Asbestos sampling procedures

The OSHA airborne asbestos limits to compare PCM results to are a Time-Weighted Average (TWA) of 0.1 fiber per cubic centimeter and a 30-minute Excursion Level of 1.0 fiber per cubic centimeter. See 1915.1001 App B for the Appendix B table and 1915.1001 for the standard context.

You must use a 25-mm diameter three-piece cassette with a mixed-cellulose ester (MCE) membrane filter and an electrically conductive 50-mm extension cowl (conductive filter holder). A 25-mm cellulose backup pad is used behind the membrane filter. See 1915.1001 App B for the equipment specifications and notes that fully conductive cassettes reduce fiber loss and that cassettes must not be reused.

For personal sampling the recommended pump flow rate is between 0.5 and 5.0 liters per minute (L/min), commonly set between 1 and 2 L/min. The recommended minimum air volume is 25 L and the maximum is 2,400 L. See 1915.1001 App B for the table of sampling rates and volumes.

Choose the sampling time and rate so the filter fiber density falls between about 100 and 1,300 fibers/mm² (the ideal counting range). Appendix B says the sampling time and rate should be selected to produce that density. Use the recommended flow rates (0.5–5.0 L/min) and calculate volume so the expected airborne concentration yields the target density on the filter. See 1915.1001 App B for the density target and sampling guidance.

The Appendix specifies mixed-cellulose ester (MCE) membrane filters, 25 mm plain white, with pore sizes between 0.4 and 1.2 μm. See 1915.1001 App B for the required filter type and pore-size range.

Use a battery-operated, self-contained pump small enough to be worn by the monitored employee and capable of the chosen flow rate and sampling time. Calibrate pumps using a stopwatch and bubble tube/burette or an electronic meter; connect the pump to a calibration cassette with the specified 6-mm bore tubing (do not use luer connectors). See 1915.1001 App B for pump and calibration details.

You must seal the point where the cassette base and cowl meet with a gel band or tape to minimize contamination and loss. Cassettes must not be reused. See 1915.1001 App B (notes (a) and 5.2.1) for the sealing instruction and the prohibition on reuse.

Prepare by clearing (making transparent) a wedge-shaped portion of the sample filter, mount it on a glass slide, and analyze by Phase Contrast Microscopy (PCM) at 400X magnification to count fibers meeting the defined criteria. Appendix B describes the clearing and PCM counting procedure and notes that PCM counts total fibers but does not positively identify asbestos—polarized light or electron microscopy is needed for positive ID. See 1915.1001 App B for the analytical procedure.

For PCM counting, a ‘fiber’ is defined as a particle at least 5 μm long with a length-to-width (aspect) ratio of at least 3:1. See 1915.1001 App B for the fiber definition used in counting.

The Walton‑Beckett graticule is an eyepiece disk with a circle of about 100 μm diameter and markings (tic-marks at 3 μm and 5 μm) used to standardize the field area for asbestos fiber counting; it corresponds to a field area of about 0.00785 mm². Appendix B specifies using this graticule so counts can be reported in fibers/mm² and fibers/field consistently. See 1915.1001 App B.

Count at least 20 fields and not more than 100 fields. Stop counting when 100 fibers have been counted provided at least 20 fields have been examined; counting more than 100 fibers gives little extra precision. Appendix B explains that precision declines rapidly when total counts fall below about 10 fibers. See 1915.1001 App B for guidance on fields and stopping rules.

The detection limit for this PCM method is approximately 4.0 fibers per 100 fields, which corresponds to about 5.5 fibers/mm². Appendix B explains how this limit was derived using counting statistics and a 95% confidence Lower Control Limit. See 1915.1001 App B for details on the detection limit calculation.

The ideal fiber density on the filter is 100 to 1,300 fibers/mm² (equivalent to about 0.8 to 10 fibers/field with the Walton‑Beckett graticule). This range balances precision and ease of counting—low densities increase statistical variability and very high densities make counting difficult. See 1915.1001 App B for the recommended density range and the corresponding fibers/field.

