Wednesday, June 17, 2020

This is why we wear masks

We have a preponderance of evidence that Covid-19 is spread by aerosols and large droplets expelled through our mouths when we talk and through our mouth and nose when we sneeze or cough.  We also have plenty of evidence that (nearly) universal mask-wearing in public is extremely effective in Reducing transmission of SARS-CoV-2.  The linked article by Prof. Kim Prather et al is written extremely clearly in plain English and I highly recommend reading it.

I'm embedding two videos because they are helpful to understand social distancing and wearing masks.  But, it's important to keep in mind that they are showing hot air instead of droplet and aerosol motion, which does not spread as far.

Schlieren or shadowgraph imagery shows changes of thermal patterns in the air. We expel heated air, which will mix with ambient air and might waft upwards (hot air rises.)  Large droplets are heavier and will fall downwards. Aerosols might hang around a few minutes to hours, but eventually fall.

With these caveats, these images nevertheless demonstrate the effectiveness of wearing face masks that cover the nose and mouth.

First, this charming* video of NIST scientist, Matthew E. Staymates, working from home. Note how his breath stays much closer to his head when he wears a mask vs when he doesn't. This is why mask wearing, along with social distancing, dramatically reduces transmission risk.  Remember, this is showing hot air; virus-containing droplets will most likely by caught in any type of mask as long as it covers the mouth and nose.

I particularly like that he wears different styles of cloth masks sewn by NIST colleagues, who did socially-distanced handoffs outside his home.  All types of masks work as long as they are comfortable enough for you to actually wear them over both your nose and mouth.

This older video demonstrates how much air you exchange with others just in normal face-to-face conversation.  It starts with normal nose breathing, then mouth breathing, talking, coughing, conversation, etc.

I'm cautious and minimize my shopping trips.  When I go into stores, I use hand sanitizer, then put on my mask and then enter.  I don't touch the mask until I get back outside.  This protects me.  It protects you.

Give people without masks a very wide berth, 2 meters or more, and minimize your exposure to them.  If someone takes off their mask to talk in a store, run away.  They've contaminated everything within droplet distance.  If they talk loudly, that can be far greater than 2 m.   If a store allows unmasked people to enter, assume all surfaces are contaminated.  (This creates so much more work and risk for retail workers.)

If all staff and shoppers are wearing masks, I would not stress about breaking the 2 m bubble for a short time. Remember, it's also the viral load in a short amount of time that is dangerous.  If you get a tiny bit of viral load, and you have a healthy immune system, your body would destroy the viruses before they can get a toe-hold in your respiratory tract.

When I see people wear masks under their nose, I give them wider berth, but don't say anything to them about it.  They tried, but maybe they are too uncomfortable.  Or maybe they are conspiracy theorists who don't understand that oxygen and carbon dioxide pass through the masks just fine.  Either way, I don't want to engage with them.  A mask that covers their mouth would still contain droplets expelled by talking unless they sneeze or cough.

For those of us in low-risk settings (not health care workers or people who work around others all day) the best mask is the one you will actually wear.  If you try to make it too efficient at filtering, it can be difficult to breath through, or get soggy from your exhaled breath.  If you take off your mask when indoors (other than in your own home,) then you lost protection.

I walk and bike outdoors without a mask, but carry one in case it gets crowded.  I'm coming around to the idea that perfect is the enemy of good and I may sew lower-coverage masks that don't extend as far across my cheeks for outdoor exercise and gardening.  The best mask is the one that you will keep on.

* Maybe I'm biased because I got my PhD at a NIST lab.

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