Exposure limits and sampling matrix

Under 1926.1101AppB the OSHA PELs are 0.1 fiber/cc as an 8-hour Time Weighted Average and 1.0 fiber/cc for a 30-minute Excursion Level.

• See the exposure limits listed in 1926.1101AppB for the official matrix and sampling guidance.

Use a 25-mm diameter mixed-cellulose ester (MCE) filter housed in a three-piece 25-mm cassette equipped with a 50-mm electrically conductive extension cowl.

• Appendix B specifies a conductive filter holder and MCE filter of 25 mm with a 0.4 to 1.2 µm pore size in 1926.1101AppB.
• Do not re-use cassettes and use fully conductive cassettes to reduce electrostatic loss (see the equipment notes in 1926.1101AppB).

The recommended personal sampling flow rate is between 0.5 and 5.0 L/min (commonly 1–2 L/min), with minimum total sample volumes of 25 L and maximum up to 2,400 L depending on environment.

• See the recommended sampling rate and air volume ranges in 1926.1101AppB.
• Choose a flow that yields a fiber density of about 100–1,300 fibers/mm2 on the filter and avoid overloading with dust (guidance and environment-specific max volumes are given in the appendix).

Place the open side of the cassette approximately 10 cm from the employee’s nose/mouth, held open-side down in the breathing zone.

• Appendix B instructs that the cassette should be secured to the collar or lapel about 10 cm from the nose/mouth during sampling (1926.1101AppB).

Calibrate each sampling pump before and after sampling with a calibration cassette in-line and, if possible, perform the calibration at the sampling site using a primary standard.

• Appendix B requires pre- and post-sampling calibration with a calibration cassette from the same lot as the sampling cassettes and recommends using a primary standard such as a bubble burette (1926.1101AppB).
• If site calibration isn’t possible, environmental conditions may affect flow and you should correct for temperature/pressure per the appendix instructions.

Seal the joint between the cassette base and cowl with a gel band or tape before sampling.

• Appendix B explicitly states to seal the point where the base and cowl meet with a gel band or tape to prevent contamination and leakage (1926.1101AppB).

For asbestos removal with visible dust, the suggested maximum sample volume is about 100 L; for removal with little dust, about 240 L; and for office environments the suggested range is 400 to 2,400 L.

• Appendix B provides environment-specific suggested maximum air volumes to avoid filter overloading and obscuring fibers on the filter (1926.1101AppB).
• CAUTION in the appendix warns that too much non-fibrous dust can obscure counting.

For a TWA determination, a suggested minimum air volume is 25 L; for a 30-minute Excursion Limit evaluation, a recommended minimum is 48 L.

• These minimum volumes are specified in 1926.1101AppB as guidance for collecting representative samples.

A wedge of the MCE filter is cleared and prepared, then asbestos fibers are counted by Phase Contrast Microscopy (PCM) at 400X magnification.

• Appendix B describes preparing a portion of the filter and analyzing it by PCM at 400X (1926.1101AppB).
• Note that PCM counts total fibers meeting size/aspect criteria but cannot positively identify mineralogical asbestos (see discussion of PCM limitations in the same appendix).

The ideal fiber density is 100–1,300 fibers/mm2, which with a Walton-Beckett graticule corresponds to about 0.8 to 10 fibers per field.

• Appendix B defines the ideal counting range and shows the Walton-Beckett graticule equivalence in 1926.1101AppB.
• NIOSH/OSHA counting statistics are referenced in the appendix to support precision expectations at these ranges.

The detection limit for the PCM method in Appendix B is 4.0 fibers per 100 fields (about 5.5 fibers/mm2).

• Appendix B explains the detection limit determination and supporting counting statistics in 1926.1101AppB.
• The appendix discusses how this limit was derived from coefficient of variation (CV) calculations and historical data.

Count at least 20 fields and not more than 100 fields; stop when 100 fibers have been counted provided at least 20 fields have been examined.

• Appendix B recommends counting between 20 and 100 fields and discontinuing after 100 fibers are counted (if at least 20 fields have been counted) to balance precision and effort (1926.1101AppB).
• The appendix notes precision decreases sharply if total counts fall below about 10 fibers.

Common interferences include non-asbestos fibrous particles such as fiberglass, plant fibers, perlite, diatoms, and some synthetic fibers; resolution may require electron microscopy or optical tests like polarized light and dispersion staining.

• Appendix B lists likely interfering materials and advises using electron microscopy or optical identification methods when differentiation is necessary (1926.1101AppB).
• Remember PCM cannot positively identify asbestos mineralogy, so follow-up analysis is needed when identification matters.

