In general, you're not supposed to wear a beard with a respirator mask (N95, P100, etc), at least not in a way where you have facial hair under the seal:

But how much worse is the fit? A P100 with a beard is going to filter less well than a P100 without a beard, but does it do as well as an N95? Or is it hopelessly compromised? And is stubble as bad as a full beard? Does length matter? In an emergency where I really need my mask for protection should I shave?

Bottom line up front: with my rough DIY test setup I got 80% filtration with a long beard, 92% with a short one, and 99.7% with stubble. But see my note at the end about a weird effect I saw.

What was my test setup? The goal is to measure the difference between filtered and unfiltered air. I have a Temtop M2000 meter, which I'd bought to measure air quality for testing my ceiling fan air purifier idea, so I can use that as a sensor. Burning matches is still a fine way of putting enough smoke in the air that I can see differences. And then I can measure the effect of the mask by measuring at my 3M HF-802SD elastomeric respirator's exhale valve, with the meter in a bag, and seeing how smoky it is (pm2.5). With the filter cartridges inserted I can measure the effect of the mask, while with them removed I'm measuring essentially no filtration. The ratio of particles with vs without the filters is my filtration efficacy.

This isn't perfect:

• My lungs or the the non-filter parts of the mask might be doing some filtration.

• My seal with the bag might not be 100%.

• The bag needs to be unsealed to let air out, or it will burst. But maybe this also lets a little smoky air in.

All of these are off in the same direction, however: my measurement is likely to be a lower bound in filtration efficacy.

To actually build this I made a small hole in a gallon ziplock, and attached it around the vent of my elastomeric respirator. The first time I did this I used packing tape, which didn't work very well and kept coming loose:

I did it again with duct tape:

I made sure to get a good seal all the way around:

Then I realized I wanted to test this with a pretty long beard to start, so put this on hold for a month while growing out my beard. I got to maybe 1 1/4":

Here's it combed to show the full length, though for all these tests I had it flat against my face.

I put on the mask, put the meter in the bag, and the bag started to inflate:

The mask has a fit test button, which blocks the inhalation valve: if you can still breathe in with it pressed you know air is making its way under the seal. I didn't feel much air coming in, but I did feel some.

I took a series of measurements, alternating filter status. Qualitatively, each time I put the filters in/out it immediately changed whether I could smell smoke.

Filters?	pm2.5	pm10
No	328.5	509.9
Yes	51.5	76.5
No	264.0	400.3
Yes	49.7	74.4
No	199.2	302.5

Since I expected the level of smoke in the room to slowly drop over time, doing interleaved measurements was quite important.

With a full beard, it looks like the mask cut pm2.5 from 263.9 to 50.6 (-79%), and pm10 from 404.2 to 75.5 (-81%).

Then I trimmed my beard with a bead trimmer set to 3/8":

This is where I normally trim by beard to:

I tried combing it out, but at this length it doesn't do anything:

I made more smoke and repeated the measurements:

Filters?	pm2.5	pm10
No	349.1	555.3
Yes	22.9	34.6
No	338.6	539.3
Yes	25.1	37.9
No	264.4	418.5

This time the filters brought pm2.5 from 317.4 to 24.0 (-92%) and pm10 from 504.4 to 36.3 (-93%).

Then I used the shaver with no guard to remove as much of my beard as I could, getting down to maybe 1/32" stubble:

And from the front:

More measurements:

Filters?	pm2.5	pm10
No	436.2	713.2
Yes	1.0	1.4
No	307.0	486.6
Yes	0.8	1.0
No	261.1	417.4

The filters now took pm2.5 from 334.8 to 0.9 (-99.7%), and pm10 from 539.1 to 1.2 (-99.8%). This is great!

Additionally, when I tried the fit test button I now couldn't breathe in at all, which actually felt a bit terrifying.

Overall I'm pretty happy with these results, except for one wrinkle: the meter reliably read a higher number when measuring my exhalant than it did in the ambient air. How could that be?

I tried a few more tests:

• Putting the meter in the bag and not blowing in: same as ambient, then slowly decreasing (probably particles settling?). It never plateaued, just kept going down slowly.

• Breathing directly into the bag with no mask: elevated from ambient

• Using an air mattress pump to inflate the bag: elevated from ambient

• Repeating my test in a room with minimal smoke: probably same results but harder to tell because the numbers were small.

Here are the numbers from what I think is the clearest test, using the pump to inflate the bag:

Status	pm2.5	pm10
Ambient	156.4	235.1
In bag, after waiting and just before pump	37.0	54.9
With pump, after plateauing	219.6	327.5
Ambient	117.5	177.7

I don't know where this is coming from, but possibly it's due to pressure? I suspect that whatever the effect is it's a scalar effect, and so is compatible with interpreting filtration ratios, but I don't know for sure.

This was also the first time my kids had seen me without a beard: