Reverse valve masking

I was exposed to Covid recently, so by University rules I need to mask for a while. I don't particularly love masking at the gym, but I found a nice solution. My go-to mask for physical activity during the pandemic was the Trend Air Stealth N100 respirator, with the valve replaced by a 3D-printed blocker. But now I don't need to protect myself, just others. So I simply put the valve back in, but in reverse, so I get clear air intake but my exhalations go through the N100 filters. The respirator was already pretty breathable, but now it's even better, though it still looks super-weird and I need to remember not to use the respirator with this modification when I actually need protection, e.g., when doing woodworking.

Personal escape?

I'm now fully vaccinated, so through the wonders of modern medicine I'm deeming myself to have likely escaped COVID-19, while sadly aware that so much of the world is still struggling (I pray that vaccine patent waivers will go through and help).

I've been so very fortunate, unlike so many. Starting with August, the pandemic had little negative personal impact. On the contrary, being masked and distanced made me feel more comfortable around other people, and having conference and lecture travel be replaced with online events was wonderful. It would be great, for convenience, cost and environmental reasons, if conferences continued online indefinitely, hopefully with technical progress on getting informal interactions working better. (My experience is that the formal discussions at conferences are intellectually every bit as good when online, at least when the same people are in attendance.)

Since April 2020, I had exactly four occasions where I was within two meters of someone not from my household for 15 minutes or more in a 24-hour period. Two of them were dental visits. And two were Uber rides between home and the car repair shop. For the two Uber rides, I wore my "industrial-looking" elastomeric 3M P100 respirator (with an added exhalation filter), so I was quite safe--alas, it wasn't practical to do that for the dental visits.

For church and grocery stores, I usually used the 3M respirator. For the gym, I have a Trend woodworking N100 respirator (eventually modified to remove valves and block the exhale port). In both cases, I was quite safe and so were others around me.

Apart from one lecture moved online due to snow and one online due to a false COVID alarm (on the basis of two negative tests, I eventually concluded it was just a bad cold), I taught in person in both fall and spring. For teaching, I used cheap but heavily modified five-layer KN95 masks, because I was much more audible through them than through cloth, not to mention elastomeric respirators. Of course, I usually had some students Zooming in, often for COVID-related reasons but sometimes probably for more minor reasons.

I am not quite sure how I will modify my protocols now that I am fully vaccinated. My 3M and Trend elastomeric masks are very breathable, and keep the filter material away from my sweaty face, so I may end up still using the Trend mask for the gym. I may replace its N100 filters with 3D printed filters based on surgical mask material, though. I have considered continuing social distancing for the rest of my life in order to reduce the transmission of all respiratory diseases. Neither the flu nor even the common cold are enjoyable, and my personal utilities are such that if social distancing were to prevent the flu, it would be worth it, since given my white collar occupation and given that I live in a smallish town, it's only a slight inconvenience to distance myself under most circumstances, and it's intrinsically pleasant to have more personal space. However, since other people in my household aren't going to distance themselves from strangers once the pandemic is over, I suspect that my personal distancing would not do much to keep me from getting respiratory diseases at that point. I may make an effort to try for at least a meter of distance in most circumstances when interacting with people outside the household--but that is anyway within the range of North American proxemic zones.

Another KN95 mask mod

As an experiment, I cut the ear loops of one of my KN95 masks, and then made them join very snugly behind the head with rubber bands. I found that both comfort and fit improved greatly: I tested this while rock climbing. I expect this might work for surgical-style masks as well.

Important note: If you do this, strengthen the ear loop joints with Shoe Goo or some other goopy adhesive first, or they will likely not withstand the added pressure.

An optional improvement I made later is to add a clasp to the lower strap to make it easier to put on despite the loop being tight. Here are my 3D printing files. If you don't have a 3D printer, you could probably make a clasp out of a paperclip.

This is a lot of work for a disposable mask. But I reuse them.

Making KN95 masks better

I have found (using a small single blind test) that disposable masks have better audio quality for teaching: cloth muffles the voice, especially I think an already somewhat muddy male voice like mine. Surgical-style masks have a lot of leaks around the edges, so I went with cheap ebay KN95 masks.

I reuse them, "disinfected" by airing for seven days (as recommended for N95 masks by the N95's inventor), leaving them hanging on a little wooden rack with nails in it. (I had some fun with my CNC router on it as you can see.)