Use filters and cassette lots that have been screened for background fibers; discard any filter lot with more than 4 fibers per 100 fields on background checks.

• Appendix B advises purchasing filters selected by the manufacturer for asbestos counting or testing representative filters and discarding lots exceeding 4 fibers/100 fields (1926.1101AppB).
• The appendix also notes preassembled sampling systems reduce contamination risk.

Yes; fully conductive cassettes are required to reduce fiber loss to cassette walls caused by electrostatic attraction.

• Appendix B explicitly states fully conductive cassettes are required to reduce electrostatic fiber loss (1926.1101AppB).

Other validated cassette types, such as the Bell-mouth, may be used provided they are within the limits of their validation.

• Appendix B allows alternative cassettes like Bell-mouth if their performance has been validated (1926.1101AppB).

Use a battery-operated, self-contained pump small enough to be worn by the employee without interfering with work and capable of sampling at the chosen collection rate for the required time.

• Appendix B lists these pump requirements and notes the pump must be able to maintain the selected flow between 0.5 and 5.0 L/min (1926.1101AppB).

No; do not use luer connectors—use tubing that fits the cassette’s built-in adapters as specified.

• Appendix B instructs not to use luer connectors because the specified cassette has built-in adapters (1926.1101AppB).

Differential counting is the practice of excluding fibers that do not appear to be asbestos from the PCM count; it should be used when an experienced analyst can reliably distinguish asbestos-like non-asbestos fibers to reduce false positives.

• Appendix B defines differential counting and notes it requires a great deal of experience to distinguish asbestos from non-asbestos using PCM (1926.1101AppB).
• When positive identification is required, use polarized light microscopy or electron microscopy as PCM cannot uniquely identify asbestos.

PCM is specific for fibers, inexpensive, quick, and compatible with historical exposure data, but its main disadvantage is it cannot positively identify asbestos mineralogy and may count non-asbestos fibers.

• Appendix B lists these pros and cons and states that the smallest visible fibers by PCM are about 0.2 µm, missing finer asbestos fibers as small as 0.02 µm (1926.1101AppB).

Use a 25-mm cellulose backup pad, gel bands for sealing cassettes, and flexible 6-mm bore tubing; the sampling system is often preassembled by the manufacturer.

• Appendix B lists these accessories (backup pad, gel bands, 6-mm tubing) and recommends preassembled sampling systems to reduce contamination (1926.1101AppB).

When sampling open-face, remove the cassette end cap and hold the cassette open-side down so air is drawn through the exposed filter in the employee’s breathing zone.

• Appendix B gives the open-face sampling orientation and placement instructions (1926.1101AppB).

If remediation activities involve asbestos-containing building materials (ACBM), the construction asbestos standard, 29 CFR 1926.1101, generally applies even for property remediation companies.

• OSHA’s letter on asbestos remediation protocols reinforces that remediation activities involving ACBM are covered by 29 CFR 1926.1101 (see the interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2024-11-14).
• For sampling and analysis details, consult 1926.1101AppB.

Discard a filter lot if representative background testing shows more than 4 fibers per 100 fields.

• Appendix B instructs discarding filter lots with more than 4 fibers/100 fields on background checks to avoid false positive counts (1926.1101AppB).

Do not re-use cassettes; Appendix B explicitly instructs to avoid re-using cassettes to prevent contamination and inaccurate sampling.

• See the equipment notes in 1926.1101AppB which state “DO NOT RE-USE CASSETTES.”

Use Phase Contrast Microscopy at 400X magnification and the Walton-Beckett graticule (100 µm projected diameter) for asbestos fiber counting.

• Appendix B describes preparing the filter and counting fibers by PCM at 400X and specifies the Walton-Beckett graticule design (1926.1101AppB).

Stop or change your sampling plan because the count may be biased low when more than about 25–30% of the field is obscured. The guidance in 1926.1101AppB explains that heavy non-asbestos dust can mask asbestos fibers and recommends using smaller sampled air volumes when excessive non-asbestos dust is present. Practical steps include:

• Reduce the sampled air volume so the filter does not overload.
• If you already see heavy dust buildup on the filter, stop sampling, seal the cassette, and replace it with a clean sampler (see the visible-dust and sealing steps in 1926.1101AppB).
• Note the dusty conditions on your field paperwork so the lab can consider interferences during analysis.

While sampling, look at the filter with a small flashlight and stop sampling if you see a visible layer of dust on the filter. The appendix states to observe the filter with a small flashlight and, if a visible dust layer appears, stop sampling, remove and seal the cassette, and replace it with a new sampling assembly (1926.1101AppB).

