Are respirators fitted with HEPA or P-100 filtered cartridges safe for DIY work, possibly in the presence of asbestos in the attic and roof? I'm in California, USA, but this Minnesota (USA) Department of Health page states:

Respirators must be equipped with HEPA filtered cartridges (color coded purple) or an N-100, P-100 or R-100 NIOSH rating. These cartridges are specific for filtering out asbestos fibers.

These filter 99.97% of airborne particles that are 0.3 microns or larger in diameter. However, this Occupational Safety and Health Administration (OSHA) page states, regarding asbestos exposure and sample fiber counting using Phase Contrast Microscopy (PCM):

A further disadvantage of PCM is that the smallest visible fibers are about 0.2 um in diameter while the finest asbestos fibers may be as small as 0.02 um in diameter. For some exposures, substantially more fibers may be present than are actually counted.

A study, entitled Size and shape of airborne asbestos fibres in mines and mills, states regarding asbestos fiber sizes during industrial fiber processing:

The median true diameters of airborne crocidolite, amosite, and chrysotile fibres, as determined by transmission electron microscopy, were 0.07 μm (initial stage) to 0.09 μm (final stage), 0.20 μm (initial) to 0.26 μm (final), and 0.05 μm (initial) to 0.06 μm (final) respectively…

Many asbestos fibers are much smaller than 0.3 μm in diameter. Does this mean there's no safe respirator filter, whether HEPA, P100, or better, to protect against asbestos? Do better air-purifying respirator filters exist? Or, is using an atmosphere-supplying respirator the only way to be safe?

EDIT: piojo's answer shows the HEPA spec requires 99.97% filter efficiency for 0.3 μm diameter particles, not for 0.3 μm diameter "or larger" particles. His source describes filter efficiency as the aggregate efficiency of several mechanisms: sieving, inertial impaction, interception, and [Brownian] diffusion. The most penetrating particle size (MPPS) is where aggregate efficiency is weakest. It states 0.3 μm is the benchmark because it is near that size. So, 99.97% efficiency is near the MPPS efficiency. Its graph assumes ≥99.97% efficiency for all particles down to 0.01 μm, but I've found no source that corroborates this efficiency for sub-0.07 μm, which is well within the range of airborne asbestos diameters. Efficiency should approach 100% with increasing diameters beyond the MPPS, due to increasing effectiveness of interception, impaction, and sieving. While efficiency increases with diameters immediately smaller than the MPPS due to diffusion, efficiency can't keep increasing when below a certain diameter, because the filter must pass air without making it too hard to breathe—the efficiency will drop below MPPS efficiency at some small diameter. MPPS efficiency should be confined between infinity and this small diameter. I've found no source that identifies this limit.

Wikipedia says the MPPS is 0.21 μm, citing the research paper Particle Size for Greatest Penetration of HEPA Filters—and Their True Efficiency. It's dated 1982, so I don't know if the HEPA spec and filter composition have since changed. The paper doesn't identify any percentage efficiency at 0.3 μm required by 1982's HEPA spec. The paper states:

The penetration at 0.21 μm is calculated to be seven times greater than at the 0.3 μm used for testing HEPA filters.

Much of it discusses efficiency of a single fiber of the filter, so I don't know how whole-filter efficiency scales. I don't know if 1982's HEPA 0.3 μm efficiency spec is 99.97% nor if whole-filter efficiency scales the same as single-fiber efficiency, i.e. I don't know if modern whole-filter MPPS efficiency is:

99.97% - [7 * (100% - 99.97%)] = 99.76% ??

The paper collected data from other research, adopted some of their assumptions and made its own, and extrapolated to establish single-fiber efficiency curves. In many of these curves, the experimental data points do not fit well with the curves it's trying to theorize. For example, Figure 4's theoretical curve is significantly translated upwards (towards higher efficiency) from the empirical data points. It glosses over the reasoning with: "The conversion was made from protection factor, P.F., to single-fiber efficiency…" but never defines P.F. or mentions it again. Even if the curve weren't translated, its shape won't fit well with an actual best-fit curve through those points. It only has 6 empirical data points, the smallest particle diameter of which is 0.07 μm, which is around the median diameter of airborne crocidolite particles and larger than the median diameter of airborne chrysotile particles. Figure 3 has no experimental data for sub-0.1 μm diameters. Yet, both Figures jump to conclusions by extrapolating ill-fitting curves to 0.02 μm. These are the only efficiency vs. diameter data this paper has on HEPA filters. Most of the data this paper uses to determine HEPA filter's MPPS (called SOMP therein) comes from non-HEPA filters!

The wiki lists electrostatic attraction as a filtration mechanism, but lacks a citation. The paper doesn't mention this mechanism, but it states that filters increase in efficiency with use due to loading (i.e. getting filled with particles), up to a certain extent before the trapped particles slip through (page 13).

Another research paper, entitled Fiberglass Vs. Synthetic Air Filtration Media, states that loading reduces electrostatic filtration effectiveness due to decreasing the filter's exposed surface area and that aerosols neutralize electrostatic charges. I don't know how well electrostatically charged filters hold asbestos particles compared to other particles of the same diameter. How would aerosol fit-tests (e.g. nebulizing saccharin or Bitrex) test for electrostatic filtration? Would the tests decrease the filter's efficacy?