The cheap KN95s have some air leaking around the edges and top, and occasionally I've had an earloop break off. So, here are three mods I've made. You can do the first two without special equipment, but the third requires a 3D printer. (But if you're someone in my social circle at Baylor, I could 3D print some for you.)

First: add a bit of Shoe Goo under, around and over where the earloops meet the mask. This seems to greatly increase the strength of the connection. No broken off earloops since.

Second: I added some rubber bands joining the earloops. The main point was to make the fit more snug, reducing air leaking. Sadly, it puts more pressure on the ears. (There is probably a sweet spot in rubber band length where it reduces pressure on the ears, but I'm putting safety over comfort.)

Third: I replaced the flimsy metal nosepiece with a hefty 3D printed one. It took me five prototypes until I got the size and shape right, but then it worked great in teaching. It's 2mm thick, printed in PLA, and glued on with Shoe Goo. (The metal strips came off very easily from at least one of the brands of cheap KN95s.) No more fiddling with fitting the nosepiece, and no more feeling of air going up and out along the nose bridge, so I expect it increased the protection for others from exhalation. I don't normally need to wear glasses with a mask, but I tested with my sunglasses while walking home from class and found no fogging. I still fiddle with and adjust the mask, but a nice bonus is that I mainly need to touch the plastic strip, which is probably cleaner than the filtering surface.

I don't know that the strip increased the protection for me as significantly, because the KN95s already would seal around the face when breathing in. (There have been a lot of claims made that masks protect others more than they protect the wearer. I am somewhat skeptical of this in the case of KN95s and well-fitted cloth masks, because my experience is that when you breathe in, fitted masks pull to the face and seal much more tightly than when breathing out.)

My 3D printing files are here. Unless I have an identical twin I don't know about, you'll need to edit the OpenSCAD files and tweak the Bezier parameters to make them work for you. Mine I did mainly by trial and error with five prototypes, but when I made one for my son, I had him press a wire around the bridge of his nose, and then scanned the wire along with a ruler for size, and traced Beziers over the wire (if you're in my Baylor social circle, I can do this for you, on the basis of a good photo of a bent wire and a ruler or other calibrating object).

Testing masks for teaching

I had three family members turn away from me while I read two prepared philosophical sentences, using three different masks and no mask, and they ranked each reading for comprehensibility. The scale is:

1. Incomprehensible
2. Somewhat comprehensible
3. Mostly comprehensible
4. Comprehensible
5. Completely clear

Here are the results:

• No mask: 5.0
• Cheap ebay "KN95": 4.3
• Sonovia cloth antiviral mask (modified with nose wire): 3.7
• Surgical mask: 4.7

The ordering between these also matches how I sounded to myself. Since the surgical masks provide very little in the way of protection--I can feel lots of air leaking around the edges--I think I will teach in the "KN95". Maybe I will try to tighten the straps on it for better fit.

For pictures and further descriptions of the masks, see my mask collection.

I would love to hear comments suggesting other options. Those of us who are not in a high risk category are expected to be teaching in person in the fall--with students and faculty all wearing masks, and with social distancing apparently less than 6 feet--and if anyone has ideas that balance comprehensibility with protection, I would love to hear them. Unfortunately, I don't project well normally when I speak.

My mask collection

Just as different pairs of shoes are useful for different occasions, different masks are useful for different occasions. Here's my collection.

Category 1: Likely to be significantly protective

1.1. 3M 6300 half-face mask with 2091 P100 filters

Summary: Should be extremely protective for inhalation. The fit of the soft silicone and the adjustable head-straps is excellent. It is easy to breathe through thanks to the large surface area of the filters. And while the original mask has an exhalation valve, and hence doesn't protect others much, I added a 3D printed filter over the exhalation valve, loaded with surgical mask material. As a result, the mask is highly protective of the wearer and probably more protective of others than many other masks, due to the much better fit.

My use cases: Church and shopping.

Down-sides: (1) thanks to the vibrating inhalation and exhalation valves and the thick silicone, I am barely audible through this mask, so I avoid wearing it in situations where I need to communicate with someone; (2) the mask is bulky, which makes it less good for sports; (3) the filters are expensive; (4) can leak around edges on exhalation when breathing extremely hard due to strenuous physical activity.

1.2. Sonovia cloth mask with antiviral and antibacterial coating

Summary: These are the most expensive cloth masks I've seen ($53-69 each, depending on quantity). I learned about these from my mom. My first thought was that they are a scam, but as far as I can tell (but then, this is not my field) they are made by a legitimate Israeli company, backed by serious scientists (including a Nobel prize winner), and their antiviral and antibacterial copper coating apparently really does work, and lasts many cycles of washing. I got a pack of three of these (at the time, they didn't sell them individually) so my daughter could attend her (outdoor, socially distanced) high school graduation.

