I've been busy traveling (as safely as I can) to and from my beta-coronavirus bioaerosol disinfection research work in Colorado since November. I've been paying a lot of attention to the people around me as I move about the world, and I've decided it's time to update my earlier recommendations on masks and the pandemic along with an important discussion of droplet vs. aerosol transmission.
Broadly, the idea behind general population masking (what CDC/WHO have kind of told us, but not very well, and based on what I’m seeing out in the world with lots of gaps around poorly fitting cloth and med/surg masks) is to prevent droplet transmission from the wearer, by catching the larger droplets. Some protection is afforded the wearer from others, but this is primarily from droplet transmission. Eye protection can also help, especially in a droplet transmission scenario; I wear a pair of tight-fitting safety glasses when I’m out in the world. (https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31142-9/fulltext).
Smaller particulate matter (this smaller PM is commonly called aerosols) can turn corners the larger droplets can't (this is actually the basis for some size selecting bioaerosol capture devices, change the speed and angle of the flow of air, from slow big turns, to fast sharp turns, to selectively capture increasingly smaller size fractions, see "cascade impactor"). Effectively, community masking is the equivalent of and enhances the protection of distancing; they both reduce the spread of large droplets (large droplets follow a ballistic trajectory and fall to the ground/surfaces within about 1m). This is considered the primary means of transmission of this virus under most circumstances, and assuming no direct contact.
If you're in an enclosed space, especially with poor ventilation, with others for extended periods (i.e. classrooms, bars, restaurants, homes), aerosol transmission can come into play. Here, masks with gaps do little to prevent spread by the smaller aerosols or to protect the wearer because most of the air you inhale and exhale will follow the path of least resistance, through gaps around the mask, and aerosols can round those corners. Maximize ventilation/clean air exchange (open windows, use high clean air delivery rate (CADR) standalone HEPA filtration) and minimize unfiltered air recirculation to reduce aerosol transmission for everyone in the room. ASHRAE has some good guidance on this. (https://www.ashrae.org/technical-resources/reopening-of-schools-and-universities) Aim for hospital levels of clean air exchange; you may not get there, but it’s the goal. Try to keep the relative humidity (RH) around 40-55%. This is the efflorescence RH around which the virus is most readily inactivated by environmental conditions. Higher and lower RH put less environmental stress on the virus, so it survives better as an aerosol.
Treat a mask like it works, that it catches and holds the virus, and because of the moisture from breathing, conditions aren't favorable to inactivate the virus in/on the mask well. Moisture can make it mobile, so touching your mask means you can now spread by touch/contact. Wash/sanitize hands before and after touching a mask. For similar reasons, your mask should sit away from your mouth and nose. (This will also help you speak more clearly.) Fit is super important. An N95 with 0.5sqcm of gap operates like a poor fitting cloth mask (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7185834/ see Table 1). No air should move around the edges of a well fitted mask. Proper fit, because the air can't move around the edges of a mask, can mean that it can become slightly harder to breathe, especially with multilayer cloth masks. N95 material should have less of this issue. Larger surface area helps here.
Learn to do a fit test if you can (https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.134AppA). Even just an understanding of the why and how of a proper fit test will help. Definitely follow the directions for a user seal check on whatever mask you're using (example on page 3 https://multimedia.3m.com/mws/media/92131O/3m-8000-series-n95-particulate-respirator-user-instructions.pdf). Find an N95 that fits your face. I tried 6 kinds before I found one that worked well for my face.
Non-“medical grade” N95s (by which I mean they aren’t certified for liquid penetration per ASTM https://www.astm.org/Standards/F1862.htm ) are available for $1.50-$3 each. They can be reused several times if they're still clean. Keep a worn mask in its own paper bag for a week or so, and it should be OK. A clothes dryer on medium or high heat will speed the virus inactivation (here are some other methods, but I would stay away from UV, it only inactivates what the light can get at, and it will degrade the polymer material of an N95 mask over time https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7153525/). Use a reputable supplier, there are poor quality counterfeit masks out there, especially with KN95s. Check for NIOSH certification (https://wwwn.cdc.gov/niosh-cel/ ). Here's a good source. https://www.grainger.com/category/safety/respiratory-protection/disposable-respirators-masks/general-purpose-disposable-respirators-dust-masks?attrs=NIOSH+Rating%7CN95&filters=attrs
Head straps will hold the mask in place better than ear loops, so most actual N95's will not have ear loops. This is a big failure in the design of a lot of the KN95s I'm seeing out there, and probably the cause of the complaints and my observations of these masks falling down all the time, requiring touching the mask to adjust it, and the requisite hand cleaning/sanitization before and after touching a mask. You also see this in the data around effectiveness of med/surg masks with ear loops vs head straps. (https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2769443)
As for vents on N95's. I personally think the CDC (and airlines, etc) got this wrong. They will work as well or better at preventing spread from you (droplets can't round corners (ie the flap of the vent), so they're as good or better than a cloth or med/surg mask for transmission), and they afford the wearer PPE level protection. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7467698/ See Figure 3, vented N95's are in the middle of the cloth mask types for droplets released when talking. The study definitely has limitations, but it's better than the NIST video of a sharp cough out of a vented N95. (https://www.nist.gov/news-events/news/2020/11/new-airflow-videos-show-why-masks-exhalation-valves-do-not-slow-spread) Their claim vented N95s won't slow the spread is not a valid conclusion based on available data and real-world scenarios. Who's coughing in public these days anyway? How much of the time are you coughing vs talking vs just breathing normally? If you need to, cough into the crook of your elbow and cover the vent; it will be better than trying to cover the gaps around a poorly fitted cloth mask. As for the perception of wearing one in public given what the CDC has said, well, that's a judgement call on your part. They are inexpensive and very easy to source (see the Grainger link above, they're below the non-vented selection). Get one on everyone (properly fitted, of course), and you solve the problem.
The news is just starting to, and I expect will continue to, push a move away from cloth masks toward PPE. (e.g. https://www.washingtonpost.com/opinions/2021/01/26/n95-masks-safest-next-best-options/) I think this is good.
As my father has been saying lately, "Don't be the last casualty in this war."
Stay safe and feel free to reach out with any questions.