EDIT 2: The research paper Total Inward Leakage of Nanoparticles Through Filtering Facepiece Respirators shows a model-unspecified P100 filter on a full-face respirator that's perfectly sealed to the face (using silicone sealant) to have >99.97% efficiency for 8 different diameters of charge-neutralized particles in the 0.08–0.4 μm range (Figure 3). The graph isn't a best-fit curve. It connects points with straight lines. Because the MPPS isn't plotted, the graph doesn't show its dip in efficiency. The author clarifies by stating: "Particle penetration was several times higher for the particles in the MPPS than for 8 and 400 nm size ranges. P100 FFR does not show the MPPS at 40–50 nm range in the figure because of the low penetration value (<0.03%)." Because the author implies his test filter has >99.97% efficiency at the 0.04–0.05 μm MPPS and this MPPS is within his 0.08–0.4 μm test range, he implies the filter has >99.97% efficiency over the 0.08–0.4 μm range. The author doesn't specify the filter's model number. Is this performance representative of other filters? Does every P100 filter's per-HEPA-spec ≥99.97% efficiency at 0.3 μm guarantee ≥99.97% efficiency for the entire 0.08–0.4 μm range? Note that this paper appears to use a filter that greatly surpasses the HEPA spec, because the spec expects the MPPS to be near 0.3 μm, whereas this paper's is much smaller.

  • 1
    You've eviscerated what we know about HEPA safety (at least, what can be found in the internet's greatest friend, Wikipedia)--so I suggest researching this from a different angle. What kind of filters to asbestos removal workers use? What is the incidence of asbestos-related diseases among these workers?
    – piojo
    Commented Jul 19, 2017 at 6:40

2 Answers 2


The HEPA spec doesn't describe "0.3 microns or larger". It just states "0.3 microns". The explanation I've always read (for example, here) is that 0.3 microns is the most difficult to capture, so you can expect the filter to be more efficient with larger or smaller particles. Larger particles are captured mechanically, and smaller particles are captured electrostatically. I'm not sure what the lower limit for particle capture is, though--obviously these filters don't capture individual molecules, so there must be some point at which a particle is too small. I don't know what that would be.

Would I do it? Not without training and a professional fit test to prove that no air is getting in around the edges of the filter.

  • +1 for filter mechanisms & the spec's not stating the "or larger." I edited my question to include your source because, like much of HEPA literature, it's informative but dubious. Its graph's assumption that a HEPA filter has asymptotic 99.99+% efficiency at 0.01–0.2 μm is dubious. You're right about min. capture limit—wearer must breathe. Approaching that limit, efficiency must drop below that of the most penetrating particle size (MPPS). I found no empirical data for efficiency vs. sub-0.07 μm, which is in range of many asbestos particles. Is the filter sufficient for most/all of this range?
    – CodeBricks
    Commented Jul 19, 2017 at 3:38
  • 1
    @CodeBricks Good question. I found a web page that claims the bottom size when a particle stops acting like a particle and starts behaving like a gas (and isn't captured) is around 0.01 μm, but I wouldn't trust this without more than one source. inspiredliving.com/airpurifiers/hepa-filters.htm
    – piojo
    Commented Jul 19, 2017 at 6:32
  • 1
    Another manufacturer says their filters can filter down to 0.12 μm, which is too large for your purposes. This is more specific (and conservative) than the first claim, so it's a little safer to trust this. nilfiskcfm.com/filtration
    – piojo
    Commented Jul 19, 2017 at 6:33
  • academic.oup.com/annweh/article/55/3/253/150869/… shows a P100 filter (model # unspecified) efficiency >99.9% at 8 sizes in 0.008–0.4 μm. Its Fig. 3 uses line segments, not best-fit curve. Given: "…MPPS [is] in the 50 nm size range. Particle penetration was several times higher for the particles in the MPPS than for 8 and 400 nm size ranges", if several times means <10, then the lowest efficiency in range is 99.6+%. Is this representative of other P100 filters, i.e. does 99.97% efficiency at 0.3 μm guarantee 99.6+% efficiency at 0.008–0.4 μm?
    – CodeBricks
    Commented Jul 21, 2017 at 1:23
  • …This whole-respirator efficiency was achieved by sealing respirator to mannequin face w/ silicone sealant. They later poked two 2.41 mm holes in sealant, which reduced efficiency by >10%. How can someone ensure perfect seal under working conditions without damaging his face or respirator? Tape? Remain expressionless? Also, your last comment is about ULPA filters, but I haven't found any I can buy for respirators. I can't find any ULPA respirator filters from 3M or Moldex. Like HEPA, I don't know if ULPA spec's 99.999% efficiency at 0.12 μm guarantees any minimum efficiency at 0.008–0.4 μm.
    – CodeBricks
    Commented Jul 21, 2017 at 1:44

No respirator is a guaranteed protection, they are all rated by protection factor.

Most 1/2 face respirators are rated at 10. Protection factor 10 means it will protect you from 10x the Personal Exposure Limit. Supplied-air and self-contained breathing apparatus are typically rated much higher. You would need to use the highest available protection factor equipment while capturing an actual representative sample to determine proper minimum protective gear.

Performance of all respirators is dependant on proper fit and use. That is why, in the state of California, DIY asbestos abatement is illegal. You must hire a licensed abatement contractor.

And by the way, licensed abatement contractors generally use full-face P100 respirators along with disposable coveralls, and have robust containment and disposal equipment and procedures.

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