Unfortunately, out of the box they don't come with a nose wire, which makes the fit poor. However, it was easy to cut a seam, insert a wire and sew it closed, and now the fit is much better. They have better fit for inhalation than exhalation in my experience: if I don't adjust the straps just right, I can feel exhaled air escaping at the cheeks, but they close tight around the nose and mouth for inhalation, providing what feels like a solid seal. A nice bonus of the antiviral coating is that I do not worry about the surface of the mask being contaminated.

My use cases: When I want significant protection but also want to be able to talk, or when the 3M mask is too bulky for convenience.

Disadvantages: More difficult to breathe through than some other options, especially if drenched with sweat. When our climbing facility reopened, I climbed in this mask the first day, but when it got wet while I was climbing laps, I felt like I was drowning for lack of air.

Category 2: Likely to be moderately protective

2.1. Home-made two-layer fitted mask often paired with surgical mask

I made this from one layer of T-shirt cotton and one layer of microfiber cleaning cloth, after looking at the sizes of holes in various fabrics under a microscope. I used this pattern, enhanced with a nose wire. The fit feels superb: I don't feel the tell-tale feel of air rushing past my skin around the edges on either inhalation or exhalation. Back when I didn't have the better masks above, when I went to high risk destinations like the grocery store or church, I would sometimes put a surgical mask under it, thereby increasing the number of layers of filtration while at the same time pressing the surgical mask to my face and hence improving the surgical mask's rather terrible fit.

2.2. Ebay "KN95"

These were so cheap ($10 for ten, and I've since gotten a similar pack of ten for $5) that notwithstanding apparent Chinese certifications, I wouldn't be surprised if they were fake KN95s (and that makes me feel good about not taking protective equipment away from medical staff). However, even if they don't reach the filtration level of a real (K)N95, they fit acceptably, and I suspect--admittedly, without testing--they provide better filtration than home-made cloth options. When breathing very hard--namely, during strenuous physical activity, I can feel air rushing out by the edges of the mask at inhalation, but the mask seems to seal to the face on inhalation. Moreover, I can breathe through it even when very sweaty. I reuse these, following the advice of the N95 inventor to just have multiple masks labeled with days of the week and let them self-disinfect over the period of a week.

My use case: Indoor rock climbing.

Disadvantages:(1) suspiciously low price; (2) poor fit on exhalation

Category 3: Less likely to be moderately protective

3.1. "Surgical" mask

Summary: We have these hanging about in various places and they are convenient to toss on quickly. I can feel they leak around the edges. They are so cheap ($10 for 50 on ebay, if memory serves me) that I doubt they are serious medical protective equipment, so I feel OK about using them.

My use cases: mainly for brief contact with someone (e.g., when picking up a drive-through order in the car)

Disadvantages: Leaks around edges.

3.2. Reebok athletic mask

Summary: These are easy to breathe through, but I have no idea how much protection they provide. They are great for athletic endeavors when one is so far from people outside one's household (e.g., when playing badminton with a family member on an indoor court) that the only reason for a mask is to satisfy the rules rather than protection.

My use cases: Indoor racquet sports when alone on a court with my son. I use a better mask when closer to staff while checking into the facility.

Disadvantages: Leaks at nose bridge causing fogging in sports that require goggles (e.g., racquetball).

Category 4: Not protective

4.1. Home-made single layer T-shirt mask

Summary: I sewed these quickly from an old T-shirt and a piece of wire. Sometimes one needs a mask to satisfy rules but where realistically protection of self or others is not an issue. For instance, an outdoor facility where one can easily distance oneself from others, or while alone on an indoor court with a family member.

Use cases: I toss these in our tennis bag in case I need to go to the bathroom in my department's building after hours when I am unlikely to meet another human in the building, but the university still requires face covering. Also, outdoors at our marina.

Disadvantages: Minimal to non-existent protection of self or others.

4.2. Bandana

Summary: This provides very little protection indeed, since I can feel most of the air coming in and out around the bottom, but it does not leak much around the top and hence works well with racquetball goggles.

Use case: When I play racquetball alone in a court with a family member, the main reason to wear the face covering is to satisfy the facility rules. I am careful to wear a better mask in other areas of the building where there is closer proximity to others, such as when signing in.

Disadvantages: Minimal to non-existent protection of self and others.