resulting TWA as calculated from the results of sampling will be higher than the actual worker exposure, which may lead to unnecessary use of more expensive and cumbersome respiratory protection equipment.

When a worker takes a lunch break, or otherwise ceases to be exposed for a period of time, the pump is stopped and the time recorded on the chain-of-custody form. When the worker returns to the containment to resume work, the pump is re-started and this time is recorded on the chain-of-custody form. When the shift ends and the worker exists the containment, the pump is stopped and the final stop time and flow rate are recorded.

If the filter falls out of the holder before, during or after sampling, the sample is invalidated and must be replaced. The appropriate information should be recorded on the chain-of-custody form. Similarly, if the cassette comes apart during sampling, the sample must be voided. However, if the cassette itself falls off the tubing, but the filter remains intact and is not contaminated, the filter can be re-attached and sampling can be continued.

Pump Maintenance

Personal air sampling pumps operate on rechargeable batteries. Depending on the model, twelve to sixteen hours of charging time are necessary for the eight to ten hours of operation. The battery should always be fully charged before the start of sampling. If pumps are consistently run for less than 8 hours, some batteries may develop a “memory” and will not retain a charge adequate for more than the time it has routinely been run. To avoid this condition, periodically allow the pump to run until the battery has fully discharged. Some chargers are equipped with a discharge/recharge feature to perform this step automatically.  Always follow the Manufacturer’s recommendations for pump and battery maintenance.

The Analytical Method

The OSHA Recommended Method (ORM) for personal air sampling in asbestos abatement is the NIOSH 7400 Method, which is provided at the end of this section. The NIOSH method specifies Phase Contrast Microscopy (PCM). The advantages of PCM is that it is relative simple and low cost. The disadvantage is its lack of specificity. This means it will not distinguish between asbestos fibers and non-asbestos fibers, thus all fibers will be counted.


  • Summary of the PCM Analytical Method – The mixed cellulose ester (MCE)  filters are collapsed by acetone vapor. This makes the filter transparent, however, some fibers are transparent also. To enable the analyst to see the transparent fibers, Phase Contrast Microscopy is used. In short, PCM makes the transparent fibers visible because they have a different refractive index (they bend light differently) than the filter media. The magnification used for PCM is 400 X.


  • Definition of a fiber – Regardless of the analytical methods used, a fiber is defined as a structure which is at least five microns (one millionth of a meter) long, and has a length to width aspect ratio of at least 3:1. Both of these criteria are based on the current knowledge of health effects of asbestos fibers. It is presently accepted that fibers less than 5 microns long do not exhibit significant health effects. The rationale for the length to width ratio limitation is the same.


  • Preparation Technique and Analysis of the Sample – A glass slide with the collapsed filter on it is viewed under the phase contrast microscope. The microscope is equipped with a device called a Walton-Beckett reticule. This device limits the field in which the fibers will be counted. The reticule is an optical piece of glass with a 100mm diameter circle etched into the glass. The area of this circle represents a “field”. After the fibers are counted in a specified number of fields, the total number of fibers on the filter can be calculated. The method requires counting 100 fields or 100 fibers, whichever comes first. A minimum of 20 fields must be counted. Dividing the resulting fiber count by the number of liters of air that was collected allows a determination of the concentration of fibers in the sampled air.


  • Transmission Electron Microscopy (TEM) – TEM is the definitive method for analysis of air samples for airborne asbestos fibers. This method is mandated by the AHERA standard for final air sampling in schools. The advantages of this method include its higher sensitivity and specificity. Higher sensitivity is explained by the higher magnification achievable with electron microscopy (4000 X common for asbestos analysis) permitting thinner fibers to be seen as compared to PCM. The specificity of the method is explained by its coupling with X-ray diffraction (XRD). The crystalline structure of the mineral can be determined by using XRD. This structure will be not only specific for each type of asbestos, but will also be specific for the same types of asbestos coming from different mines. Therefore, a positive identification of asbestos structures is possible.

The disadvantages of TEM are the longer time required for analysis (time for the filter dissolution alone is 24 to 32 hours) and the higher cost, resulting from the higher costs of equipment, labor and exhaustive quality control work. Concerns are being expressed that with market pressures and falling prices, the quality of analysis may be compromised.


  • Summary of the TEM procedure – Filters to be analyzed are coated with a thin layer of platinum or graphite in a vaporizer, then dissolved in an acetone vapor and placed in the electron microscope and viewed. X-ray diffraction on selected structures is performed. The resulting diffraction pattern is analyzed. Determination of airborne fiber concentration from the analytical results is identical to that of the PCM method.


  • Scanning Electron Microscopy (SEM) – Scanning electron microscopy is more expensive and time-consuming than PCM, but less so than TEM.


Both TEM and SEM utilize electron microscopy. However, only SEM shows the surface of the sample, whereas TEM creates the image of the whole depth of the sample. As XRD is not used with SEM, this method in contrast to TEM, is not specific for asbestos fibers because it relies on the morphology (shape) of the fiber and does not provide information on the crystalline structure of the material. As a consequence, SEM is rarely used for asbestos analysis.


Choosing an Appropriate Method


Without specifically stating it, the State and Federal Government, as well as the Industrial Hygiene community, have tacitly agreed that Phase Contrast Microscopy provides an index number that can be used both for worker and public protection. Indeed, when counting all fibers, rather than just asbestos fibers, we seem to err on the side of overprotection, rather than under protection. This statement is supported by practical experience that shows that a site which had a satisfactory PCM result very seldom fails under side by side TEM sampling, while the reverse may happen quite often.


TEM analysis, however, is mandated under EPA (AHERA) regulations. Therefore, in addition to the New York State Department of Health ELAP accreditation, final air sampling in school projects has to be performed by TEM. In New York State, the project cannot be cleared by TEM alone unless the pre-airs and dailys were also analyzed by TEM methodology. Therefore, because New York State requires consistency of methodology, school projects typically have to be sampled by both PCM and TEM methodology on a side-by-side basis, with the TEM samples usually held pending satisfactory clearance by PCM analysis.


TEM analysis may also be mandated by contract specifications for any project if the architect or project designer decides it is necessary or desirable.   For example, TEM sampling is frequently specified for health care facilities, pharmaceutical manufacturing plants and large office complexes undergoing abatement projects.


The Analytical Laboratory


OSHA requires that the analytical laboratory successfully participate in a national quality assurance program. For work performed in New York State, the analytical laboratory selected must be accredited/certified by the American Industrial Hygiene Association (AIHA) and the New York State Department of Health Environmental Laboratory Approval Program (NYSDOH ELAP).







Area air sampling is conducted to comply with New York State Code Rule 56 requirements and/or AHERA requirements. Four types of air sampling are detailed in Code Rule 56 and are described below.  Area air samples, except for Background and Clearance air samples, shall be collected and air samples run for each entire work shift. Area air samples must be collected with a minimum flow rate capacity of two (2) liters per minute (lpm) and a maximum flow rate consistent with the applicable accepted air sampling and analysis methodology. The flow rate for each air sample shall be pre-calibrated and post-calibrated at the beginning and end of each air sample collection.  The calibrations shall be recorded.  Primary and secondary calibration devices shall be calibrated as per NYS DOH ELAP requirements.  The air sampling technician shall be on-site to observe and maintain air sampling equipment for the duration of air samples collection.


