It is now clear that SARS-CoV-2 transmits mostly between people at close range through inhalation. Aerosol scientists have shown that even talking and breathing are aerosol generating procedures. 11 12 13 As airborne spread of SARS-CoV-2 is fully recognised, our understanding of activities that generate aerosols will require further definition. As it is, healthcare workers wearing surgical masks have become infected without being involved in aerosol generating procedures. If the virus is transmitted only through larger particles (droplets) that fall to the ground within a metre or so after exhalation, then mask fit would be less of a concern. However, both high filtration efficiency and a good fit are needed to enhance protection against aerosols because tiny airborne particles can find their way around any gaps between mask and face. Masks usually impede large droplets from landing on covered areas of the face, and most are at least partially effective against inhalation of aerosols. 8 People are much more likely to become infected in a room with windows that can’t be opened or lacking any ventilation system.Ī second crucial implication of airborne spread is that the quality of the mask matters for effective protection against inhaled aerosols. 6 7 This means opening windows or installing or upgrading heating, ventilation, and air conditioning systems, as outlined in a recent WHO document. If we accept that someone in an indoor environment can inhale enough virus to cause infection when more than 2 m away from the original source-even after the original source has left-then air replacement or air cleaning mechanisms become much more important. One crucial difference, however, is the need for added emphasis on ventilation because the tiniest suspended particles can remain airborne for hours, and these constitute an important route of transmission. Wearing masks, keeping your distance, and reducing indoor occupancy all impede the usual routes of transmission, whether through direct contact with surfaces or droplets, or from inhaling aerosols. Why does it matter? For current infection control purposes, most of the time it doesn’t. Traditionalists will refer to the larger short range particles as droplets and the smaller long range particles as droplet nuclei, but they are all aerosols because they can be inhaled directly from the air. Some of these will be inhaled almost immediately by those within a typical conversational “short range” distance (2 m). People infected with SARS-CoV-2 produce many small respiratory particles laden with virus as they exhale. 3 Essentially, if you can inhale particles-regardless of their size or name-you are breathing in aerosols. Although this can happen at long range, it is more likely when close to someone, as the aerosols between two people are much more concentrated at short range, rather like being close to someone who is smoking. This created poorly defined divisions between “droplet,” “airborne,” and “droplet nuclei” transmission, leading to misunderstandings over the physical behaviour of these particles. The confusion has emanated from traditional terminology introduced during the last century. Over a year into the covid-19 pandemic, we are still debating the role and importance of aerosol transmission for SARS-CoV-2, which receives only a cursory mention in some infection control guidelines. Improving indoor ventilation and air quality will help us all to stay safe
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