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Short answer
NIOSH Method 7400 is a published analytical method for counting airborne fibers on a filter sample under a phase contrast microscope (PCM). It reports an index of airborne fibers, mainly used to estimate asbestos concentrations, but PCM cannot tell asbestos from other fibers. The result is expressed in fibers per cubic centimeter of air (f/cc).
Definition and scope
Method 7400, “Asbestos and Other Fibers by PCM,” sits in the NIOSH Manual of Analytical Methods (NMAM), 4th edition. The version most often cited is Issue 2, dated 15 August 1994. The method lists its technique as phase contrast light microscopy, its analyte as fibers counted manually, and its evaluation status as full.
The method describes its own output as “an index of airborne fibers.” It is used primarily to estimate asbestos concentrations, yet it counts any airborne fiber that meets the counting criteria. For that reason NIOSH advises using it together with electron microscopy, for example Method 7402, when fibers need to be identified. Fibers thinner than about 0.25 µm in diameter will not be detected by PCM.
NIOSH revised Method 7400 on 14 June 2019. According to the AIHA publication The Synergist (August 2019), the revision allowed alternative slide preparation and mounting procedures, recommended optional gridded coverslips so analysts can return to the same field, allowed dimethylformamide and Euparal resin in place of acetone and triacetin, and introduced a phase-shift test slide that may replace the HSE/NPL slide, which is no longer available. The same report states that the rules for which fibers count as asbestos are unchanged.
How the method is used in practice
Air is drawn through a 25-mm cellulose ester membrane filter in a cassette with a conductive cowl; the 1994 text specifies pore sizes of 0.45 to 1.2 µm and notes that 0.8 µm filters are used for personal sampling. The listed flow rate is 0.5 to 16 L/min. The minimum volume is 400 L at 0.1 fiber/cc, and the sample is adjusted to give a filter loading of 100 to 1,300 fibers/mm². A set of samples includes 2 to 10 field blanks.
An analyst then examines the prepared filter by PCM and counts fibers by a fixed set of rules rather than by judgment. The counting rules matter more than any other detail, because the number is only comparable between laboratories when the same rules are used (see the table below).
What the numbers mean
- Working range: 0.04 to 0.5 fiber/cc for a 1,000 L air sample, per the method’s applicability statement.
- Detection limit: an estimated 7 fibers/mm² of filter area, or below 0.01 fiber/cc in atmospheres free of interferences.
- Why volume is adjusted: the method instructs the sampler to adjust volume to give 100 to 1,300 fibers/mm² on the filter. High non-fibrous dust can obscure fibers and raise the detection limit.
Quality control built into the method
A field blank with more than 7 fibers per 100 graticule fields is reported as possible contamination. Analysts perform blind recounts on 10% of filters, and filter lots are discarded if a laboratory blank check averages 5 or more fibers per 100 fields.
Counting rules and related methods
NIOSH recommends the “A” rules, which match the older P and CAM 239 rules. The method also describes alternative “B” rules and states that hybridizing different sets of counting rules is not permitted, and that the report must say which rules were used.
| Feature | “A” rules (NIOSH recommended) | “B” rules (alternate) |
|---|---|---|
| Minimum length | Longer than 5 µm | Longer than 5 µm |
| Aspect ratio | 3:1 or greater | 5:1 or greater |
| Maximum diameter | Not specified as a separate criterion | Under 3 µm |
| What is counted | Fibers (a fiber with one end in the graticule area counts as one half) | Ends |
Other points from the method: curved fibers are measured along the curve; bundles count as one fiber unless both ends of individual fibers can be seen; fields are counted until 100 fibers are reached, with a minimum of 20 fields and a stop at 100 fields.
Electron microscopy methods such as 7402 are the companion route when the identity of the fibers matters, since PCM counts by shape alone. The method also notes that it can be used for fibers other than asbestos, such as fibrous glass, with alternate counting rules.
Where it matters for labs and clinics
For facilities staff and safety officers, the method matters because it defines what an airborne fiber result actually represents. The OSHA asbestos standard, 29 CFR 1910.1001, sets a permissible exposure limit of 0.1 fiber per cubic centimeter of air as an eight-hour time-weighted average, and an excursion limit of 1.0 fiber/cc averaged over 30 minutes. It defines a fiber as a particulate form of asbestos 5 micrometers or longer with a length-to-diameter ratio of at least 3 to 1, which lines up with the “A” counting rules.
The standard requires samples to be evaluated using the OSHA Reference Method in its Appendix A, or an equivalent counting method. An equivalent method must show that 90% of samples collected in the range of 0.5 to 2.0 times the permissible limit fall within plus or minus 25 percent of the reference method results at a 95% confidence level. The OSHA text does not name NIOSH 7400, so a laboratory’s reference to 7400 is a statement about the analytical method it follows, not a regulatory citation.
Air sampling in a facility sits alongside the rest of a safety program. Related CASRAI pages cover the laboratory safety manual and chemical hygiene plan, the OSHA chemical hygiene plan and the respiratory protection program for laboratories. For the optics behind the method, see what a research microscope is, microscope objective selection and microscope slides.
Common misunderstandings
- “A 7400 result is an asbestos result.” It is a fiber count. The method states that PCM does not differentiate between asbestos and other fibers.
- “The 2019 revision changed which fibers count.” The report on the revision states that the rules for which fibers count as asbestos are unchanged; the changes concern slide preparation, mounting media and test slides.
- “OSHA requires 7400 by name.” The OSHA standard requires the OSHA Reference Method or an equivalent counting method and does not mention 7400.
- “PCM sees every fiber.” Fibers below about 0.25 µm in diameter are not detected, and chain-like particles can look fibrous.
- “Counting rules are interchangeable.” The method forbids hybridizing sets of rules, and the report must state which rules were used.
FAQ
Is NIOSH 7400 the OSHA method?
No. NIOSH 7400 is a NIOSH analytical method. OSHA 29 CFR 1910.1001 requires the OSHA Reference Method (Appendix A) or an equivalent counting method with the accuracy criterion described above, and does not name 7400.
What fibers can’t it see?
Fibers thinner than about 0.25 µm in diameter are not detected by PCM, and the method cannot identify a fiber’s composition. Electron microscopy such as Method 7402 is the companion technique for identification.
What does it measure?
The count of fibers longer than 5 µm with an aspect ratio of at least 3:1 under the “A” rules, converted to fibers per cubic centimeter of air from the sampled volume.
Sources
- NIOSH, NIOSH Manual of Analytical Methods, 4th ed., Method 7400, “Asbestos and Other Fibers by PCM,” Issue 2, 15 August 1994 (archived copy: webharvest.gov archive of the CDC PDF), checked 29 October 2026.
- OSHA, 29 CFR 1910.1001, Asbestos, checked 29 October 2026.
- AIHA, The Synergist, August 2019: revised method for asbestos analysis, reporting the 14 June 2019 NIOSH revision, checked 29 October 2026.