Background Samples


Background samples are collected before any other work relating to the abatement project begins. The purpose of background sampling is to determine the background or ambient level of airborne fibers prior to the start of any work which might artificially increase these levels. The background samples will be used later to determine if the airborne fiber concentrations have changed as a result of pre-abatement site prep work or by the abatement work itself. It is obvious that it will be impossible to determine such changes without results of the analysis of these samples taken prior to the start of other field activities. Background samples are collected both inside and outside the prospective work area.


Work Area Preparation Samples


Work Area Preparation (Prep) air samples are collected during abatement site prep work but prior to the start of actual abatement activities. When required, “Work Area Prep” samples are collected outside the prospective work area and must be collected during each shift, for the entire workshift, throughout the prep phase of the project.


Asbestos Handling Air Samples


Air samples collected while the abatement project is in progress are referred to by several names including “work in progress samples (WIPs), environmentals, dailys, etc. This type of air monitoring is performed every day while abatement activities occur.


When required, Asbestos Handling, or WIP, air samples are collected outside the work area only, including the clean room of the decontamination facility and at each negative air exhaust or 1 sample collected at the terminating point of a bank of up to a max of five negative air exhausts. The purpose of this air monitoring is to document the integrity of the containment barriers and the proper functioning of the negative air machines.


Should the barriers become damaged, or the pressure differential between the work area and adjacent areas disappear, a possibility is created for the airborne fibers within the containment to escape and contaminate the surroundings. A comparison is made between these daily samples and samples taken outside the work area prior to the start of abatement (backgrounds).


If an elevated fiber concentration is detected, one of the possible conclusions is that the contamination of the outside area is due to faulty abatement practices. When elevated airborne fiber levels are detected, the work must stop and the reasons for the elevated fiber levels shall be determined. The area outside the work area must be cleaned prior to the restart of abatement practices.


It should be noted that many activities could result in elevated fiber levels including vacuuming of carpets, cutting wood, re-insulation work involving fiberglass and other similar activities. These possibilities must also be investigated. If it is believed that the source of the elevated fiber levels is not associated with the abatement project, transmission electron microscopy (TEM) analysis may be performed to verify that the fibers are non-asbestos.


Post Abatement (Final Clearance) Sampling


Air samples collected at the conclusion of the abatement project are referred to as final or clearance samples. Final samples are collected inside and outside the abatement containment in the identical positions as the original background samples.

The purpose of these samples is to determine if the abatement project work area satisfies the New York State clearance criteria of <0.01F/CC or the established background, whichever is greater. If the abatement project is a school, the AHERA clearance criteria of less than or equal to 70 structures/mm2 also applies.


If the final air samples are below the appropriate clearance criteria, the area can be released to the building occupants following removal of the containment and all asbestos waste.


Clearance samples must be collected using aggressive air sampling techniques. This means that the air is agitated prior to and during the air sampling process with air moving devices such as a leaf blower and fans.


Aggressive Sampling Techniques


Aggressive sampling shall be performed in the following manner:


Pre-sampling Agitation:   Before starting the sampling pumps, direct the exhaust of

forced air equipment against all walls, ceilings, floors, ledges

and other surfaces. Continue for at least five minutes per 1000

square feet of floor space within the enclosure.


On-going Agitation:           Use a 20-inch fan placed in the center of each room. Use

one fan per 10,000 cubic feet of space within the enclosure.

Operate the fan(s) on slow speed and point towards the ceiling

until the sampling is finished. During clearance air sampling, the

negative air equipment must be operated at a rate of two air

changes per hour.


Determining the Number of Samples to Be Collected


The following table summarizes the types of air sampling to be performed for each size of abatement project and the minimum number of samples, which must be collected according to NYS CRR 56.








The table above summarizes the requirements of State of New York regulations. You can collect additional samples if you think it is necessary, but you cannot collect any less samples than specified in the table.


For a minor project, no area sampling is mandated, except if there is a breach or loss of integrity in the glovebag or tent, or upon an incidental disturbance or if the minor project is part of a small or large project. However, if you do make a decision to perform area air sampling on a minor project, make sure you collect background samples. Otherwise you will have no baseline for the results of final air sampling.





Equipment for Area Air Sampling


The following equipment is necessary for conducting area air sampling in support of abatement projects:


  • High volume stationary pumps, associated flexible tubing and telescopic
  • A calibrated rotometer
  • Fans, leaf blower
  • Power cords & GFCI
  • 25 mm air sampling cassettes

–   0.8 um Mixed Cellulose Ester Filter (MCEF) for PCM analysis

–   0.45 um MCEF or 0.45 um Polycarbonate for TEM analysis


High Volume Sampling Pumps – High volume pumps are available from a variety of manufacturers and are very similar in design. The basic parts of the pump are the electrical motor (120VAC) and the vacuum pump mounted on the rotor of the motor. The vacuum pump is equipped with a regulating valve, a vacuum gauge (manometer) and a nipple for connecting the sample tubing. This assembly may also include a critical orifice (described below).


The pump is usually placed in a protective case, which is used for pump transportation, prevents mechanical and water damage to the pump, and can in some designs be used for mounting the telescopic stand. However, the stand must always be separate from the pump to prevent vibration of the air cassette.

Some pump models include multiple vacuum pumps mounted on one electric motor. Lengths of tubing are then used to provide for air sampling in appropriate locations. This version of air sampling equipment lacks the flexibility necessary for most typical asbestos abatement projects.






                FIGURE 12-1                                                          FIGURE 12-2

                Proper Sampling Technique                     Air Sampling Cassette

















Critical Orifices


A critical orifice is a metal plug with a carefully machined hole in it. The principle of the critical orifice is that when air pressure downstream of the orifice falls below 53% of the air pressure upstream from the orifice, the air flowing through the critical orifice reaches the speed of sound, which it cannot exceed. Thus, as long as the vacuum pump maintains the air pressure downstream from the orifice below 53% of the upstream pressure, the flow rate through the orifice will be constant. The orifices can be manufactured and pre-calibrated to deliver a specific flow rate of 10 lpm, for example. Critical orifices have to be periodically calibrated and cleaned, because of dust accumulation in the orifices, which changes the effective diameter and consequently, the flow rate. The primary advantage of the critical orifice is the fact that the calibration of the pump will not vary during sampling (however, pump calibrations must still be documented before and after sampling.




A rotometer is a device for accurately measuring flow rate. The flow rate is the rate at which the pump is adjusted to draw air through the filter. This rate is usually expressed in liters per minute (l pm).

The body of the rotometer is often made of a block of clear plastic. Some are constructed with a glass tube. The diameter of the channel increases from the bottom to the top.

Inside the channel, a bore or float is placed. When air flows through the channel, it raises the bore to a level that is proportional to the volume of air flowing through the  channel. The higher the flow rate, the higher the bore will be suspended in the channel. The rotometer is graduated in liters per minute or in low flow rotometers, in cubic centimeters per minute (cc/minute). The readings provided by a rotometer will change over time as a result of wear and accumulation of dust on the channel walls and bore. This makes it necessary to periodically calibrate the rotometer against a primary standard.

A primary standard flow meter is a glass burette or electronic calibrator. These devices use a large diameter channel and a soap bubble film instead of a metal, glass or plastic bore. A stopwatch is used in the glass burette system to measure the time necessary for the soap film to travel from a zero mark to a one-liter mark. The actual flow rate measured in liters per minute can then be calculated.

Calibration through the range of gradations on the rotometer is desirable, with a minimum of 5 rates being adequate, for example 2,4, 6, 8 and 10 liters per minute.

Electronic calibrators also use a soap film and electronically time the flow with the aid of an infrared detector at the zero and end points of the channel.