Steps to follow:

• Shine a small flashlight on the filter periodically.
• If a visible dust layer is present, stop the pump immediately.
• Remove the cassette, seal it (do not touch the filter), and replace with a new cassette.
• Record the condition and cause (e.g., heavy non-asbestos dust) on the chain-of-custody and sample paperwork.

Do not let the total dust loading on the filter exceed 1 mg. The appendix warns that the total dust loading should not exceed 1 mg and instructs stopping sampling and sealing the cassette if visible dust accumulates (1926.1101AppB).

Practical notes:

• Plan air volume so expected non-asbestos dust will not exceed 1 mg on the 25-mm filter.
• If in doubt, take shorter samples and run multiple shorter samples rather than one long one that could overload the filter.

Prepare blanks as described: two blanks for the first 1–20 samples and blanks equal to 10% of the samples for sets larger than 20, and handle blanks like field samples except do not draw any air through them. 1926.1101AppB explains blank handling: open the blank cassette where the sample cassettes are mounted, hold it open about 30 seconds, then close and seal it. Store blanks with the samples for shipment.

Quick checklist:

• Two blanks for 1–20 samples; for >20 samples use blanks = 10% of total.
• Do not draw air through blanks.
• Open blank in the same location where you mount samplers, hold open ~30 seconds, then seal.
• Ship blanks with the rest of the samples and label them accordingly.

Immediately close and seal each cassette using its base and plastic plugs, and attach a sample seal that cannot be removed without destroying it; also tape the ends of the seal and wrap tape around cassette joints. The appendix requires sealing the cassette so end cap and base plugs cannot be removed without destroying the seal, and instructs taping the ends of the seal and wrapping tape at each joint (1926.1101AppB).

Key do's and don'ts:

• Do not touch or puncture the filter membrane while sealing—the analysis will be invalidated.
• Use a seal that visibly shows tampering; tape the seal ends together.
• Wrap tape around cassette joints to keep the seal secure.

Send samples with paperwork requesting asbestos analysis, list any known fibrous interferences and the workplace operations sampled, and package cassettes so they won't rattle or be exposed to static during shipment. 1926.1101AppB specifies these shipment practices and warns not to use expanded polystyrene peanuts, vermiculite, paper shreds, or excelsior. Tape sample cassettes to sheet bubbles and place in a cushioned container; ship bulk samples in separate containers to avoid contamination.

Packaging checklist:

• Complete chain-of-custody and note any fibrous interferences and sampled operations.
• Secure cassettes to prevent rattling and static (use sheet bubbles, tape).
• Avoid loose packing materials that can abrade or contaminate samples.
• Ship bulk samples separately from air samples.

Treat acetone as extremely flammable and handle it in a ventilated laboratory hood away from open flames, using small containers and a spark-free heat source for any vapor generation. 1926.1101AppB warns about acetone's flammability and prescribes transferring solvent in a hood and avoiding open flames. It also advises prudence to prevent asbestos spills and contamination of lab personnel or facilities.

Practical lab controls:

• Use acetone only in a fume hood and in minimal quantities.
• Keep ignition sources away and use spark-free heaters if needed.
• Have spill cleanup procedures and PPE ready to prevent fiber dispersal.

Clean up asbestos spills immediately using wet methods and/or a HEPA-filtered vacuum; do not use a vacuum without a HEPA filter because it will disperse fine fibers. The appendix explicitly directs wet cleanup and/or HEPA vacuums and cautions against using non-HEPA vacuums (1926.1101AppB).

Tips for safe cleanup:

• Wet down the material to keep fibers from becoming airborne.
• Use a commercially certified HEPA vacuum for final cleanup.
• Isolate the area and use appropriate PPE during cleanup to protect personnel.

Use a phase contrast microscope with a binocular or trinocular head, a Walton-Beckett Graticule type G-22 (about 100 ±2 µm projected diameter), a 40X phase objective with a numerical aperture of 0.65–0.75, and 10X widefield or Huygenian eyepieces (with the eyepiece containing the graticule being a focusing eyepiece). 1926.1101AppB lists these equipment requirements and alignment recommendations (Kohler illumination, phase telescope, etc.).

Other useful equipment noted in the appendix includes:

• Mechanical stage and stage micrometer,
• Phase-shift test slide (mark II),
• Precleaned slides and #1 1/2 cover glasses,
• Fine forceps and scalpels for sample preparation.