Common interferences include non-asbestos fibrous materials such as fiberglass, plant fibers, perlite, diatoms, and some synthetic fibers. When differentiation is necessary, polarized light microscopy, electron microscopy, or dispersion staining can be used to identify non‑asbestos fibers. See 1915.1001 App B (section on interferences) for the list and suggested methods for resolving interferences.

Differential counting is the practice of excluding fibers that appear not to be asbestos (based on morphology) from the PCM count; it should be used when experienced analysts can reliably distinguish non‑asbestos fibers to avoid overestimating asbestos concentrations. However, PCM cannot positively identify asbestos, so confirmatory methods (PLM or TEM) are needed when positive identification is required. See 1915.1001 App B for the discussion on differential counting and PCM limitations.

Purchase filters selected by the manufacturer for asbestos counting or analyze representative filters from a lot for background fiber counts; discard the lot if background exceeds 4 fibers per 100 fields. Appendix B specifically advises discarding filter lots with more than 4 fibers/100 fields. See 1915.1001 App B (notes to 5.1.1).

Other cassette designs (for example, Bell-mouth) may be used only within the limits of their validation; the Appendix emphasizes fully conductive cassettes to reduce electrostatic fiber loss. If you use an alternate cassette, ensure it has been validated for asbestos sampling equivalency. See 1915.1001 App B for the note on alternative cassettes and conductivity.

Yes—Appendix B recommends using sampling systems preassembled by the manufacturer (filter, backup pad, cassette) to reduce the possibility of contamination and handling errors. Preassembly helps ensure consistent filter orientation and reduces background contamination risk. See 1915.1001 App B (note (d) to 5.1.1).

Personal samples should be collected with the pump worn on the employee (pump small enough not to interfere with work) and the sampling inlet cassette positioned in the employee’s breathing zone; Appendix B frames the procedure for personal sampling using battery-operated wearable pumps. See 1915.1001 App B for guidance on pump size and placement.

Appendix B provides non-mandatory sampling procedures under the shipyard asbestos standard, but for most building remediation work involving asbestos-containing building materials (ACBM), OSHA has clarified that the construction asbestos standard [29 CFR 1926.1101] generally applies rather than the general-industry standard. See the OSHA interpretation on asbestos remediation protocols at https://www.osha.gov/laws-regs/standardinterpretations/2024-11-14 and refer to 1915.1001 App B for the sampling procedure when operating under 1915.1001.

Always choose a flow rate and sample volume that will not produce overloaded filters; stop sampling and replace the cassette if dust obscures about 25–30% of the field. See the guidance in 1915.1001AppB.

• Observe the filter periodically with a small flashlight while sampling (per 1915.1001AppB).
• If more than about 25–30% of the field area is obscured by non‑asbestos dust, the count may be biased low and you must stop sampling, seal the cassette, and use a new cassette.
• Use the suggested maximum air volumes for different environments in 1915.1001AppB (e.g., 100 L for visible dust during asbestos removal) and reduce volume further if dust levels are high.

You must calibrate each sampling pump before and after sampling with an in-line calibration cassette from the same lot and use a primary standard (for example, a bubble burette) to set the flow rate. See 1915.1001AppB.

• Calibrate the pump at the sampling site when possible to account for environmental influences.
• If on-site calibration is not possible, consult the pump manufacturer about temperature and pressure effects and correct the flow rate using the "Sampling Pump Flow Rate Corrections" formula in 1915.1001AppB.
• Always use a calibration cassette from the same lot used for sampling to avoid cassette-to-cassette variation.

Place each open-face cassette held open side down in the employee's breathing zone about 10 cm from the nose/mouth and secure it to the collar or lapel so it remains in that position during sampling. See 1915.1001AppB.

• Connect the pump to the cassette base with flexible tubing and remove the end cap before sampling (per 1915.1001AppB).
• Hold the cassette open side down and secure with spring clips or similar devices so it stays in the breathing zone during the entire sample.

A suggested minimum air volume for TWA compliance sampling is 25 liters. See 1915.1001AppB.

• This is a suggested minimum; increase volume based on analytical needs but avoid filter overload by following the volume guidance for the environment.

For a 30‑minute excursion limit evaluation, a minimum air volume of 48 liters is recommended. See 1915.1001AppB.

• Use this recommended volume to ensure the sampling is representative for the short (30‑minute) period.