Rotometers should be calibrated against primary standards on a monthly basis, quarterly at minimum. These calibrations should be documented.

Air Sampling Cassettes

Two types of filter material are used for asbestos air sampling. For PCM analysis, MCEF cassettes are used. For TEM analysis MCEF or Polycarbonate may be used, however, the pore size is smaller than for PCM analysis. The smaller pore size allows these filters to capture fibers of a smaller diameter, which cannot be seen under PCM analysis. The cassette body is identical in all cases.


Each cassette must be labeled with a minimum of a sample ID and date.


Extension Cords and Power Strips

Providing sources of electrical power on an asbestos abatement project is traditionally the responsibility of the contractor. However, the person performing air sampling will need adequate amounts of extension cords and power strips to set up the required number of sampling pumps in all the selected locations. The length and number of cords will be determined based on the size of the area to be covered and the availability of  power source(s). It is important to assure that there is an adequate power supply prior to starting pumps. To determine if the power source is sufficient for the number of pumps needed, the following formula can be used:


AMPS X VOLTS = WATTS. For example, a 15-amp circuit at 120 volts will provide a maximum of 1800 watts (15 X 120 = 1800). The power requirements (in watts) of each sampling pump should be found on the motor nameplate.

Extension cord use should also be considered in power calculations. Use only cords rated to carry the load anticipated and be aware that long lengths of cord and numerous connections will provide some resistance to current flow, reducing the maximum useable power at the end point.  All circuits must be protected by GFCIs at the power source. OSHA regulations stipulate that extension cords not exceed two hundred (200’) feet in length.



The locations for project area air samples are selected at the time of background and pre-abatement air sampling. Subsequently, air sampling during abatement and final clearance is performed in the same locations as the pre-abatement and background sampling.

The locations for sampling inside the future work area should be selected so that there is no restriction or obstruction of the airflow at the sampling point. The samplers should not be placed in corners or near walls. Within these constraints, the samplers should be placed at random in the work area. If the work area contains the number of rooms equivalent to the required number of samples, collect a sample in each room. When the number of rooms is greater than the required number of samples, a representative sample of rooms should be selected.

When selecting the locations outside the work area, one sampler should be placed at the entrance to the decontamination unit, one next to (at a maximum of 10 feet) the exhaust of each negative air machine, and 1 for a bank of up to five machines max and the rest equally spaced near the critical barriers. In doing so, all locations where the possibility of airflow from the containment out will be monitored.



  • Select the air sampling locations as described above.


  • Determine power requirements and availability.


  • Set up samplers.


  • Label cassettes.


  • Calibrate each sampler with the cassette in-line; adjusting the flow is necessary to
    obtain the desired flow rate.
  • Remove the end cap of the cassette and begin sample collection.
    • Fill out the chain of custody form (data sheet) by recording all data pertinent to the
      project, sample numbers, flow rates, time that the samplers were turned on, and
      the name of the individual conducting the sampling.
    • Perform sampling for the time determined to be adequate to collect the desired
      volume of air (daily air samples must be full shift in New York State).
    • Repeat the calibration procedure (do not adjust the flow rate at this time).
    • Turn sampler off.
    • Record the end flow rate and time off.
      • Calculate the amount of time the pump was on in minutes and record it on the
        chain-of-custody form.
      • Calculate the volume of air sampled and record it on the chain-of-custody form.
      • Sign and date the form.


In New York State, the person performing project air sampling must hold a valid Air Sampling Technician license (Restricted II), since each individual must hold a valid license in the class for which the work is being performed. A project monitor would, therefore, be required to also hold a license as an air-sampling technician to conduct project air sampling. It should be noted that personal air sampling, since it is performed to comply with OSHA requirements, does not require a NYS air sampling technician license, but must be performed by a “competent person”.


Figure 12-3

 Sample Chain-of-Custody/Air Sample Data Sheet





CLIENT: REQUESTED T/A: 1. Immediate(Circle one)               2. 2 4 Hour3. 72 Hour♦Please                  4. Other (*)







PROJECT #:                                                                                LAB QUOTE #:
METHOD REFERENCE: PHONE #:  _____________________FAX #:  ________________________NUMBER OF SAMPLES:



  TIME   X   FLOW     =  VOLUME
Total Min. X  Liters/Min.  = Liters


COMMENTS ______________________________________________________________________




Formula: various                                                                                 Fibers

Method:          7400

M.W.: various                                                                                       Issued:           2/15/84

Revision #3   5/15/89


OSHA:   0.2 asbestos fiber (25 чm long) /cc:                                              Properties:     solid, fibrous

1 asbestos fiber/cc 30-minute excursion [1]

MSHA:   2 asbestos fibers (>5 чm long) /cc [2]

NIOSH:  carcinogen; control to lowest level possible [3]; 3 glass fibers (>чm x<3.5 чm /cc[4]

ACGIN:  0.2 crocidolite; 0.5 amosite; 2 chrysotile and other asbestor, fibers/cc [5]



SYNONYMS: actinolite [CAS #13768-00-8] or ferroactinolite; cummingtonite-grunerite (amosite) crocidolite [CAS # 12001-28-4] or riebeckite; tremolite [CAS #14567-73-8]; amphibole asbestos; fibrous glass.





(0.45 to 1.2 чm cellulose ester membrane, 25 mm diameter; conductive cowl on cassette).


FLOW RATE: 0.5 TO 16 L/min


VOL-MIN:       400 l @ 0.1 fiber/cc

VOL-MAX       (step 4, sampling)

Adjust to give 100 to 1300 fibers/mm2


SHIPMENT: routine (pack to reduce shock)


FIELD BLANKS: 10% of samples




RANGE STUDIED: 80 to 100 fibers


OVERALL PRECISION (S): 0.115 to 0.13 [7]












ANALYTE: fibers (manual count)


SAMPLING PREPARATION: acetone/triacetin “hot block” method [6]


COUNTING RULES: described in previous version of this method as A rules [1,7]


EQUIPMENT: 1. Positive phase-contrast


2. Walton-Beckett graticule (100

чm field of view) Type G-22

3. phase-shift test slide




RANGE:  100 TO 1300 fibersmm2 filter area


ESTIMATED LOD: 7-fibers/mm2-filter area




APPLICABILITY: The quantitative working range is 0.04 to 0.5 fiber/cc for a 1000-L air sample.  The LOD depends on sample volume and quantity of interfering dust, and is, 0.01 fiber/cc for atmospheres free of interferences.  The method gives an index of airborne fibers.  It is primarily used for estimation asbestos concentrations, though PCM does not differentiate between asbestos and other fibers.  Use this method in conjunction with electron microscopy

(e.g. Method 7402) for assistance in identification of fibers.  Fiber, ca, 0.25 mm diameter will not be detected by this method [8].  This method may be used for other materials such as fibrous glass by using alternate counting rules (see Appendix C).


INTERFERENCES:  Any other airborne fiber may interfere since all particles meeting the counting criteria are counted.  Chain-like particles may appear fibrous.  High levels of non-fibrous dust particles may obscure fibers in the field of view and increase the detection limit.


OTHER METHODS:  This method introduces changes for improved sensitivity and reproducibility.  It replaces P& CAM 239 [7.9] and NIOSH Method 7400.  Revision #2 (dates 8/15/87).




FIBERS                                                                                             METHOD: 7400




  1. Acetone*
  2. Triacetin (glycerol triacetate), reagent grade.





  1. Sampler: field monitor, 25 mm, three-piece cassette with ca. 50 mm electrically conductive extension cowl and cellulose ester filter, 0.45 to 1.2 чm pore size, and backup pad.


