Let’s talk droplets.
Everyone knows that coughs and sneezes are “dirty.” Not only do they indicate that someone might be unwell, but they often involve a visible spray of mucus and saliva. As it turns out, most of those large and visible droplets land quickly nearby, and are also too large for a nearby person to inhale; they require intermediate step—a handshake or doorknob, then touching of face—to get someone else sick. According to https://bmcpulmmed.biomedcentral.com/articles/10.1186/1471-2466-12-1, invisible droplets less than 2.5 microns may pose the greater threat, as they can both remain in the air for extended periods and also be inhaled deeply into lungs.
Take a look at the coughing in this video, starting at about 4:20 to 5:20. The smaller red “micro-droplets” form a fog near the cougher that persists for a surprisingly long time.
Such a “viral fog” may have been how one Chinese coronavirus patient managed to infect nine other people on a four-hour bus ride, including people who sat several rows away and one person who got on the bus after the patient got off. (https://www.scmp.com/news/china/science/article/3074351/coronavirus-can-travel-twice-far-official-safe-distance-and-stay)
But it doesn’t stop with coughing. People also expel large quantities of respiratory droplets when they talk and sing. According to one study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2843952/), respiratory droplet count is the same from a person counting out loud from 1 to 100 as a person coughing 20 times, or ratio of about two seconds of talking to one cough. Another (https://www.nature.com/articles/s41598-019-38808-z) reported that saying “aah” for only thirty seconds releases as many droplets as a full minute of continuous coughing, and that singing causes SIX TIMES the number of droplets as normal speech. Talking loudly and laughing may also cause larger quantities of droplets, and a reader noted that the University of Hong Kong study cited above suggests that respiratory droplets may be greater when people eat and talk at the same time. Some people are “speech supermitters,” releasing ten times more droplets than average (might such a person be at the heart of every viral supercluster?).
Take a look at the same video, this time around 2:50 to 3:30. Speech droplets are generally smaller than cough droplets, and therefore more prone to floating around in the air and being directly inhaled by others.
Seeing this, it is obvious that places where crowds gather indoors to talk or sing (churches, choirs, conferences, and parties, which have all resulted in COVID-19 superclusters) are the most dangerous when it comes to spreading respiratory disease (unless perhaps you have a silent church service without hymns or call and response). Other places where people gather indoors, such as public transit, workplaces, and stores, also pose great risk. This risk can be mitigated by mandatory public mask use, which filters the respiratory droplets from getting into the air in the first place. Since most virus-containing respiratory droplets are much larger than the sub-micron particles for which N95s are rated, many materials will likely do a decent job of keeping them out of the air.
Given that eating and talking may pose special risk, and masks aren’t really compatible with eating, restaurants may sadly require a longer period of closure during this pandemic, and maybe meetings should not include meals.
Unless they’re outdoors, maybe. Take a look at the video, again, from 5:50 to 6:10, and see how quickly the droplets disperse when a window is opened.
We should try for fresh air whenever people are gathered—a special concern during winter and the height of summer—and be less afraid of outdoor activities if they are not crowded (in which case masks can be used to reduce risk) and there isn’t risk of transmission by physical contact with contaminated surfaces (although of course hand washing is always a good idea). One article (https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-019-3707-y) explains that airborne droplets can be produced by the desiccation of larger droplets, which would happen more quickly in drier environments, and so perhaps greater precautions are necessary when it is less humid.
Thinking of respiratory illnesses through the lens of respiratory droplets may help us return to some version of normal life. This is all common sense, and doesn’t even require knowledge of how the virus works. We know that it travels in respiratory droplets, and so we just need to consider how to stop one person’s respiratory droplets from getting into another person. Mandatory public mask use and fresh air are, along with hand washing and some amount of social distancing, likely our two best and cheapest tools.