Suggested maximum air volumes depend on dust levels because excessive non‑asbestos dust can obscure fibers and bias counts low; follow the environment-specific volumes in 1915.1001AppB.

• Asbestos removal operations (visible dust): 100 L
• Asbestos removal operations (little dust): 240 L
• Office environments: 400 to 2,400 L

Why it matters:

• High non‑fibrous dust loading can obscure fibers; if >25–30% of the field is obscured, results may be biased low.
• If dust is heavy, reduce sample volume below the suggested maximum and visually inspect the filter while sampling (per 1915.1001AppB).

If you observe a visible layer of dust on the filter during sampling, stop sampling immediately, remove and seal the cassette, and replace it with a new sampling assembly. See 1915.1001AppB.

• Use a small flashlight to inspect the filter intermittently while sampling (per 1915.1001AppB).
• If more than 25–30% of the field is obscured, the result may be biased low and the sample is invalid.
• Replace the cassette and document the interruption and reason on your sampling paperwork.

Prepare two blanks for the first 1–20 samples, and for sets over 20 samples prepare blanks equal to 10% of the sample count; handle blanks like air samples but do not draw air through them. See 1915.1001AppB.

• Open the blank cassette where the sample cassettes are mounted and hold it open for about 30 seconds, then close and seal it.
• Store blanks with the sample cassettes and ship them with the batch to the laboratory so contamination during handling can be detected (per 1915.1001AppB).

Immediately after sampling you must close and seal each cassette with the base and plastic plugs without touching or puncturing the filter membrane. Then attach a sample seal around the cassette so the end cap and base plugs cannot be removed without destroying the seal. See 1915.1001AppB.

• Do not touch or puncture the filter membrane—doing so invalidates the analysis.
• Use a seal that requires destruction to remove the plugs; tape the ends of the seal together and wrap tape around cassette joints to keep the seal secure (per 1915.1001AppB).

Package samples so they cannot rattle, are protected from static electricity, and are cushioned; do not use expanded polystyrene peanuts, vermiculite, paper shreds, or excelsior. See 1915.1001AppB.

• Tape sample cassettes to bubble wrap and place them in a container that cushions and prevents movement (per 1915.1001AppB).
• Ship bulk samples in separate mailing containers to avoid cross‑contamination.
• Include paperwork requesting asbestos analysis and list any known fibrous interferences and the workplace operation(s) sampled (per 1915.1001AppB).

Laboratory staff must handle acetone as an extremely flammable solvent in a ventilated hood away from open flames and clean asbestos spills immediately using wet methods or a HEPA‑filtered vacuum. See 1915.1001AppB.

• Transfer acetone in a ventilated laboratory hood and avoid use near open flame; use spark‑free heat sources for acetone vapor generation (per 1915.1001AppB).
• Clean asbestos spills immediately with wet methods and/or a HEPA‑filtered vacuum; do not use a vacuum without a HEPA filter because it will disperse fibers (per 1915.1001AppB).

Use a phase contrast microscope with a 40X phase objective (numerical aperture 0.65–0.75), Walton‑Beckett Graticule type G‑22 (100 ±2 μm projected diameter), and widefield or Huygenian 10X eyepieces (with a focusing eyepiece for the graticule). See 1915.1001AppB.

• Kohler illumination with a green or blue filter is recommended if possible (per 1915.1001AppB).
• Other useful equipment: mechanical stage, phase telescope, stage micrometer, phase‑shift test slide, precleaned slides and cover glasses, scalpel, fine forceps (see 1915.1001AppB).

Mount a triangular wedge (about 1/6 to 1/4 of a 25‑mm filter) on a clean glass slide, clear it with acetone on a 70 °C aluminum block, apply 2.5–3.5 μL triacetin within 30 seconds, and lower a cover slip gently to produce a permanent, smooth mount. See the sample mounting steps in 1915.1001AppB.

• Do not touch the filter with your fingers; grasp at the perimeter with forceps (per 1915.1001AppB).
• Inject about 200 μL acetone into the aluminum block slot, wait 3–5 seconds for clearing, then add triacetin within 30 seconds to avoid increased refractive index and poor mounts (per 1915.1001AppB).
• If more than 30 seconds elapse before triacetin, glue the cover slip edges with lacquer or nail polish to secure it (per 1915.1001AppB).