NOTE 1:  Analyze representative filters for fiber background before use.  Discard the filter lot if mean is 25 fibers per 100 graticule fields.  These are defined as laboratory blanks.  Manufacturer-provided quality assurance checks on filter blanks are normally adequate as long as field blanks are analyzed as described below.


NOTE 2:  The electrically conductive extension cowl reduces electrostatic effects.  Ground the cowl when possible during sampling (10).


NOTE 3:  Use 0.8 чm pore size filters for personal sampling.  The 0.45 чm filters are recommended for sampling when performing TEM analysis on the same samples.  However, their higher-pressure drop preludes their use with personal sampling pumps.


  1. Sampling pump, 0.5 to 16 L/min (see step 4 for flow rate), with flexible connecting tubing.
  2. Microscope, positive phase (dark) contrast, with green or blue filter, adjustable field iris, 8 to 10x eyepiece, and numerical aperture = 0.65 to 0.75.
  3. Slides, glass, frosted-end, pre-cleaned, 25 x 75 mm.
  4. Cover slips, 22 x 22 mm, No. 1-1/2 unless otherwise specified by microscope manufacturer.
  5. Lacquer or nail polish.
  6. Knife, #10 surgical steel, curved blade.
  7. Tweezers.
  8. Heated aluminum block for clearing filters on glass slides (see ref. [6] for specifications or see manufacturer’s instructions for equivalent devices).
  9. Micropipettes, 5-uL and 100 to 500 uL.
  10. Graticule, Walton-Beckett type, 100 чm diameter circular field (area =
    0.00785 mm2 ) at specimen plane (type G-22).  Available form PTR Optics LTD., 145 Newton Street, Accessories and Components, 850 Pasquinelli Drive, Westmont, IL 60559 (phone 312-887-7100).


NOTE: The graticule is custom-made for each microscope. See Appendix A for the custom ordering procedure).


  1. HSE/NPL phase contrast test slide, Mark II.  Available from PTR Optics LTD. (address above).
  2. Telescope, ocular phase-ring centering.
  3. Stage micrometer (0.01 mm divisions).
  4. Wire, multi-stranded, 22-gauge.
  5. Tape, shrink or adhesive.




*SPECIAL PRECAUTIONS:  Acetone is extremely flammable.  Take precautions not to ignite it.  Heating of acetone in volumes greater than 1mL must be done in a ventilated laboratory fume hood using a flameless, spark-free source.





  1. Calibrate each personal sampling pump with a representative sampler in line.


  1. For personal sampling, fasten sampler to the worker’s lapel near the worker’s mouth.  Remove top cover from cowl extension (open face) and orient face down.  Wrap joint between cowl and monitor body with tape to help hold the cassette together, keep the joint free of dust, and provide a marking surface to identify the cassette.

NOTE:  If possible, ground the cassette to remove any surface charge, using a wire held in contact (e.g. with a hose clamp) with the conductive cowl and an earth ground such as a cold-water pipe.


  1. Submit at least two field blanks (or 10% of the total samples, whichever is greater) for each set of samples.  Handle field blanks in the same fashion as other samplers.  Open field blank cassettes at the same time as other cassettes just prior to sampling.  Store top covers and cassettes in a clean area with the top covers from the sampling cassettes during the sampling period.
  2. Sample at 0.5 L/min or greater [11].  Adjust sampling flow rate, q (L/min), and time,

t (min), to produce a fiber density, E, of 100 to 1300 fibers /mm² (3.85 – 104 to 5 – 105 fibers per 25 mm filter with effective collection area Aƈ= 385 mm2) for optimum accuracy.  These variables are related to the action level (one half the current standards), L (fibers/cc), of the fibrous aerosol being sampled by:


, min

t = Aƈ– E


      Q – L – 103



 NOTE 1:  The purpose of adjusting sampling times is to obtain optimum fiber loading on the filter.  A sampling rate of 1 to 4 L/min for 8 hrs. is appropriate in atmospheres containing ca. 0.1 fiber/cc in the absence of significant amounts of non-asbestos countable samples.  In such cases take short, consecutive samples and average the results over the total collection time.  For documenting episodic exposures, use atmospheres, where targeted fiber concentrations are much less than 0.1-fiber/cc use larger sample volumes (3,000 to 10,000 L) to achieve quantifiable loadings.  Take care, however, not to overload the filter with background dust.  If ≥ 50% of the filter surface is covered with particles, the filter may be too overloaded to count and will bias the measured fiber concentration.


NOTE 2:  OSHA regulations specify a maximum sampling rate of 2.5 L/min [1].


NOTE 3:  OSHA regulates specify a minimum sampling volume of 48 L for an excursion measurement [1].


  1. At the end of sampling, replace top cover and end plugs.


  1. Ship samples with conductive cowl attached in a rigid container with packing material to prevent jostling or damage.

     NOTE:  Do not use untreated polystyrene foam in shipping container because
electrostatic forces may course fiber loss from sample filter.




NOTE 1:  The object is to produce samples with a smooth (non-grainy) background in a medium with refractive index of 1.46.  This method collapses the filter for easier focusing and produces relatively permanent mounts, which are useful for quality control and inter-laboratory comparison.  The aluminum “hot block” or similar flash vaporization techniques may be used outside the laboratory [6].  Other mounting techniques meeting the above criteria may also be used (e.g. the laboratory fume hood procedure for generating acetone vapor as described in Method 7400 – revision of 5/15/85, or the non-permanent field mounting technique used in P&CAM 239 (3,7,9, 12).  A videotape of the mounting procedure is available form the NIOSH Publication Office [13]




NOTE 2:  Excessive water in the acetone may slow the clearing of the filters, causing material to be washed off the surface of the filter.  Also, filters that have been exposed to high humidity’s prior to clearing may have a grainy background.


  1. Ensure that the glass slides and cover slips are free of dust and fibers.


  1. Adjust the rheostat to heat the “hot block” to ca. 70 •C [6].

NOTE: If the “hot block” is not used in a fume hood, it must rest on a ceramic plate and be isolated from any surface susceptible to heat damage.


  1. Mount a wedge cut from the sample filter on a clean glass slide.
    1. Cut wedges of ca. 25% of the filter area with curved-blade knife using a rocking motion to prevent tearing.  Place wedge, dust side up, on slide.

      NOTE: Static electricity will usually keep the wedge on the slide.


    1. Insert slide with wedge into the receiving slot at the base of “hot block”.  Immediately place tip of a micropipette containing ca. 250 uL acetone (use the minimum volume needed to consistently clear the filter sections) into the inlet port of the PTFE-cap on top of the “hot block” and inject the acetone into the vaporization chamber with a slow, steady pressure on the plunger button while holding pipet firmly in place.  After waiting 3 to 5 seconds for the filter to clear, remove pipet and slide from their ports.


      CAUTION:  Although the volume of acetone used is small, use safety precautions.  Work

in a well-ventilated area (e.g. laboratory fume hood).  Take care not to ignite the acetone.

Continuous, frequent use of this device in an unventilated space may produce explosive

acétone vapor concentrations.


    1. Using the 5-uL micropipette, immediately place 3.0 to 3.5 uL triacetin on the wedge.  Gently lower a clean cover slip onto the wedge at a slight angle to reduce bubble formation.  Avoid excess pressure and movement of the cover glass.

