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1/ Confused about the statement that N95 use might be risky with pregnant women? I was too, so I started looking into the evidence.
2/ First, here's a recent review that concludes there is no significant risk for pregnant women using an N95, at least in the short term (few hours). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7416223/
3/ So that mostly answers the question, at least for short durations. But this paper ( https://aricjournal.biomedcentral.com/articles/10.1186/s13756-020-00779-6) specifically mentions a risk for pregnant women.
4/ Conly et al cite this paper (or at least they meant to, I think there was a typeset error in there) https://aricjournal.biomedcentral.com/articles/10.1186/s13756-015-0086-z as evidence supporting there being a risk to N95 use with pregnant women.
5/ So, let's dig into this one and see what's going on. https://aricjournal.biomedcentral.com/articles/10.1186/s13756-015-0086-z. All in all it's fine, relatively few subjects (n = 28) but whatever. But I have a major issue with how they ran their tests - it doesn't reflect real world use of N95s at all.
6/ They use a rigid Hans-Rudolph mask with the outlet covered by a snipped out circle of N95 mask material to simulate an N95:
7/ What's the problem with this? Well, with a normal N95 you breathe across the entire surface of the mask (or most of it, anyway), something that has an area of ~150 cm^2. The cross sectional area of this "N95-equivalent" is much smaller - lets call it 30 cm^2.
8/ Let's say you're generating an inhalation flow rate of 85 L/min (something you'd generate during moderate exertion). With a regular N95, the velocity of air going through the mask is ~ 0.001417 m^3/s / 0.015 m^2 = 0.094 m/s, or about 10 cm/s.
9/ That matches what Rengasamy et al summarize as the typical velocity generated during the NIOSH test meant to provide a "worst case scenario" for particle filtration. Pass that test, and you're golden. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7157953/
10/ What happens if you cut down the area you're breathing through by a factor of 5? If you keep breathing at the same flow rate, the velocity increases by a factor of 5 due to conservation of mass. So 10 cm/s becomes 50 cm/s.
11/ Pressure drop across a mask/respirator relates to a few things including the "resistance" of the material and the velocity of air flowing through it. If you have a material with a given resistance and you increase the velocity of air flowing through it by a factor of 5...
12/ You'll get a massive increase in the pressure drop. If airflow stays laminar, the pressure drop is linearly proportional to velocity. Roughly speaking, you've made the mask 5 times harder to breathe through by cutting down it's surface area by a factor of 5.
13/ What's the problem? Well, N95s are designed to never exceed a particular pressure drop during use, 343 Pa during inhalation per NIOSH 42 CFR Part 84 https://www.cdc.gov/niosh/npptl/topics/respirators/pt84abs2.html - because the designers know that when you make something too hard to breathe through, you have issues.
14/ What this experiment is doing is taking an N95, making it stupidly hard to breathe through, then saying that it looks like there are issues with N95s in pregnant women when they wear an "N95" while exercising.
15/ The authors jeopardized their conclusions via the experimental design. Probably unintentionally, because this is a pretty honest mistake to make if you don't know the physics that govern filtration of aerosols. Unfortunately, this means their results are not generalizable.
16,f/ You have to be careful with how you run these sorts of studies so that you don't screw up your measurements before you take them. Perhaps adding more basic scientists into the mix could help reduce wasted effort on studies that have major design/methodological flaws.
