Subpart Z
Use a 25-mm diameter, three-piece mixed-cellulose ester (MCE) filter cassette with a 50-mm electrically conductive extension cowl and a 25-mm cellulose backup pad. The appendix specifies this sample assembly and notes that fully conductive cassettes are required to reduce fiber loss to the cassette sides and that cassettes should not be reused (1910.1001 App B).
Select a sampling flow rate between 0.5 and 5.0 liters per minute (L/min) and choose an air volume between 25 L and 2,400 L depending on the situation. The appendix recommends 0.5–5.0 L/min (commonly 1–2 L/min) for personal sampling and lists a minimum volume of 25 L and a maximum of 2,400 L (1910.1001 App B).
Place the cassette in the worker’s breathing zone about 10 cm from the nose/mouth, held open side down on the collar or lapel. The appendix instructs that the cassette should be approximately 10 cm from the nose/mouth and secured so it remains open-face down in the breathing zone during sampling (1910.1001 App B).
Yes, seal the junction of the cassette base and cowl with a gel band or tape, and do not reuse cassettes. The appendix requires sealing the base and cowl with a gel band or tape to prevent loss or contamination and explicitly notes "Do not re-use cassettes" (1910.1001 App B).
Calibrate each sampling pump before and after sampling with a calibration cassette in-line, using a primary standard such as a bubble burette; calibrate at the sampling site if possible. The appendix instructs pre- and post-sampling calibration with a calibration cassette from the same lot and recommends site calibration with a primary standard, and notes environmental influences may require corrections described in the appendix (1910.1001 App B).
Use mixed-cellulose ester (MCE) filters with pore sizes between 0.4 and 1.2 µm and discard a filter lot if background counts exceed 4 fibers per 100 fields. The appendix specifies 0.4–1.2 µm pore sizes and directs purchasers to select filters screened for asbestos counting or to analyze representative filters and discard the lot if more than 4 fibers/100 fields are found (1910.1001 App B).
Aim for an on-filter fiber density of about 100 to 1,300 fibers/mm2 because that range gives optimal counting precision and accuracy for phase contrast microscopy. The appendix states the sampling time and rate should be chosen to yield 100–1,300 fibers/mm2 on the filter, which corresponds to about 0.8–10 fibers/field with a Walton‑Beckett graticule and supports reliable counting statistics (1910.1001 App B).
Count at least 20 fields and stop when 100 fibers are reached (provided at least 20 fields have been counted); do not count more than 100 fields routinely. The appendix explains that a general rule is to count at least 20 and not more than 100 fields, and to discontinue counting when 100 fibers are counted if at least 20 fields have been examined (1910.1001 App B).
The detection limit for the PCM method is about 4.0 fibers per 100 fields (≈5.5 fibers/mm2). The appendix explains this detection limit and the statistical basis for it, citing a 4–4.5 fibers per 100 fields value as the supported lower limit (1910.1001 App B).
PCM cannot positively identify asbestos and may count non-asbestos fibers and cannot see the very finest fibers (down to ~0.02 µm). The appendix notes that PCM is a fiber-counting technique that does not positively identify asbestos (other fibers may be included unless differential counting is used) and that PCM's smallest visible fibers are about 0.2 µm in diameter while some asbestos fibers can be as small as 0.02 µm (1910.1001 App B).
Use phase contrast microscopy at 400X with a Walton‑Beckett graticule (100 µm projected diameter circle) for counting. The appendix specifies preparing the filter for PCM at 400X and describes the Walton‑Beckett graticule design used to define the field area for counting (1910.1001 App B).
Use electron microscopy or optical tests (e.g., polarized light) when positive identification of asbestos is needed or when interfering fibrous materials are suspected. The appendix states PCM does not positively identify asbestos and recommends electron microscopy or optical tests like polarized light and dispersion staining to differentiate asbestos from other fibers when necessary (1910.1001 App B).
Common interferences include fiberglass, plant fibers, perlite, diatoms, wollastonite, gypsum, and certain synthetic fibers. The appendix lists these and advises using electron microscopy or optical tests to distinguish them from asbestos when needed (1910.1001 App B).
If the pump is influenced by temperature or pressure, consult the pump manufacturer and correct the flow rate using the appendix's "Sampling Pump Flow Rate Corrections" guidance. The appendix advises consulting the pump manufacturer about environmental dependence and applying the flow rate corrections provided in the appendix when necessary (1910.1001 App B).