NOTE:  If too many bubbles form or the amount of triacetin is insufficient, the cover slip may become detached within a few hours.  If excessive triacetin remains at the edge of the filter under the cover slip, fiber migration may occur.


  1. Glue the edges of the cover slip to the slide using lacquer or nail polish [14] counting may proceed immediately after clearing and mounting are completed.

                  NOTE:  If clearing is slow, warm the slide on a hotplate (surface temperature

50 •C) for up to 15 minutes to hasten clearing.  Heat carefully to prevent gas bubble





  1.  Microscope adjustments.  Follow the manufacturer’s instructions.  At least once daily

use the telescope ocular (or Bertrand lens, for some microscopes) supplied by the manufacturer to ensure that the phase rings (annular diaphragm and phase-shifting elements) are concentric.  With each microscope, keep a logbook in which to record the dates of calibrations and major servicing.


    1. Each time a sample is examined, do the following:
  1. Adjust the light source for even illumination across the field of view at the condenser iris.  Use Kohler illumination, if available.   With some microscopes, the illumination may have to be set up with bright field optics rather than phase contrast optics.


  1. Focus on the particulate material to be examined.


  1. Make sure that the field iris is in focus, centered on the sample, and open only enough to fully illuminate the field of view.


    1. Check the phase shift detection limit of the microscope periodically for each analyst/microscope combination:
  1. Center the HSE/NPL phase contrast test slide under the phase objective


  1. Bring the blocks of grooved lines into focus in the graticule area.

NOTE:  The slide contains seven blocks of grooves (ca. 20 grooves per block) in descending order of visibility.  For asbestos counting the microscope optics must completely resolve the grooved lines in blocks 6 and 7 must be invisible when centered in the graticule area.  Blocks 4 or 5 must be at least partially visible but may vary slightly in visibility between microscopes.  A microscope, which fails to meet these requirements, has resolution either too low or too high for fiber counting.


  1. If image quality deteriorates, clean the microscope optics.  If the problem persists, consult the microscope manufacturer.


  1.  Document the laboratory’s precision for each counter for replicate fiber counts.


    1.  Maintain as a part of the laboratory quality assurance program a set of reference slides to be used on a daily basis [15].  These slides should consist of filter preparations including a range of loadings and background dust levels from a variety of sources including both field and PAT samples.  The Quality Assurance Officer should maintain custody of the reference slides and should supply each counter with a minimum of one reference slide per workday.  Change the labels on the reference slides periodically so that the counter does not become familiar with the samples.
    2. From blind repeat counts on reference slides; estimate the laboratory intra and inter-counter s г (step 21).  Obtain separate values of relative standard deviation for each sample matrix analyzed in each of the following ranges:  5 to 20 fibers in 100 graticule fields, .20 to 50 fibers in 100 graticlue fields, .50 to 100 fibers in 100 graticule fields, and 100 fibers in less than 100 graticule fields.  Maintain control charts for each of these data files.

NOTE:  Certain sample matrices (e.g. asbestos cement) have been shown to give poor precision [16].


  1. 12.  Prepare and count field blanks along with the field samples.  Report counts on

each field blank.

NOTE 1:  The identity of blank filters should be unknown to the counter until all counts have been completed.


NOTE 2:  If a field blank yields greater than 7 fibers per 100 graticule fields, report possible contamination of the samples.


13. Perform blind recounts by the same counter on 10% of filters counted (slides relabeled by a person other than the counter).  Use the following test to determine whether a pair of counts by the same counter on the same filter should be rejected because of possible bias:  Discard the sample if the absolute value of the difference between the square roots of the two counts (in fiberr/mm2) exceeds 2.8 (X) sr, where X = the average of the square roots of the two fiber counts (in fiber/mm2) and sr = one-half the intra-counter relative standard deviation for the appropriate count range (in fibers) determined, from step 11.  For more complete discussions see reference [15].


NOTE 1: Since fiber counting is the measurement of randomly placed fibers which may be described by a Poisson distribution, a square root transformation of the fiber count data will result in approximately normally distributed data [15].


NOTE 2:  If a pair of counts is rejected by this test, recount the remaining samples in the set and test the new counts against the first counts.  Discard all rejected paired counts.  It is not necessary to use this statistic on blank counts.


14. The analyst is a critical part of this analytical procedure.  Care must be taken to provide a non-stressful and comfortable environment for fiber counting.  An ergonomically designed chair should be used, with the microscope eyepiece situated at a comfortable illumination level in the microscope to reduce eye fatigue.  In addition, counter should take 10 to 20 minute breaks from the microscope every one or two hours to limit fatigue [17].  During these breaks, both eye and upper back/neck exercises should be performed to relieve strain.


  1. 15.   All laboratories engaged in asbestos counting should participate in a proficiency-testing program such as the AIHA/NIOSH Proficiency Analytical Testing (PAT) Program or the AIHA Asbestos Analyst Registry and routinely exchange field samples with other laboratories to compare performance of counters.

NOTE:  OSHA requires that each analyst performing this method take the NIOSH direct training course #582 or equivalent [1].  Instructors of equivalent courses should have attended the NIOSH #582 course at NIOSH within three years of presenting an equivalent course.





  1. 16.   Center the slide on the stage of the calibrated microscope under the objective lens.  Focus the microscope on the plane of the filter.


17. Adjust the microscope (Step 10).

NOTE:  Calibration with the HSE/NPL test slide determines the minimum detectable fiber diameter (ca. 0.25 чm) [8]


  1. 18.   Counting rules:  (same as P&CAM 239 rules [3,7 9]; see APPENDIX 8).
    1. Count only fibers longer than .5 чm .  Measure length of curved fibers along the curve.
    2. Count only fibers with a length to width ratio equal to or greater than 3:1.
    3. For fibers which cross the boundary of the graticule field:
      1. Count any fiber longer than 5 чm, which lies entirely within the graticule area.
      2. Count as ½ fiber any fiber with only one end lying within the graticule area, provided that the fiber meets the criteria of rules a and b above.
      3. Do not count any fiber, which crosses the graticule boundary more than once.
      4. Reject and do not count all other fibers.
      5. Count bundles of fibers as one fiber unless individual fibers can be identified by observing both ends of a fiber.
      6. Count enough graticule fields to yield 100 fibers.  Count a minimum of 20 fields.  Stop at 100 graticule fields regardless of count.


  1. 19.  Start counting from the tip of the filter wedge and progress along a radial line to  the outer edge.  Shift up or down on the filter, and continue in the reverse direction.  Select graticule fields randomly by looking away from the eyepiece briefly while advancing the mechanical stage.  Ensure that, as a minimum, each analysis covers one radial line from the filter center to the outer edge of the filter.  When an agglomerate covers ca. 1/6 or more of the graticule field, reject the graticule field and select another.  Do not report rejected graticule fields in the total number counted.


NOTE 1:  When counting a graticule field, continuously scan a range of faocal planes by moving the fine focus knob to detect very fine fibers which have become embedded in the filter.  The small diameter fibers will be very faint but are an important contribution to the total count.  A minimum counting time of 15 seconds per field is appropriate for accurate counting.


NOTE 2:  This method does not allow for differentiation of fibers based on morphology.  Although some experienced counters are capable of selectively counting only fibers, which appear to be asbestiform, there is presently no accepted method for ensuring uniformity of judgment between laboratories.  It is, therefore, incumbent upon all laboratories using this method to report total fiber counts.  If serious contamination form non-asbestos fibers occurs in samples, other techniques such as transmission electron microscopy must be used to identify the asbestos fiber fraction present in the sample (see NIOSH Method 7402).  In some cases (i.e., for fibers with diameters .1 чm ), polarized light microscopy techniques may be used to identify and eliminate interfering non-crystalline fibers [18].


