Covid, Aerosols, Indoor Air
Sneezing, coughing, singing, talking, breathing. Which spreads Covid-19? Probably all of them. Sneezing and coughing should be obvious to everyone. Official guidance about "covering your cough" has been consistent since the beginning of the pandemic. It seems obvious (at least to most people) that a mask helps reduce the droplet ejection from coughing and sneezing, but a mask doesn't catch it all.
Beyond coughing and sneezing the scientific data is less clear, but it's gradually coming into focus. Proof does not come easily. In contrast to the scientific method of controlled experimentation taught in high school, it is not ethical to do the obvious experiments with Covid-19 using human subjects. The potential consequences, death or lifelong disability, are too dire for the participants. Furthermore, any direct experiment would need to consider a lot of variables. For example, there are likely individual differences among infected people in how much virus they exhale--and certainly there are differences in immune resistance among those exposed. On account of those variations a definitive experiment would require hundreds of participants. Not practical or ethical.
We are left with inductive reasoning. Scientists study respiratory droplets and aerosols, the distances over which they spread, and the likely virus load (dose) they contain. They link that data with the results of what we might call "natural experiments," the careful study and reconstruction of events recognized to have spread the disease (that's the realm of epidemiology and contact tracing). All of that data is pooled, discussed, and hashed out to produce the best recommendations possible based on what we know at the time. But here's the thing: People like easy-to-remember absolutes while the best science can offer is an assessment of the probability of transmission.
Stand cross-wise to the sun's rays. Talk for a while. You may notice the occasional tiny speck of light traveling away from your mouth. Those are droplets. Everyone produces some of them. Most of them land on the ground within 3 feet (1m) and an even higher percentage hit the ground within 6 feet. "Droplets" include particles down to 5 micrometers (5 one thousandths of a millimeter. A 5 micron droplet is not visible.) Hence the recommendation we've heard in this country since the beginning: 6 feet social distancing. Six feet does not ensure perfect protection, but a compromise between risk and workability. As you can see from the two demonstrations below, masks help against droplet spread--a lot.
So for droplets, six feet is good and a mask definitely helps, but what about aerosols, specifically, bio-aerosols, fine particles emitted when someone breathes, particles that remain suspended in the air? What's the risk and what might this risk mean for indoor gatherings?
The Skagit Chorale incident in March was an early cautionary tale, a "natural experiment", we all should heed. Sixty-one singers practiced indoors (with no masks) for 2.5 hours, Fifty-three participants tested positive or were deemed positive by symptoms for Covid-19 within a couple of weeks, three were hospitalized--and two died. There was some moving around during the practice and distancing was not consistent (both facts cloud the conclusions a bit), but the attack rate strongly implies that forceful singing in an indoor environment produces aerosols that travel or linger in the air and are infectious beyond six feet. (Think singing hymns in church.)
For an excellent discussion of this complexity I recommend an article from National Public Radio aired way back on April 3, entitled "Scientists Probe How Coronavirus Might Travel Through The Air."
In February and March in the U.S., the early part of the Covid-19 pandemic, mask wear was not encouraged for the general public. We later learned this was thanks to the Trump administration's denial of disease and its flat-footedness in encouraging a ramp up of mask production. There was fear that an early broad recommendation for mask wear would result in a run on masks that would leave hospital staff without protection in the face of intense exposure to the disease. (I have not read an estimate of how much spread occurred and how many people died as result of lack of an early mask wear recommendation, but in those days of e.) Mask wearing for the general public was first recommended by the Centers for Disease Control and Prevention (CDC) way back in early April. Leadership in encouraging this clear recommendation could have saved lives.
To anyone paying attention to the science it has been clear for months: Wear a mask! This a predominantly respiratory disease. It is primarily spread by respiration. Any mask is better than no mask.(Except those that have a bypass that exhausts your breath without filtration. Such masks don't protect others from you--a bypass subverts the primary purpose of a mask.)
Some people, most notably our science-denier in chief, are painfully rigid, congenitally unwilling to admit they were wrong. Mr. Trump can afford to not wear a mask while telling his followers that wearing one is optional. He, after all, has everyone who enters his presence at the White House tested for Covid-19. His daughter-in-law, who tested positive, is sent away for fear of infecting him. Meanwhile, he conducts rallies with hundreds or thousands of mask-less followers, united in their denial and exposure.
If we all wore masks indoors (when in contact with people outside our pod) and outdoors in crowds or anywhere outdoors where social distancing cannot be maintained (these are the current Washington State regulations), we might regain the control of viral spread. We might get back to something close to normal sooner. But, alas, we lack intelligent leadership at the top.
Being indoors with people from outside your "Covid pod" carries risk. Bars, crowded restaurants, dance venues, especially without masks, tend to spread the virus, probably by aerosols, probably well beyond 6 feet. Air conditioning systems, heating systems, and reduced circulation of clean outside air all likely contribute to the risk. Take advantage of nice outdoor weather. Socially distanced gatherings outdoors are likely the next safest thing to being isolated. As high or low temperatures tempt us to socialize indoors, staying healthy will be more of a challenge. Winter is going to be tough.
Keep to the high ground,
A few related thoughts and observations:
The use of masks in east and southeast asia is well accepted as a necessary public health measure at the first hint of respiratory disease spread. Mask use is at least part of the reason many countries in that region have been able to tamp down viral spread far better than we. We would do well to learn from the rest of world.
Indoor v. outdoor air:
Measles virus, said to be the most infectious of human pathogens, can remain infectious hanging in indoor air for up to two hours.
In the recent past people were more attuned to the risks of indoor airborne spread of disease, even if the details were unclear or even mistaken. The word malaria, for instance, simply means "bad air" (in Medieval Italian). The disease was associated with the bad air of swamps and marshes. It wasn't until the early 1900s that it became clear that a mosquito vector was to blame for transmission, not just "bad air,"--but the name stuck. One wonders how many people contracted malaria because they left their windows open to mosquitos trying to avoid the "bad air".
Perched on top of a cliff overlooking Spokane Valley from the north, the Royal Riblet Estate's Cliff House, built in 1925, nods to the cult of fresh air. Four bedrooms occupy four corners of the structure, allowing for cross ventilation while sleeping.
My parents (of the same vintage as the Cliff House) always left bedroom windows partly open for fresh air even in the depths of Wisconsin winters.
When I was in medical training in Boston in the late 1970s the remnants of the mechanisms used to open the ceiling louvers in several of the ground floor open wards of the old Peter Bent Brigham Hospital were still visible. Decades before these had been the wards in which patients suffering from tuberculosis were housed. Such wards were constructed in the belief (quite probably correct) that fresh air was good for TB sufferers (and perhaps that fresh air also reduced the spread of the disease). Tuberculosis, once endemic to European and U.S. populations, is now treated and fairly well controlled with antibiotics. However, it remains endemic in much of the world.