Cut a triangular wedge (about one-sixth to one-fourth of the filter), place it sample-side up on a clean slide, briefly clear it with acetone on a heated aluminum block, add 2.5–3.5 µL triacetin within 30 seconds, then lower a cover slip gently to make a permanent mount. The appendix gives step-by-step mounting instructions including heating the aluminum block to ~70°C, using about 200 µL acetone to clear the filter for 3–5 seconds, and the time-sensitive addition of triacetin (1926.1101AppB).

Important cautions:

• Do not touch the filter with your fingers.
• If more than 30 seconds elapse between acetone clearing and triacetin application, the preparation may have reduced contrast—glue the edges of the coverslip if delay occurs.
• If clearing is slow, warm the slide (about 50°C) for up to 15 minutes to hasten clearing.

Count only fibers equal to or longer than 5 µm, with a length-to-width ratio of 3:1 or greater; count fibers in at least 20 fields and continue until either 100 fibers are counted or 100 fields are viewed. 1926.1101AppB lists these counting conventions and additional rules such as counting curved fibers by their curve length and the Walton‑Beckett graticule boundary rules (fibers entirely inside = 1, crossing once with one end inside = 1/2, do not count fibers crossing more than once).

Other counting specifics:

• Spend 5–15 seconds per field and scan multiple focal planes (upper 10–15 µm).
• Count bundles as one fiber unless individual fibers can be clearly distinguished and are not connected.
• Record fiber counts per field consistently so non-uniformity can be assessed.

Reject any field where an agglomerate covers more than 25% of the view and select another field; perform a blind recount on 1 in every 10 filter wedges. The appendix instructs rejecting fields with agglomerates covering >25% and conducting a blind recount of 1 in every 10 wedges, relabeling slides by a person other than the original counter (1926.1101AppB).

Quality control tips:

• Keep accurate records to support blind recounts.
• Make the blind-relabeling process independent of the original counter to assess repeatability and bias.

No—PCM cannot provide positive confirmation of asbestos; when identification is required, use differential or electron microscopy techniques, and if there is doubt about a fiber, count it ("WHEN IN DOUBT, COUNT"). The appendix states that PCM does not positively identify asbestos and recommends alternative differential counting (e.g., polarized light, SEM/TEM) when discrimination is necessary, and it explicitly instructs that if in doubt the fiber should be counted (1926.1101AppB).

If you need definitive identification:

• Request differential analysis or TEM/SEM confirmation from the lab.
• Note on the chain-of-custody that PCM was used and indicate any suspected interferences so the lab can apply appropriate confirmatory testing.

Analysts should have taken the NIOSH course for asbestos sampling and evaluation (or equivalent), participate in Proficiency Analytical Testing (PAT) or equivalent, join slide exchanges with other labs, and maintain a blind slide‑bank QA program with performance targets. The appendix requires NIOSH training or equivalent and participation in interlaboratory slide trading and PAT, and explains a slide bank and quality assurance program for establishing historical means and standard deviations (1926.1101AppB).

Performance expectation:

• A counter must have 95% of all quality control samples within three standard deviations of the historical mean, per the appendix guidance.

Calculate airborne fiber concentration using the formula in the appendix that combines the total fibers counted, fields counted, blank counts, effective collecting area of the filter, pump flow rate, microscope field area, and sample time; also measure the actual collection area of the filter because it is seldom exactly 385 mm². 1926.1101AppB provides the calculation (AC = function of FB, FL, BFB, BFL, ECA, FR, MFA, and T) and notes the nominal 385 mm² for a 25-mm filter but recommends laboratories routinely measure the filter diameter to compute the exact collection area using Area = pi(d/2)^2.

Key variables to record:

• FB = fibers >5 µm counted on sample;
• FL = fields counted on sample;
• BFB/BFL = blank fibers and blank fields;
• ECA = effective collecting area (measure filter diameter);
• FR = pump flow rate (L/min);
• MFA = microscope field area (0.00785 mm² for Walton‑Beckett graticule);
• T = sample collection time (min).

Work activities that involve asbestos-containing building materials (ACBM), including many residential remediation tasks, are covered by OSHA's construction asbestos standard, 29 CFR 1926.1101, not the general industry asbestos standard. OSHA's interpretation on asbestos remediation protocols confirms that remediation activities involving ACBM are covered by the construction standard and references application guidance (Asbestos remediation protocols and 1926.1101AppB).

Practical implications:

• If your remediation work may disturb ACBM, follow the requirements in 1926.1101, including exposure assessment, controls, respiratory protection, and monitoring obligations.
• Document assumptions and sampling procedures, and consult 1926.1101AppB for accepted sampling and analysis practices.