NOTE 3:  Under certain conditions, electrostatic charge may affect the sampling of fibers.  These electrostatic effects are most likely to occur when the relative humidity is low (below 20%), and when sampling is performed near the source of aerosol.  The result is that deposition of fibers on the filter is reduced, especially near the edge of the filter.  If such a pattern is noted during fiber counting, choose fields as close to the center of the filter as possible. [10].





  1. 20.   Calculate and report fiber density on the filter, E (fibers/mm2), by dividing the average fiber count per graticule field, f/nf, minus the mean field blank count per graticule field, B/nb, by the graticule field area, Af (approx. 0.00785 mm2):


( f B)

nF   nb

                                           E=              AF         fibers/mm2



NOTE:   Fiber counts above 1300 fibers/mm2 and fiber counts from samples with >50%

of filter area covered with particulate should be reported as “uncountable” or

“probably biased.”


  1. Calculate and report the concentration, C (fibers/cc), of fibers in the air volume sampled, V (L), using the effective collection area of the filter, Ac (approx. 385 mm2 for a 25-mm filter:

C = (E) (Ac)

V • 103


NOTE: Periodically check and adjust the value of Ac, if necessary.


  1. Report intralaboratory and interlaboratory relative standard deviations (Step 11) with each set of results.


NOTE:  Precision depends on the total number of fibers counted [7,10].  Relative standard deviation is documented in references [7,18,19,20] for fiber counts up to 100 fibers in 100 graticule fields.  Comparability of interlaboratory results is discussed below.  As a first approximation, use 213% above and 49% below the count as the upper and lower confidence limits for fiber counts greater than 20

(Fig. 1).







A.  This method is a revision of P&CAM 239 [3, 7, 9].  A summary of the revisions is as



      1.  Sampling:

The change from a 37-mm to a 250mm filter improves sensitivity for similar air volumes.  The change in flow rates allows for 2-m3 full-shift samples to be taken, providing that the filter is not overloaded with non-fibrous particulates.  The collection of efficiency of the sampler is not a function of flow rate in the range 0.5 to 16L/min [11].


  1. 2.     Sample Preparation Technique:

The acetone vapor-triacetin preparation technique is a faster, more permanent mounting technique than the dimethyl phthalate/diethyl oxalate method of P&CAM 239 [6,8,9].  The aluminum “hot block” technique minimizes the amount of acetone needed to prepare each sample.


  1. 3.     Measurement:
    1. The Walton-Beckett graticule standardizes the area observed [21, 22, 23].
    2. The HSE/NPL test slide standardizes microscope optics for sensitivity to fiber diameter [8, 21].
    3. Because of past inaccuracies associated with low fiber counts, the minimum recommended loading has been increased to 100-fibers/mm2 filter area (80 fibers total count).  Lower levels generally result in an overestimate of the fiber count when compared to results in the recommended analytical range [25].  The recommended loadings should yield intracounter sr in the range of 0.10 to 0.17 [7, 24, 26].



  1. Interlaboratory Comparability:


An international collaborative study involved 16 laboratories using prepared slides

from the asbestos cement, milling, mining, textile, and friction material industries

[16]. The relative standard deviations (sr) varied with sample type and laboratory.

The ranges were:


Intralaboratory          Interlaboratory          Overall


AIA (NIOSH Rules)              0.12 to 0.40               0.27 to 0.85               0.46


* Under AIA rules, only fibers having a diameter less that 3чm are counted and fibers attached to particles larger than 3чm are not counted.  NIOSH rules are otherwise similar to the AIA rules.



A NIOSH study was conducted using field samples o asbestos [24].  This study indicated intralaboratory sr in the range 0.17 to 0.25 and an interlaboratory sr of 0.45.  This agrees well with other recent studies [16, 19, 21].


At this time, there is no independent means for assessing the overall accuracy of this method.  One measure of reliability is to estimate how well the count for a single sample agrees with the mean count from a large number of laboratories.  The following discussion indicates how this estimation can be carried out based on measurements of the interlaboratory variability, as precision and to measured intra- and interlaboratory sr.  (NOTE: The following discussion does not include bias estimated and should not be taken to indicate that lightly loaded samples are as accurate as properly loaded ones).


Theoretically, the process of counting randomly distributed (Poisson) fibers on a filter surface will give a sr that depends on the number, N, of fibers counted:


sr = 1/(N)½                                                                                     (1)


Thus sr is 0.1 for 100 fibers and 0.32 for 10 fibers counted.  The actual sr found in a number of studies is greater than these theoretical numbers [16, 19, 20, 21].


An additional component of variability comes primarily from subjective interlaboratory differences.  In a study of ten counters in a continuing sample exchange program, Ogden [18] found this subjective component of intralaboratory sr to be approximately 0.2 and estimated the overall sr by the term:


N + (0.2 • N)2)1/2                                                                    (2)



Ogden found that the 90% confidence interval of the individual intralaboratory counts in relation to the means were +2sr and –1.5sr.  In this program, one sample out of ten was a quality control sample.  For laboratories not engaged in an intensive quality assurance program, the subjective component of variability can be higher.


In a study of field sample results in 46 laboratories, the Asbestos Information Association also found that the variability had both a constant component and one that depended on the fiber count [21].  These results gave a subjective interlaboratory component of sr(on the same bases as Ogden’s) for field samples of ca. 0.45.  A similar value was obtained for 12 laboratories analyzing a set of 24 field samples [24].  This value falls slightly above the range of sr(0.25 to 0.42 for 1984-85) found for 80 reference laboratories in the NIOSH Proficiency Analytical Testing (PAT) program for laboratory-generated samples [20].


A number of factors influence for a given laboratory, such as that laboratory’s actual counting performance and the type of samples being analyzed. In the absence of other information, such as from an interlaboratory quality assurance program using field samples, the value for the subjective component of variability is estimated a 0.45.  It is hoped that laboratories will carry out the recommended interlaboratory quality assurance programs to improve their performance and thus reduce the sr.



The above relative standard deviations apply when the population mean has been determined.  It is more useful, however, for laboratories to estimate the 90% confidence interval on the mean count from a single sample fiber count (fiber 1).  These curves assume similar shapes of the count distribution for interlaboratory and intralaboratory results [19].


For example, If a sample yields a count of 24 fibers, Figure 1 indicates that the mean inter-laboratory count will fall within the range of 227% above and 52% below that value of 90% of the time.  We can apply these percentages directly to the air concentrations as well.  If, for instance, this same (24 fibers counted) represented a 500-L volume, then the measured concentration is 0.02 fibers/mL (assuming 100 fields counted, 25-mm filter, 0.00785mm2 field counting area).  If this same sample were counted by a group of laboratories, these are a 90& probability that the mean would fall between 0.01 and 0.08 fiber/mL.  These limits should be reported in any comparison of results between laboratories.


Note that the sr of 0.45 used to derive Figure 1 is used as an estimate for a random group of laboratories.  If several laboratories belonging to a quality assurance group can show that their interlaboratory sr is smaller, then it is more correct to use that smaller sr.  However, the estimated sr of 0.45 isto be used in the absence of such information.  Note also that it has been found that sr can be higher for certain types of samples, such as asbestos cement [16].


