Two characters on a respirator filter tell you everything: a letter for oil resistance and a number for efficiency. Get them right and you match the filter to the hazard; get them wrong and you either under-protect a worker or hand out gear that clogs in an hour. This is the whole system — the nine NIOSH classes, N95 vs P100 in the field, and a common misconception: the filter does not change your assigned protection factor. Respiratory protection is adopted for construction unchanged through 29 CFR 1926.103, and OSHA requires particulate filters to be NIOSH-certified under 42 CFR Part 84 (1910.134(d)(3)(iv)).
At a glance: The letter = oil resistance (N = no oil aerosols; R and P = rated for oil). The number = efficiency (95 = ≥95%, 99 = ≥99%, 100 = ≥99.97%, the HEPA-equivalent class). Nine classes total. The filter doesn't set your APF — a half-mask is APF 10 with an N95 or a P100. On a NIOSH respirator filter the letter N, R, or P is the oil resistance and the number 95, 99, or 100 is the minimum efficiency; a P100 is rated for oil and at least 99.97 percent efficient, but the assigned protection factor is set by the facepiece, so a half-mask is a factor of 10 whether it uses an N95 or a P100.
The letter: oil resistance (N, R, P)
Oil aerosols — cutting and machining fluids, some paints and solvents, certain lubricants — degrade many filter media over time. NIOSH sorts filters into three series (42 CFR 84.170(a)(2)):
- N — Not resistant to oil. Use only where no oil aerosols are present. Most construction dust — silica, wood, drywall, concrete — is a fine fit for an N-series filter.
- R — Resistant to oil. Intended for particulates that include oil-based liquids.
- P — oil-Proof. Also intended for oil-based particulates, and the series to choose when oil exposure is heavy or long; follow the manufacturer's time-use instructions for either series.
The rule of thumb: if there's any chance of oil in the air, skip N and reach for R or P. When you're not sure, P is the safe default.
The number: efficiency (95, 99, 100)
The number is the minimum filter efficiency NIOSH measures under 42 CFR 84.181: a solid sodium chloride aerosol (count median diameter 0.075 µm) for N filters and a liquid DOP aerosol (0.185 µm) for R and P filters, at 85 liters per minute through a single filter:
- 95 — at least 95%
- 99 — at least 99%
- 100 — at least 99.97% (the HEPA-equivalent class: OSHA defines HEPA as at least 99.97% efficient against 0.3-micrometer particles and names N100, R100, and P100 as the equivalents, 1910.134(b); a P100's filter is often color-coded magenta)
Higher isn't always necessary — a 95 is efficient by any ordinary standard — but a 100-class filter buys margin for high-toxicity or high-concentration work.
The nine classes, in one grid
Three series times three efficiencies gives the nine NIOSH classes:
| 95 (≥95%) | 99 (≥99%) | 100 (≥99.97%) | |
|---|---|---|---|
| N (not oil-resistant) | N95 | N99 | N100 |
| R (oil-resistant) | R95 | R99 | R100 |
| P (oil-proof) | P95 | P99 | P100 |
The nine classes and their efficiency levels come from 42 CFR 84.170(a)(3).
The misconception that matters: the filter is not your APF
This is where crews go wrong. People treat P100 as simply "more protection" than N95 — so they upgrade the filter and assume they've upgraded the respirator. They haven't, in the way that counts for exposure.
The assigned protection factor (APF) — the number OSHA lets you rely on to reduce exposure (APF × PEL = the maximum use concentration) — is a property of the facepiece, not the filter. A half-mask is APF 10 whether it holds an N95 pad or a P100 cartridge, because the dominant leak path isn't the filter media — it's the face seal. A P100 filters more efficiently and shrugs off oil, but if the mask leaks at the cheek, the highly efficient filter is bypassed.
Filter class vs. assigned protection factor — two different things
To get more protection you move up the facepiece (half → full → PAPR/SCBA), not just the filter. APFs from Table 1, 1910.134(d)(3)(i)(A).
So the two decisions are independent: the filter class matches the contaminant (is there oil? how toxic?), and the facepiece sets the protection factor (how far below the exposure limit do I need to get?). You choose both.
Fit testing doesn't care which filter you picked
Upgrading from an N95 to a P100 changes nothing about the seal. If the respirator is tight-fitting, it still needs a medical evaluation, a fit test, and a user seal check under 29 CFR 1910.134 — the filter class is irrelevant to whether the mask seals on the face. (A disposable N95 filtering facepiece is itself tight-fitting and must be fit tested when it's required; only genuinely voluntary use skips that, with the Appendix D information.) The one class of respirator that skips fit testing is a loose-fitting hood or helmet — because it has no seal, not because of its filter.
Putting it together on the jobsite
- Name the contaminant and ask about oil. Silica, wood, or drywall dust with no oil in the air → an N-series filter is fine. Oil mist, cutting fluids, oil-based coatings → R or P.