Use the Appendix B short-cut by combining constants for a fixed analyst and filter so the fields-per-filter factor becomes a single number (49 in the example). For a consistent microscope/reticle/analyst setup the exposed filter area, the microscope field area, and the air-volume conversion from liters to cc are constant and can be combined so that ECA/(1,000 × MFA) = 49 in the example.

• Example: if the exposed filter area is 385 mm² and the field area is 0.00785 mm², then the number of fields per filter is 385 / 0.00785 = 49,000; combined with the liters→cc factor the example short-cut reduces to the stated constant (49).

See the worked example and explanation in 1926.1101 App B for the full derivation and the context for using this short-cut.

Calibrate the eyepiece-objective-reticle combination and use a field diameter of 100 ± 2 µm (0.1 mm) when specified; the corresponding field area is calculated as pi × (D/2)², which equals 0.00785 mm² when D = 100 µm.

• To order a Walton–Beckett graticule disc, measure the magnified grid length (PL, µm) using a stage micrometer and the actual grid length on the graticule (AL, mm). Then calculate the required circular counting-area disc diameter d(c) in mm with the formula d(c) = (AL × D) / PL, where D = 100 µm in the example.

Follow the instructions and acceptance tolerances in 1926.1101 App B for graticule sizing, calibration, and when recalibration is required after any change.

Apply the Appendix B recount statistical test: reject a pair of counts when the difference between the two concentration estimates exceeds the criterion based on AC(1), AC(2), AC(avg), and CV(FB). If a pair is rejected, you must recount the remainder of the filter set and apply the same test to any other pairs.

• Definitions you must use: AC(1) = lower estimated airborne fiber concentration, AC(2) = higher estimated airborne fiber concentration, AC(avg) = average of the two estimates, CV(FB) = coefficient of variation for the average.
• The criterion is set at a 5% type‑I error (maximum 5% risk of falsely rejecting counts that differ by chance alone).

See the full recount formula, variables, and required actions in 1926.1101 App B.

Report asbestos lab results as fibers/cc using two significant figures, and if multiple analyses are done on the same sample report the average unless a result is rejected for cause.

• If a sample fails the statistical test for homogeneity, the laboratory must include a memo to the industrial hygienist stating the sample failed homogeneity testing and that the true air concentration may be significantly different than the reported value.
• Follow the Appendix B guidance on averaging multiple analyses and on documentation when rejecting results.

See the reporting, averaging, and rejection/memo requirements in 1926.1101 App B (see the section on Reporting Results and Rejection).

Use the Appendix B flow‑rate correction that adjusts the calibrated flow for ambient pressure and temperature differences: calculate Q(act) from Q(cal) and the ratios of pressures and absolute temperatures at calibration and sampling.

• Variables to use (as named in the Appendix): Q(act) = actual flow rate, Q(cal) = calibrated flow rate (or rotameter value), P(cal) = uncorrected pressure at calibration, P(act) = uncorrected pressure at sampling site, T(cal) = calibration temperature (K), T(act) = sampling temperature (K).
• The Appendix gives Equation D to correct Q(cal) for P and T differences when the pump does not compensate and when differences exceed about 5%; follow that formula and use absolute temperatures in kelvins and absolute pressures consistent with how the equation is written.

See 1926.1101 App B for the formal Equation D and the specific correction procedure.

Use the Appendix B counting rules: single fibers fully inside the counting circle count as 1, fibers that cross the circle once count as 1/2, fibers too short or entirely outside count as 0, and crossing fibers or two separate fibers inside the circle count as 2 (per the illustrated examples).

• Common examples from the Appendix table: structures 1–6 = 1 (single fibers fully inside the circle); structure 7 = 1/2 (fiber crosses circle once); structure 8 = 0 (fiber too short); structure 9 = 2 (two crossing fibers inside); structure 10 = 0 (fiber outside circle); structure 11 = 0 (fiber crosses twice); structure 12 = 1/2 (split fiber that only crosses once).

Follow these illustrated rules and the full discussion in 1926.1101 App B when performing PCM counting.

Work activities that involve asbestos‑containing building materials (ACBM) during property remediation are covered by the construction asbestos standard, 29 CFR 1926.1101, not the general industry standard 1910.1001.

• OSHA's November 14, 2024 interpretation to the property remediation industry explicitly states that even if a company is not a typical construction firm, remediation operations involving ACBM are covered by 29 CFR 1926.1101 (see the letter, Asbestos remediation protocols).

Use the construction standard's requirements for work practices, monitoring, and worker protection when asbestos is involved in remediation work.