Quite often the estimated airborne concentration from an asbestos analysis is used to compare to a regulatory standard.  For instance, if one is trying to show compliance with an 0.5 fiber/mL standard using a single sample on which 100 fibers have been counted, then figure 1 indicates that the 0.5 fiber/mL standards must be 213% higher than the measured air concentration.  This indicates that if one measures a fiber concentration of

0.16 fiber/mL (100 fibers counted), then the mean fiber count by a group of laboratories (of which the compliance laboratory might be one) has a 95% chance of being less than 0.5 fibers/mL; i.e., 0.6 + 2.13 X 0.16 = 0.5.


If can be seen from Figure 1 that the Poisson component of the variability is not very important unless the number of fibers counted is small.  Therefore, a further approximation is to simply use +213% and –49% as the upper and lower confidence values of the mean for a 100-fiber count.













The curves in Figure 1 are defined by the following equations


(insert formulas)


where sr = subjective inter-laboratory relative standard deviation, which is close to the

total inter-laboratory sr when approximately 100 fibers are counted.

X = total fibers counted on sample

LCL = lower 95% confidence limit

UCL = upper 95% confidence limit

Note that the range between these two limits represents 90% of the total range.




[1]        Occupational Safety and Health Administration, U.S. Department of Labor, Occupational Exposure to Asbestos, Tremolite, Anthophyllite, and Actinolite Asbestos; Final Rules, 29 CFR Part 1910. 1001 Amended June 20, 1986; Sept 14, 1988. Final Rules 29 CFR 1926. 58 Amended Sept 14, 1988.


[2]        Mine Safety and Health Administration, U.S. Department of Commerce, Exposure Limits for Airborne Contaminants; Part 56.001 Amended July 1, 1988.


[3]        Revised Recommended Asbestos Standard, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-169 (1976); as amended in NIOSH statement at OSHA Public Hearing, June 21, 1984.


[4]        Criteria for a Recommended Standard…Occupational Exposure to Fibrous Glass, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-152 (1977).


[5]        American Conference of Governmental Industrial Hygienist.  “Threshold Limit Values and Biological Exposure Indices for 1988-1989,” ACGIH (1988).


[6]        Baron, P.A. and G.C. Pickford. “An Asbestos Sample Filter Clearing Procedure,” Appl. Ind. Hyg. 1:169-171, 199 (1986).


[7]        Leidel, N.A., S.G. Bayer, R.D. Zumwalde, and K.A. Busch.  USPHS/NIOH Membrane Filter Method for Evaluating Airborne Asbestos Fibers, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 79-127 (1979).


[8]        Rooker, S.J., N.P. Vaughn, and J.M. LeGuen. “On the Visibility of Fibers by Phase Contract Microscopy,” Amer. Ind. Hyg. Assoc. J., 43, 505-515 (1982).


[9]        NIOSH Manual of Analytical Methods, 2nd ed., Vol. 1., P&CAM 239, U.S. Department of Health, Education, and Welfare Pub. (NIOSH) 11-157-A (1977).


[10]      Baron, P. and G. Deye, “Electrostatic Effects in Asbestos Sampling,” Parts I and II Am. Ind. Hyg. Assoc. J. (submitted for publication) (1989).




[11]      Johnston, A.M., A.D. Jones, and J.H. Vincent. “The Influence of External Aerodynamic Factors on the Measruement of the Airborned Concentration of Asbestos Fibers by the membrane Filter Method,” Ann. Occup. Hyg., 25, 309-316 (1982).


[12]      Jankovic, J.T., W. Jones, and J. Clere.  “Field Techniques for Cleaning Cellulose Ester Filters Used in Asbestos Sampling,” Appl. Ind. Hyg., 1:145-147 (1986).


[13]     Sinclair, R.C. “Filter Mounting Procedure,” NIOSH Publication Videotape No. 194

(1984 [updated 1986]).


[14]      Asbestos International Association, AIA Health and Safety Recommended Technical Method #1 (RTMI).  “Airborne Asbestos Fiber Concentraions at Workplaces by Ligh Microscopy” (Membrane Filter Method), London (1979).


[15]      Abell, M., S. Shulman and P. Baron.  The Quality of Fiber Count Data, Appl. Ind. Hyg. (in press) (1989).


[16]     Crawford, N.P., H.L. Thorpe, and W. Alexander,   “A Comparison of the Effects of Different Counting Rules and Aspect Ratios on the Level and Reproducibility of Asbestos Fiber Counts, Part I: Effects on Level” (Report No TM/82/23). “Part II: Effects on Reproducibility” (Report No. TM/82/84).  Institute of Occupational Medicine, Edinburgh, Scotland (December, 1982).


[17]     “Potential Health Hazards of Video Display Terminals, “ NIOSH Research

Report, June 1981.


[18]     McCrone, W. L. McCrone and J. Delly, “Polarized Light Microscopy,” Ann Arbor

Science (1978).


[19]     Ogden, T.L. “The Reproducibility of Fiber Counts, “  Health and Safety Executive

Research Paper 18 (1982).


[20]     Schlecht, P.C. and S.A. Schulman. “Performance of Asbestos Fiber Counting

Laboratories in the NIOSH Proficiency Analytical Testing (PAT) Program, “ AM.

Ind. Hyg. Assoc. J., 47, 259-266 (1986).


[21]      “A Study of the Empirical Precision of Airborne Asbestos Concentration

Measurements in the workplace by the Membrane Filter Method,”  Air Monitoring

Committee Report, Asbestos Information Association, Arlington, VA (June,



[22]      Chatfield, E. J. Measurement of Asbestos fiber Concentrations in Workplace

Atmospheres, Royal Commission on Matters of Health and Safety Arising from

the Use of Asbestos in Ontario, Study No. 9, 180 Dundas Street West, 22nd

Floor, Toronto, Ontario, Canada M5G 1Z8.


[23]      Walton, W.H. “The Nature, Hazards, and Assessment of Occupational Exposure

`           to Airborne Asbestos Dust: A Review, “ Ann. Occup. Hyg., 25, 115-247 (1982).


[24]      Baron, P.A. and S. Shulman.  “Evaluation of the Magiscan Image Analyer for

Asbestos fiber Counting.”  Am. Ind. Hyg. Assoc. J. 48:39-46.


[25]      Cherrie, J.A. Jones, and A. Johnston.  “The Influence of Fiber Density on the

Assessment of Fiber Concentration Using the Membrane Filter Method.”

Am. Ind. Hyg. Assoc. J. 47:465-74 (1986).


[26]      Taylor, D.G., P.A. Baron, S.A. Shulman and J.W. Carter.  “Identification and

Counting of Asbestos Fibers.”  Am. Ind. Hyg. Assoc. J. 45 (2), 84-88 (1984).


[27]      “Reference methods for Measuring Airborne Man-Made Mineral Fibers (MMMF)”

WHO/EURO Technical Committee for Monitoring an Evaluating Airborne MMMF,

World Health Organization, Copenhagen (1985).







Before ordering the Walton-Beckett graticule, the following calibration must be done to obtain a counting area, (D) 100 чm in diameter at the image plane.  The diameter, dc (mm), of the circular counting area and the disc diameter must be specified when ordering the graticule.



  1. Insert any available graticule into the eyepiece and focus so that the graticule lines are sharp and clear.


  1. Set the appropriate interpupillary distance and if applicable reset the binocular head adjustment so that the magnification remains constant.