Hold the sampling cassette open-face down during collection so it samples in the worker’s breathing zone. The appendix specifically directs taking each air sample open face and assuring each cassette is held open side down during sampling (1910.1001 App B).
A suggested minimum air volume for TWA compliance sampling is 25 liters. The appendix explicitly recommends a minimum air volume of 25 L when sampling to determine TWA compliance (1910.1001 App B).
Differential counting means excluding fibers from the count that do not appear to be asbestos (based on analyst experience) and is used when non-asbestos fibers are present to reduce false positives. The appendix defines differential counting as excluding certain kinds of fibers that do not appear to be asbestos and notes that doing this correctly requires considerable experience (1910.1001 App B).
Stop counting when 100 fibers are reached (if at least 20 fields counted) because counting more gives little precision gain; counts below about 10 fibers show rapidly worsening precision. The appendix explains the general rule of counting 20–100 fields, stopping at 100 fibers for efficiency, and that precision depends on total fibers counted and distribution uniformity; lower counts (below ~10 fibers) lead to an accelerated loss of precision (1910.1001 App B).
The asbestos remediation protocols letter clarifies that remediation activities involving asbestos-containing building materials (ACBM) are generally covered by the construction asbestos standard, 29 CFR 1926.1101, rather than the general industry standard 29 CFR 1910.1001. OSHA's interpretation explains that many remediation activities, even by non‑construction companies, are subject to the construction standard and refers readers to the application discussion (see Asbestos remediation protocols and 1910.1001).
To reduce contamination risk, use preassembled sampling systems from the manufacturer when possible, purchase filters selected for asbestos counting, seal cassette joints, and avoid reusing cassettes. The appendix recommends preassembly to decrease contamination, selecting filters with low background or testing lot background (discard if >4 fibers/100 fields), sealing cassette seams with gel bands, and not reusing cassettes (1910.1001 App B).
Positive identification is required when you need to know if fibers are asbestos and should be done using polarized light microscopy or electron microscopy techniques. The appendix states PCM does not positively identify asbestos and recommends polarized light or electron microscopy for positive identification (1910.1001 App B).
The recommended minimum air volume for an Excursion Limit (30-minute) asbestos sampling is 48 liters. This recommendation appears in the sampling guidance in 1910.1001AppB.
The suggested maximum air sample volumes listed in 1910.1001AppB 5.2.8 are:
These are guidance values to help avoid overloading the filter with non-asbestos dust; adjust downward if non-asbestos dust is excessive.
You must avoid overloading the filter because heavy non-asbestos dust can obscure fibers and bias the asbestos count low (when about 25–30% or more of the field area is obscured the result may be biased low). The guidance in 1910.1001AppB recommends observing the filter with a small flashlight while sampling to check for a visible layer of dust. If a visible dust layer appears, stop sampling and follow the cassette change procedure.
If you see a visible layer of dust on the filter while sampling, stop sampling, remove and seal the cassette, and replace it with a new sampling assembly. 1910.1001AppB specifically advises this action and cautions that high non-fibrous dust can obscure fibers and invalidate or bias results.
The total dust loading on a sampled filter should not exceed 1 milligram. This limit is stated in the guidance in 1910.1001AppB to prevent obscuration of fibers and invalid counts.
Prepare two blank cassettes for the first 1–20 samples and then prepare blanks equal to 10% of the samples for sets greater than 20; do not draw air through blank samples. 1910.1001AppB directs that blanks are handled the same as air samples except no air is drawn through them. For blank preparation, open the blank cassette where sample cassettes are mounted on the employee, hold it open about 30 seconds, then close and seal it for shipment.
Immediately after sampling, close and seal each cassette with its base and plastic plugs and attach a sample seal so that the end cap and base plugs cannot be removed without destroying the seal; tape the seal ends and wrap tape around cassette joints. 1910.1001AppB warns not to touch or puncture the filter membrane because that will invalidate analysis.
Ship air samples cushioned so they will not rattle or be exposed to static electricity and do not use expanded polystyrene peanuts, vermiculite, paper shreds, or excelsior; ship bulk samples in separate mailing containers to avoid contamination. 1910.1001AppB recommends taping sample cassettes to sheet bubbles and placing them in a container that cushions them against rattle, and always packing bulk samples separately.