- Pick an efficiency for the risk. 95 for routine nuisance-to-moderate dust; 99 or 100 for high-toxicity or high-concentration work. P100 is the do-everything default when in doubt.
- Set the protection factor with the facepiece, not the filter. If a half-mask (APF 10) can't get you below the limit, move to a full facepiece (50) or a PAPR/SAR — don't just bump the filter number.
- Then handle the seal. Medical evaluation, the right fit-test method, a clean-shaven seal, and a user seal check every time.
Match the respirator to the hazard, then keep the proof
Picking the filter is the easy half; the citations come from the seal — medical clearance, the right fit-test method, and current dates per worker. The free Fit-Test Method Selector tells you whether you need a QLFT or a QNFT; HazComFast then logs each worker's fit test, with the method, the result, and a one-year expiration date by default. Plan it with the free tools, then keep the record on trial.
The standard, tools & related reading
- The hub: Respiratory Protection (OSHA 1910.134) — filters, fit testing, APFs, and the seal in one place
- The standard: Respiratory Protection — 29 CFR 1910.134
- Related: Respirator Fit Testing: QLFT vs QNFT & Who Pays · Facial Hair & Respirators: the beard rule + PAPR fix · Respiratory Protection Program — Complete Guide
- Tools: Fit-Test Method Selector · PPE Selector · Respiratory Program Builder
- Glossary: Assigned Protection Factor (APF) · Permissible exposure limit
- 2026 OSHA penalty schedule · penalties by state
Sources & verification (read on the eCFR, October 6, 2026): the NIOSH particulate-filter classification — series N/R/P (N restricted to workplaces free of oil aerosols; R and P for oil-based particulates) and efficiency levels 95/99/100 (≥95% / ≥99% / ≥99.97%), nine classes total, per 42 CFR 84.170(a)(2)-(3); test aerosols and the 85 L/min flow per 42 CFR 84.181; HEPA definition per 29 CFR 1910.134(b). OSHA requires NIOSH-certified particulate filters (42 CFR 84) or HEPA under 29 CFR 1910.134(d)(3)(iv); assigned protection factors from Table 1, 1910.134(d)(3)(i)(A); adopted for construction by 29 CFR 1926.103. General guidance, not legal advice.*
Frequently Asked Questions
What is the difference between N95 and P100?
The letter is oil resistance and the number is efficiency. N95 is not resistant to oil aerosols and filters at least 95% of particles; P100 is rated for oil aerosols and filters at least 99.97%, the class OSHA treats as equivalent to HEPA (29 CFR 1910.134(b)). Use an N-series filter only where no oil aerosols are present; use R or P where oil mists, cutting fluids, or certain paints are in the air. P100 is the safe default when you're unsure about oil.
Does a P100 give more protection than an N95?
Not in the way most people think. The assigned protection factor (APF) is set by the facepiece, not the filter — a half-mask is APF 10 whether it holds an N95 or a P100. A P100 filters more efficiently and resists oil, but the dominant leak path is the face seal, so both half-masks are rated to reduce exposure by the same factor of 10 (Table 1, 29 CFR 1910.134(d)(3)(i)(A)). Moving up in protection means a better facepiece (full facepiece APF 50, PAPR, SAR), not just a better filter.
What do the NIOSH letters N, R, and P mean?
They describe oil resistance under 42 CFR 84.170(a)(2): N-series filters are restricted to workplaces free of oil aerosols, and R- and P-series filters are intended for particulates that include oil-based liquids. The usual mnemonic reads N as not resistant to oil, R as resistant, and P as oil-proof; with oil present, follow the manufacturer's time-use instructions. The numbers 95, 99, and 100 are minimum filter efficiencies of 95%, 99%, and 99.97% (42 CFR 84.170(a)(3)).
Do I still need a fit test if I upgrade to a P100?
Yes, if it's a tight-fitting respirator. The filter class has nothing to do with the seal. A tight-fitting half-mask or full facepiece — whatever filter it carries — still needs a medical evaluation, a fit test, and a user seal check under 29 CFR 1910.134(e), (f), and (g)(1)(iii). Only loose-fitting hoods and helmets skip the fit test, because they form no seal.
How many NIOSH filter classes are there?
Nine: N95, N99, N100, R95, R99, R100, P95, P99, and P100 — three oil-resistance series (N, R, P) times three efficiency levels (95, 99, 100), certified by NIOSH under 42 CFR Part 84.
OSHA figures and citations here come from our regulatory source-of-truth modules, last checked against the eCFR, OSHA.gov, and the Federal Register on October 5, 2026. Last reviewed October 6, 2026.
About This Article
Published by: HazComFast
Published: July 21, 2026
Last Updated: October 6, 2026
This content is for informational purposes only and does not constitute legal advice.