  1. Install the 40 to 45X phase objective.


  1. Place a stage micrometer on the microscope object stage and focus the microscope on the graduated lines.


  1. Measure the magnified grid length of the graticule, Lo (чm) , using the stage micrometer.


  1. Remove the graticule from the microscope and measure its actual grid length, La (mm).  This can best be accomplished by using a stage fitted with verniers.


  1. Calculate the circle diameter, dc (mm), for the Walton-Beckett graticule:


Dc  =  La  x  D.



Example :  If lo  = 112 чm, La  = 4.5 mm and D  =  100 чm, then dc  = 4.02 mm.


  1. Check the field diameter, D (acceptable range 100 чm + 2 чm) with a stage micrometer upon receipt of the graticule for the manufacturer.  Determine field area (acceptable range 0.00754 to 0.00817 mm2).






Figure 2 shows a Walton-Beckett graticule as seen through the microscope.  The rules will be discussed as they apply to the labeled objects in the figure.



Walton-Beckett Graticule


Fiber Count

Object                 Count                                     Discussion


1                    1 fiber                                  Optically observable asbestos fibers are

actually bundles of fine fibrils.  If the fibrils

seem to be from the same bundle the object is

counted as a single fiber.  Note, however, that

all objects meeting length and aspect ratio

criteria are counted whether or not they appear

to be asbestos.


2                 2 fiber                                     If fibers meeting the length and aspect ratio

criteria (length>5 чm and length-to-width ratio >3 to 1) overlap, but do not seem to be part of the same bundle, they are counted as separate fibers.


3                 1 fiber                                     Although the object has a relatively large

diameter (>3 чm), it is counted as fiber under

the rules.  There is no upper limit on the fiber

diameter in the counting rules.  Note that fiber

width is measured at the widest compact

section of the object.


4                 1 fiber                                     Although long fine fibrils may extend from the

body of a fiber, these fibrils are considered part of  the fiber, these fibrils are considered part of the fiber if they seem to have originally been part of the bundle.


5                 Do not count                         If the object is ≤5 чm long, it is not counted.


6                 1 fiber                                     A fiber partially obscured by a particle is

counted as one fiber.  If the fiber ends

emanating from a particle do not seem to be

from the same fiber and each end meets the

length and aspect ratio criteria, they are

counted as separate fibers.


7                 ½ fiber                                    A fiber which crosses into the graticule area

one time is counted as ½ fiber.


8                 Do not count                         Ignore fibers that cross the graticule boundary

more than once.


9                 Do not count                         Ignore fibers that lie outside the graticule








  1. These units are high accuracy electronic bubble flow-meters that provide instantaneous air flow readings and a cumulative averaging of multiple samples. These calibrators measure the flow rate of gases and report volume per unit of time.


  1. The timer is capable of detecting a soap film at 80 microsecond intervals. This speed allows under steady flow conditions an accuracy of + or – 0.5% of any display reading. Repeatability is + or-0.5% of any display.


  1. The range with different cells is from 1 cc/min to 30 Lpm.


  1. Battery power will last 8 hours with continuous use. Charge for 16 hours. Can be operated from A/C charger.

Maintenance of Calibrator

1. Cleaning before use:

Remove the flow cell and gently flush with tap water. The acrylic flow cell can be easily scratched. Wipe with doth “only.” Do not allow center tube, where sensors detect soap film to be scratched or get dirty. NEVER clean with ACETONE. Use only soap and warm water. When cleaning prior to storage, allow flow cell to air dry. If stubborn residue persists, it is possible to remove the bottom plate. Squirt a few drops of soap into the slot between base and flow cell to ease removal.


2.  Leak Testing:

The system should be leak checked at 6″ H2O by connecting a manometer to the outlet boss and evacuate the inlet to 6″ H2O. No leakage should be observed.


3.  Verification of Calibration:

The calibrator is factory calibrated using a standard traceable to National Institute of Standards and Technology, formerly called the National Bureau of Standards, (NBS). Attempts to verify calibrator against a glass one liter burette should be conducted at 1000 cc/min. for maximum accuracy. The calibrator is linear throughout the entire range.


  1. When transporting, especially by air, it is important that one side of the seal tube which connects the iniet and outlet boss, be removed for equalizing internal pressure within the calibrator.


  1. Do not transport unit with soap solution or storage tubing in place.



  1. Avoid the use of chemical solvents on flow cell, calibrator case and faceplate. Generally, soap and water will remove any dirt.


  1. Never pressurize the flow cell at any time with more than 25 inches of water pressure.


  1. Do not charge batteries for longer than 16 hours.


  1. Do not leave A/C adapter plugged into calibrator when not in use as this could damage the battery supply.


5. Black close fitting covers help to reduce evaporation of soap in the flow cell when not

is use.


  1. Do not store flow ceil for a period of one week or longer with soap. Clean and store



7. The Calibrator Soap is a precisely concentrated and sterilized solution formulated to provide a dean, frictionless soap film bubble over the wide, dynamic range of the calibrator. The sterile nature of the soap is important in the prevention of residue build-up in the flow cell center tube, which could cause inaccurate readings. The use of any other soap Is not recommended.





The following circumstances often require special consideration to effectively collect air samples.



Final sampling must be performed inside the work area. In instances where the crawlspace floor is dirt and where abatement has been conducted to remove asbestos contamination from the dirt or from substrates adjacent to the dirt, consideration should be made to encapsulate the dirt floor. This practice locks down any residual asbestos contamination unidentified in the visual inspection, and provides a more suitable environment for clearance air sampling. As in all environments where non-asbestos particulate may present an issue in sample collection, samplers may consider lower volumes with longer sampling episodes to avoid pulling larger particulate into the filter.

Occupied Buildings


Occupied buildings present a particular challenge to the sampler. Consideration must be given to samples collected outside the work but in occupied spaces. This activity may be a source of discomfort for the occupants, which may require additional support and input from the sampler. Occupied spaces also may be a source of non-asbestos or non-project related contamination that may be collected on air samples. Occupants should be directed to avoid interference or horseplay around sampling pumps. Sampling should be conducted in areas where as little occupant disturbance may occur, but in consideration for sampling requirements for work area barrier sampling detailed in the State standard. Of course, analysis information should be made available to building occupants as soon as possible and within 48 hours as required by State requirements.


Analysis Considerations

The accuracy of sample analysis is often compromised when samples are collected in “dirty” environments and/or sampling is preformed without consideration for requirements in the NIOSH and AHERA methodologies. In the first case, the sampler must adjust his/her activities to limit non-asbestos contamination on the sample. This can be achieved by either running samples at a lower volume for a longer sample episode or by using multiple cassettes to collect a single episode.

Samplers may also consider “close-faced” sampling wherein only the inlet plug is removed, leaving the cassette cap to reduce overloading. Sampling personnel should be familiar with the NIOSH and AHERA methodologies. At a minimum, consideration should be given to PCM analysis requirements, per the NIOSH 7400 method, specifying flow rates between .5 and 16 liters per minute with a minimum volume of 400 liters (per NISOH – 3 “sampling”). Likewise, the AHERA method for TEM specifies sample volumes above 1200 liters.




Blanks serve to identify pre-existing contamination on the sample media. The NIOSH 7400 method (PCM) specifies that unopened “box” blanks should represent 10% of samples submitted to the lab. Practically speaking, PCM blanks should be submitted with every sampling episode. AHERA (TEM) blank submittals must follow a specific protocol. AHERA (TEM) finals must be accompanied by three (3) blanks; one opened inside the work area for thirty (30) seconds, one opened outside the work area for thirty (30) seconds and one unopened box blank.