Laboratory staff must treat acetone as extremely flammable: use small quantities, transfer solvent in a ventilated laboratory hood, avoid open flame, and use a spark-free heat source if generating acetone vapor. These precautions are specified in 1910.1001AppB.
Asbestos spills should be cleaned up immediately using wet methods and/or a High Efficiency Particulate-Air (HEPA) filtered vacuum; do not use a vacuum without a HEPA filter because it will disperse fine asbestos fibers. 1910.1001AppB gives these safety precautions for preventing contamination and exposure.
PCM asbestos counting requires a phase contrast microscope with binocular or trinocular head; widefield or Huygenian 10X eyepieces (the eyepiece with the graticule must be focusing); a 40X phase objective with numerical aperture 0.65–0.75; and a Walton-Beckett Graticule, type G-22, with a 100 ± 2 µm projected diameter. These specifications are listed in 1910.1001AppB.
Mounting procedure: heat the aluminum block to ≈70 °C, use about 200 µL acetone delivered into the block slot with a micropipette and wait 3–5 seconds for clearing, then within 30 seconds place 2.5–3.5 µL triacetin on the filter and lower a coverslip gently. If more than 30 seconds pass after acetone exposure, glue the coverslip edges with lacquer/nail polish. These steps and timings are described in 1910.1001AppB to produce a smooth mount with appropriate refractive index.
The microscope/microscopist combination must see line set 3 and at least parts of sets 4 and 5, but must not see set 6 on the phase-shift test slide; if it fails this test it may not be used. This performance criterion is required in 1910.1001AppB.
Count only fibers equal to or longer than 5 µm and with a length-to-width ratio of at least 3:1; measure the length of curved fibers along the curve. These counting rules are specified in 1910.1001AppB.
An analyst must count all fibers in at least 20 fields and continue counting until either 100 fibers have been counted or 100 fields have been viewed, whichever occurs first; count all fibers in the final field viewed. See 1910.1001AppB for the stopping rules.
Scoring rules: a fiber lying entirely within the Walton-Beckett graticule field scores 1; a fiber crossing the graticule boundary once (one end inside) scores 1/2; do not count fibers crossing the boundary more than once or those touching the circle more than once; if a fiber touches the circle it is considered to cross. Bundles count as one fiber unless individual fibers are clearly separable and not connected to another counted fiber. These rules are in 1910.1001AppB and relate to graticule counting conventions.
Reject and do not count a field if an agglomerate (mass of material) covers more than 25% of the field of view; then select another field. This rejection criterion is stated in 1910.1001AppB.
Perform a blind recount of 1 in every 10 filter wedges (slides); have someone other than the original counter re-label the slides to preserve blindness. This blind recount procedure is specified in 1910.1001AppB.
No—PCM does not provide positive confirmation of asbestos fibers; if discrimination is required use alternate techniques (polarized light, electron microscopy, or differential counting), and if you are uncertain whether a fiber is asbestos, follow the rule: "When in doubt, count." This guidance is in 1910.1001AppB.
Analysts must take the NIOSH course for sampling and evaluating airborne asbestos or an equivalent course; each laboratory must participate in the Proficiency Analytical Testing (PAT) Program or equivalent, set up slide-trading with other labs at least semiannually, maintain a slide bank for blind quality assurance counts, and require that a counter have 95% of quality control samples within three standard deviations of the historical mean. These quality system requirements are stated in 1910.1001AppB.
Property remediation activities involving asbestos-containing building materials are generally covered by OSHA's construction asbestos standard, 29 CFR 1926.1101, not the general industry standard [1910.1001]. OSHA confirmed this point in the November 14, 2024 Letter of Interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2024-11-14, which states that remediation activities of the type described (e.g., property remediation after fires, floods, or crime scenes) involving ACBM would be covered under 1926.1101 rather than 1910.1001.
Use the membrane-filter formula in 1910.1001AppB Section 7.1 that converts your microscope counts to fibers per cubic centimeter (fibers/cc). In plain terms, you: (1) compute the average fibers per field on your sample and on the blank, (2) subtract the blank average from the sample average, and (3) multiply by the area and conversion factors that convert counts to fibers/cc.