Clean optics, rough‑focus the objective, close and focus the field iris, center it with the condenser, install and center the phase rings with a phase telescope, and verify performance using a phase‑shift test slide before each counting session. See the alignment routine in 1915.1001AppB.

• Recheck alignment regularly during counting sessions because misalignment degrades image quality and counting reliability (per 1915.1001AppB).

Count only fibers 5 μm or longer, with a length‑to‑width ratio ≥3:1; count all fibers in at least 20 fields and continue until either 100 fibers are counted or 100 fields have been viewed, whichever occurs first. See the counting rules in 1915.1001AppB.

• Measure curved fibers along the curve.
• Spend 5–15 seconds per field and scan the upper 10–15 μm of the filter surface while focusing through the field (per 1915.1001AppB).

Fibers entirely within the graticule receive a count of 1; fibers crossing the boundary once (one end inside the circle) receive a count of 1/2; do not count any fiber that crosses the boundary more than once. See 1915.1001AppB.

• If a fiber merely touches the circle, it is considered to cross the line and should be treated accordingly (per 1915.1001AppB).

Count a bundle of fibers as one fiber unless you can clearly resolve and count the individual fibers as separate, unconnected fibers; reject any field where an agglomerate covers more than 25% of the field and select another field. See 1915.1001AppB.

• Agglomerates that obscure the field may bias results; do not include rejected fields in your field count (per 1915.1001AppB).

Yes; perform a blind recount on one in every ten filter wedges (slides) by re‑labeling the slides using someone other than the original counter. See 1915.1001AppB.

• Blind recounts are a quality control measure to assess counting reproducibility and reduce bias (per 1915.1001AppB).

No; PCM does not provide positive identification of asbestos—differential counting or electron microscopy techniques are required for positive identification when discrimination is needed. See the discussion in 1915.1001AppB.

• If discrimination is legally necessary, consider primary morphology discrimination, polarized light analysis, or modification of PCM data by Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) (per 1915.1001AppB).
• Differential counting requires significant experience and is discouraged unless necessary (per 1915.1001AppB).

About one‑fourth of a 25‑mm sample membrane is required for an asbestos count, and PAT program samples can serve as replicate "standards" for interlaboratory comparisons. See 1915.1001AppB.

• PAT (Proficiency Analytical Testing) samples are distributed to participating laboratories and provide nearly equal concentration replicates useful for quality control (per 1915.1001AppB).

Work that involves asbestos‑containing building materials during building remediation is generally covered by OSHA's construction asbestos standard, 29 CFR 1926.1101, rather than the general‑industry asbestos standard. See the OSHA interpretation letter "Asbestos remediation protocols" for details: Asbestos remediation protocols and consult 1915.1001 when maritime/shipyard situations apply.

• OSHA explains that many remediation activities—and especially building repair, cleanup, and demolition—fall under the construction standard [29 CFR 1926.1101] (see the interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2024-11-14).
• Use the standard that matches the work activity: consult 1915.1001 for shipyard/marine situations and [29 CFR 1926.1101] when the work is construction/remediation in buildings (see the OSHA interpretation linked above).

All individuals who perform asbestos analysis must complete the NIOSH course for sampling and evaluating airborne asbestos (or an equivalent course) before analyzing samples. This is a mandatory requirement to meet the quality control expectations in 1915.1001AppB.

• This requirement is stated in the Analytical Recommendations—Quality Control System (Section 6.8.1).
• "Equivalent course" means documented training with content and skills comparable to the NIOSH course so the analyst can reliably identify and count asbestos fibers.

A laboratory engaged in asbestos counting must set up semiannual slide exchanges with at least two other labs and participate in a recognized proficiency program (for example, the Proficiency Analytical Testing Program or the Asbestos Analyst Registry). These actions are required by 1915.1001AppB as part of the quality control system.

• Slide trading with at least two other laboratories helps compare performance and reduce systematic errors (Section 6.8.2).
• Participation in a formal proficiency testing or analyst registry program is required (Section 6.8.3).
• Round-robin (slide exchange) results should be posted so analysts can see individual performance trends.

