Several months into a pandemic that has claimed hundreds of thousands of lives and decimated economies around the world, scientists still lack a complete understanding of how the virus that caused it is transmitted. Lockdowns are already easing in some places, and people are preparing to return to a version of work and social life. For months, the U. But anecdotal reports hint that it could be transmissible through particles suspended in the air so-called "aerosol transmission". At least two died. After dining at an air-conditioned restaurant in China in late January, three families at neighboring tables became sickened with the virus—possibly through droplets blown through the air. In reality, though, the distinction between droplets and aerosols is not a clear one. But much remains unknown about whether the aerosolized virus is infectious and what amount of virus one needs to be exposed to in order to become sick, known as the minimal infectious dose. Even if aerosol transmission does occur, it is not clear how common it is, compared with other transmission routes, such as droplets or surfaces. Uncovering this information is vital, especially given the fact that people can spread SARS-CoV-2 when they have no symptoms. Some of the strongest evidence that airborne transmission of the new coronavirus may be possible comes from a study published late last month in Nature. In it, researchers measured the virus's genetic material, or RNA, in aerosols sampled in February and March at two hospitals in Wuhan, China—the city where the outbreak is widely believed to have begun. They also found high levels of viral RNA in places where medical workers remove protective gear, as well as in two crowding-prone locations near the hospitals. A preprint not yet published study led by Santarpia and his colleagues similarly found evidence of viral contamination in air samples and surfaces from rooms where COVID patients were being kept in isolation. Another paper, recently published in the New England Journal of Medicine , showed that infectious SARS-CoV-2 virus can remain in aerosols for at least three hours—and for several days on various surfaces—in a laboratory setting. But the amount of viable virus diminished significantly during that time. For influenza, studies have shown that just three virus particles are enough to make someone sick. Overall, most of the evidence that SARS-CoV-2 can become airborne comes from clinical settings—which tend to have a lot of sick people and may host invasive procedures, such as intubations, that can cause patients to cough, generating aerosols. It is not clear how representative of everyday environments these areas are. That assessment does not mean it is not occurring, however. Like cigarette smoke, aerosol particles spread around a person in a cloud, with the concentration being highest near the smoker and lower as one gets farther away. If there is a fan or air conditioner, infectious aerosols or even droplets, as was suspected in the case of that restaurant in China could potentially sicken someone farther away who is downwind. Some evidence suggests that talking could be a significant mode of viral transmission. Cowling hypothesizes that many respiratory viruses can be spread through the airborne route—but that the degree of contagiousness is low. For seasonal flu, the basic reproduction number, or R 0 —a technical designation for the average number of a people a sick person infects—is about 1. Compared with measles, which has an R 0 in the range of 12 to 18 , these values suggest most people with the disease caused by SARS-CoV-2 are not extremely contagious. But there are seeming exceptions, such as the choir practice in Washington State, Cowling says. These so-called superspreaders were documented in the SARS outbreak of , too. Ventilation likely also plays an important role in how easily the virus can be transmitted through the air. Indoor spaces probably pose a higher risk than outdoor ones, especially if they are poorly ventilated, Cowling and others say.

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