When you use a 25-mm filter with an effective collecting area of 385 mm2 and a Walton–Beckett graticule with field area 0.00785 mm2, the constant ECA/(1000 × MFA) equals 49 and the equation simplifies to:
AC = 49 × [ (FB/FL) − (BFB/BFL) ] ÷ (FR × T)
Where FB = fibers (>5 µm) counted on the sample, FL = fields counted on the sample, BFB = fibers counted on the blank, BFL = fields on the blank, FR = pump flow rate (L/min), and T = sample time (min). Always use the full formula and variable definitions in 1910.1001AppB Section 7.1 when your filter size or field area differ from the example above.
Measure the filter’s actual exposed diameter with an inside micrometer and calculate area as a circle using Area = π(d/2)2, then use that value as the effective collecting area (ECA). The guidance in 1910.1001AppB explains that the nominal 25‑mm filter collection area (385 mm2) is seldom exact and laboratories should routinely determine the actual diameter and compute ECA = π(d/2)2 for more accurate concentration calculations.
The microscope field area (MFA) is the area of one counting field (in mm2) defined by your graticule and optical setup, and you must measure and use the MFA for your specific eyepiece/objective/reticle combination. 1910.1001AppB Section 27–28 gives step‑by‑step instructions: focus a stage micrometer, measure the magnified grid length (PL, µm), measure the graticule’s actual grid length (AL, mm), and then calculate the field diameter and area. For example, a field diameter D = 100 µm gives an MFA = Δ(D/2)2 = 0.00785 mm2 for the Walton–Beckett disc; but you must calibrate and re-measure whenever you change eyepiece, objective, reticle, or interpupillary distance as required by 1910.1001AppB.
You may use the short‑cut constant 49 when you always use the same microscope/graticule combination and a 25‑mm filter with an exposed area of 385 mm2 and a Walton‑Beckett field area of 0.00785 mm2. 1910.1001AppB Section 7.2 explains that because the analyst’s interpupillary distance and the field size stay constant, ECA/(1,000 × MFA) equals 49 for that setup, which simplifies the full formula (so you can use AC = 49 × [ (FB/FL ) − (BFB/BFL ) ] ÷ (FR × T)). If you change filter size, graticule, microscope, or reticle, you must re-calculate MFA and ECA and cannot rely on the 49 constant.
Reject the pair if the difference between the two concentration estimates exceeds the statistical criterion given in 1910.1001AppB Section 7.3; if that happens you must recount the rest of the samples in the set, apply the same test to other pairs, and document the failure. The appendix explains the recount rule at a 5% type‑I error level and requires that any rejected pair be followed by recounting all remaining filters in the submitted set; rejection requires a memo to the industrial hygienist that the sample failed a test for homogeneity and the true air concentration may be significantly different than the reported value.
Report results as fibers per cubic centimeter (fibers/cc) using two significant figures. 1910.1001AppB Section 7.4 says to report fibers/cc with two significant figures and to average multiple analyses on a sample unless one or more results are rejected for cause (for example, by the recount criteria in Section 7.3). Include any notes about rejected counts or quality‑control issues with the report.
You must correct the pump flow rate if ambient temperature and/or pressure at the sampling site differ by more than 5% from the conditions at calibration and the pump does not automatically compensate. 1910.1001AppB (Sampling Pump Flow Rate Corrections) provides the correction formula: Qact = Qcal × (Pcal/Pact) × (Tact/Tcal), where Qact is the actual flow rate, Qcal is the calibrated flow rate, Pact and Pcal are sampling and calibration pressures, and Tact and Tcal are sampling and calibration temperatures in Kelvin. Apply this correction before you calculate airborne concentrations if the pump isn’t pressure/temperature compensating.
Property remediation work that involves asbestos‑containing building materials is generally covered by the construction asbestos standard, 29 CFR 1926.1101, not the general industry standard 1910.1001. OSHA’s Letter of Interpretation on asbestos remediation protocols explains that remediation activities (including many repair, cleanup, or demolition tasks in buildings) are covered under 1926.1101 rather than the general industry rule; see also the general asbestos sampling and counting guidance in 1910.1001AppB for laboratory and sampling procedures. Employers performing remediation should follow the construction standard unless the specific work is clearly within general industry operations and applicable guidance or state plans indicate otherwise.