The CDC now recognizes the coronavirus « can sometimes be spread by airborne transmission. »
Since the beginning of the pandemic, the World Health Organization has stressed that Covid-19 primarily spreads through very close personal contact. The virus-laden droplets exhaled from a sick person’s mouth and nose, the thinking goes, are heavy, and fall to the ground before they can get much farther than 6 feet. But as the pandemic has unfolded, contact tracing studies have shown this isn’t always the case. In China, an air conditioner pushed virus-laden air across three tables in a restaurant, infecting people seated at each one. Researchers reviewed video from the restaurant and saw many of these patrons were more than 6 feet apart from one another, suggesting that the virus traveled through the air. In Washington state, another study showed one person at a choir practice infected 52 of the other participants; it’s thought the singing could have led more virus to linger in the air. Carefully controlled laboratory studies are also suggesting that under the right indoor conditions, SARS-CoV-2, the virus that causes Covid-19, can float in the air and, to a certain degree, spread that way. In July, the WHO changed its language to recognize that fact. “Short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out,” the WHO now states. Previously, the organization maintained that airborne transmission was unlikely to occur outside the hospital setting, where some procedures can generate super-small particles that linger in the air longer than large respiratory drops. Then, on October 5, the CDC updated its guidance to say that “Covid-19 can sometimes be spread by airborne transmission,” explaining that “there is evidence that under certain conditions, people with COVID-19 seem to have infected others who were more than 6 feet away.” With recognition of this mode of transmission, scientists hope, communities can think more about the ventilation of indoor spaces and perhaps engineer solutions to make these spaces safer. The CDC had posted a similar update on September 20, which it quickly took down (the agency claimed the update had been posted in error). Overall, it’s been clear for quite some time that the 6 feet guidance was insufficient to explain all the facets of the risk in catching Covid-19. Overall, the public communications on the aerosol issue have been something of a mess. It turns out scientists have been debating whether respiratory diseases like the flu and coronavirus should be labeled “airborne” for a century. The “airborne” designation is more commonly applied to extremely contagious diseases, like measles. A lot of these terms are confusing (even to the scientists), and they don’t answer the question laypeople care about: Which air is safe to breathe during the Covid-19 pandemic, and which air is not safe? To answer that question, it’s helpful to understand two different scientific perspectives on the matter. One is: What physically happens when a sick person breathes, sneezes, or coughs in a room? The other is: What patterns have epidemiologists observed in the way people are exposed to the virus and get sick? Let’s start with the first. Big drops versus little drops There’s a relatively simple, if outdated, way of thinking about how respiratory diseases can spread. It starts like this: When you exhale, sneeze, or cough, you release a cloud of gas and liquid droplets. If those droplets are relatively big, they’re heavy and fall to the ground before evaporating, like raindrops do. Others are smaller, and made smaller still by evaporation. These smaller droplets can linger in the air longer, drifting on air currents for perhaps hours. If a droplet is small enough, the moisture in it evaporates before it has the chance to reach the ground (provided the humidity in the room is average). If there’s stuff like germs in that droplet, they become light enough to float on air currents, like the dust you can see suspended in the air. These particles are often called aerosols. This framework is old, and it comes from William Wells, a scientist who studied tuberculosis transmission in the 1930s. In a 1934 paper, Wells concluded: “It appears, therefore, that transmission of infection through air may take one of two forms depending upon the size of the infected droplet.” When it comes to Covid-19, one of the first questions epidemiologists tried to answer was: Is the virus spread in the dust that floats or the big drops that fall? If it’s the small dust, alarm bells go off. These are the prototypical airborne diseases. They include measles, chickenpox, and tuberculosis, and they are extremely contagious. On average, one person infected with measles will infect 12 to 18 others. If it’s the big drops, it’s still concerning. Diseases like the flu, whooping cough, the common cold, and coronaviruses are primarily large droplet-borne. With these diseases, only the closest contacts to an infected person get infected because large droplets quickly fall to the ground (within 6 feet or so). It’s also possible for these big drops to fall on surfaces, and those surfaces can become contaminated too. Luckily, in the case of Covid-19, there’s a growing consensus that getting sick from touching contaminated surfaces is rare. But keep washing your hands! (Interestingly, there’s no one reason a new virus, like SARS-CoV-2, would go one route or the other. It’s possible, says University of Leicester virologist Julian Tang, that there may be a difference in the immune response with these airborne diseases that makes saliva and mucous less viscous, leading to more virus in small drops. Also, Tang writes in an email, “it is possible (though no one has studied this yet) that exhaled breath from measles/chickenpox cases may just be shedding much more virus (maybe millions of virus per minute) compared to respiratory viruses, which only exhale 100s to 1000s of viruses per minute.”) A part of why the terminology here is so confusing, and there’s so much debate about it, is that the words “aerosol” and “droplet” mean different things to different scientific disciplines.
