The severity of the COVID-19 pandemic and the subsequent torrent of research has brought a simmering debate about how respiratory infectious diseases are transmitted to a boil, in full view of the public. The words airborne, aerosol, and droplets are now part of the daily news—but, why?
Over the last decade there have been calls within the scientific community to change the vocabulary of disease transmission routes for respiratory infectious diseases because the definitions of airborne and droplet transmission routes, which came into use in the mid-1950s, are not supported by emergent evidence. People with respiratory infectious disease release pathogens from the respiratory tract, typically in droplets of respiratory fluids. Airborne transmission is the event that small droplets containing pathogens are inhaled by a susceptible person who is far away from the infectious person. Droplet transmission is the event that large droplets project onto the facial mucous membranes when the susceptible person is close to the infectious source, e.g. a cough in the face. These airborne and droplet transmission definitions fail because we now have very good evidence that the distinctions by droplet size and distance from the infectious source are artificial. Respiratory fluids are emitted in droplets that vary widely in size, small droplets can be inhaled by people close to the source, and the distance over which small and large droplets are transported is highly variable, dependent upon environmental conditions. To address the gap between airborne and droplet transmission, some scientists have proposed a third route: short-range airborne transmission. Others, myself included, have advocated replacing the two routes with a new concept: aerosol transmission.
Simultaneously, there has been a debate about which transmission route or routes are dominant for certain infectious diseases. For most viral respiratory pathogens, the convention is that droplet and contact transmission are more important than airborne transmission. Evidence, particularly that which has emerged from research on influenza and SARS, has led to recognition that some viral respiratory pathogens may be transmitted through the airborne or aerosol route. That is, virus inhalation can result in infection. Regarding COVID-19, the evidence of airborne or aerosol transmission has become overwhelmingly persuasive to many in the scientific community, leading to vocal criticism of public health organizations like the World Health Organization and the United States Centers for Disease Control and Prevention which argued, at least initially, against this possibility.
Identifying the transmission route of an infectious disease, including COVID-19, is not an esoteric scientific question because it is the transmission route that drives the infection prevention and control strategies. Consistent with initial determinations that COVID-19 is transmitted through the droplet and contact routes, the primary public health interventions were physical barriers, physical distancing, face shields, and hand hygiene. Later, cloth masks were added. These strategies have little impact on the movement of small droplets that can be inhaled, though new research suggests that cloth masks can prevent the emission and inhalation of small droplets. The performance of cloth masks (and surgical or medical masks), however, is meaningfully inferior to that of respirators, including N95 or FFP2 filtering facepiece respirators.
Acknowledging the contribution of airborne or aerosol transmission to COVID-19 requires that we take different precautions to protect workers, including the use respirators to prevent inhalation of respiratory droplets (large and small), and ventilation or filtration devices that capture respiratory droplets at the source or while traveling through the air. While updating the disease transmission paradigm during a global pandemic may seem overwhelming, we owe it to the workers who sustain the functions of daily living and are at risk of COVID-19—and a myriad of endemic and future pandemic diseases—to use the best, current scientific evidence to guide prevention strategies.
Feature image: CDC/ Debora Cartagena 2013.