A "slide bank" is a set of prepared slides made from uniformly distributed samples across a laboratory’s workload that analysts count blind for ongoing quality assurance; each analyst must count 95% of those quality-control slides within three standard deviations of the historical mean. This requirement and performance criterion are described in 1915.1001AppB (Section 6.8.4).

• Slide bank slides should cover the full range of fiber densities the lab normally analyzes.
• The lab uses these blind counts to establish and monitor an analyst’s historical mean and standard deviation.
• If an analyst falls outside the criterion, the lab should investigate for bias, retrain, or take corrective actions.

Calculate AC using the formula in 1915.1001AppB: AC = (FB/FL − BFB/BFL) × (ECA/(1000 × MFA × FR × T)), and if the actual collection area of the filter differs from the nominal 385 mm², you must measure the filter diameter and use Area = π(d/2)² to get the exact ECA. See Section 7.1 and the note about collection area in 1915.1001AppB.

• FB = fibers >5 µm counted on sample; FL = fields counted on sample; BFB/BFL = blank-corrected counts.
• MFA is the microscope field area (e.g., 0.00785 mm² for a 100 µm Walton–Beckett graticule).
• Always measure the exposed filter diameter with an inside micrometer rather than assume 385 mm² when precision matters.

You may use the short-cut calculation when an analyst always uses the same microscope/ocular-reticle combination and the exposed filter area is constant; in the example where ECA = 385 mm² and MFA = 0.00785 mm², the combined constant equals 49, so the simplified equation uses 49 in place of ECA/(1000 × MFA) as shown in 1915.1001AppB (Section 7.2).

• Short-cut depends on fixed interpupillary distance, fixed reticle/eye-piece, and a constant exposed filter area.
• If you change microscope optics, reticle, or the exposed area, you must recalculate the constant.

Reject a pair of counts when the difference between them exceeds the statistical criterion given in 1915.1001AppB (Equation C in Section 7.3); if a pair is rejected you must recount the rest of the filters in the submitted set, apply the same test to other pairs, and document the failure with a memo to the industrial hygienist.

• The recount criterion is set at a type‑I error level (maximum 5% chance of wrongly rejecting due to random variability).
• When a pair fails, recount and flag any additional pairs that fail; prepare a memo stating the sample failed the homogeneity test and the true air concentration may differ from the reported value.

Report airborne asbestos concentration in fibers/cc using two significant figures and, if multiple analyses exist for a sample, report the average unless one or more results are rejected for cause. This reporting rule is specified in 1915.1001AppB (Section 7.4).

• Use two significant figures (for example, 0.12 fibers/cc).
• If any analysis is rejected (for cause or by the recount criterion), do not include it in the reported average; document the rejection and reason.

Apply the pump flow rate correction when ambient temperature or pressure at the sampling site differs from calibration by more than 5%; use the correction formula Qact = Qcal × (Pcal/Pact) × (Tact/Tcal) given in 1915.1001AppB (Section on Sampling Pump Flow Rate Corrections).

• Qcal is the calibrated flow (or rotameter reading), Qact is the actual flow.
• Temperatures must be in Kelvin for the ratio Tact/Tcal.
• Make this correction when pumps do not automatically compensate for site temperature/pressure differences to ensure accurate volume measurement.

When ordering a Walton–Beckett graticule, specify the eyepiece disc diameter and the circular counting-area diameter; calibrate the eyepiece-objective-reticle combination with a stage micrometer and verify the field diameter (acceptable range 100 ± 2 µm), then compute the field area by Field Area = π(D/2)² as described in 1915.1001AppB (Sections on the Graticule and calibration). Partial fibers that cross the counting circle are scored according to the counting rules illustrated in the figure (for example, a fiber crossing once is 1/2, crossing twice is 0) and those rules are detailed in the Appendix.

• Steps: insert a graticule, align microscope, place stage micrometer and measure PL, measure AL, compute circle diameter using the provided equation, and verify the field diameter.
• Example field area: D = 100 µm gives MFA = 0.00785 mm².
• The Appendix includes the counting examples (e.g., fiber crossing once = 1/2, fiber outside = 0) to maintain consistency among analysts.