News Current Events CDC Acknowledges Aerosol Transmission of Coronavirus (Again) POOF! It's Back. By Lloyd Alter Design Editor University of Toronto Lloyd Alter is Design Editor for Treehugger and teaches Sustainable Design at Ryerson University in Toronto. our editorial process Facebook Facebook Twitter Twitter Lloyd Alter Published October 6, 2020 09:03AM EDT ventilation do's and don'ts. Lidia Morawska et al Share Twitter Pinterest Email News Environment Business & Policy Science Animals Home & Design Current Events Treehugger Voices The Centers for Disease Control and Prevention (CDC) has revised its guidance on how COVID-19 spreads, once again, to include the fact that it is spread by airborne transmission. This was noted previously on September 18 but the information was removed three days later, to some controversy. The CDC previously wrote that the disease was transmitted in respiratory droplets, which are relatively big and heavy and generally fall to the ground within six feet. However, engineers and building scientists have been saying for months that it was transmitted by aerosol (very tiny particles) that move more like smoke. This turns it from a distancing problem to a building design problem, which is why we have been covering it here on Treehugger. The CDC still equivocates and hedges, but finally admits: "There is evidence that under certain conditions, people with COVID-19 seem to have infected others who were more than 6 feet away. These transmissions occurred within enclosed spaces that had inadequate ventilation. Sometimes the infected person was breathing heavily, for example while singing or exercising." The reason this is so contentious is that it really changes the guidance about how to deal with the virus. The CDC has now added the advice: "Avoid crowded indoor spaces and ensure indoor spaces are properly ventilated by bringing in outdoor air as much as possible. In general, being outdoors and in spaces with good ventilation reduces the risk of exposure to infectious respiratory droplets." As we noted in the earlier post, this means a total rethink: "Basically, you do not want to go to a Trump rally, or any other crowded, poorly ventilated indoor space where people don't wear masks. This also doesn't bode well for the restaurant industry, where people are close together, talking, and not wearing masks." Everyone has been carefully sitting six feet apart with plastic shields, when in fact the separation distance is not nearly as relevant as the number of air changes per hour, something that may well be hard to adjust (although it will be much easier through the fall when the outside air is cooler). A recent British report covered in Treehugger suggested that the plastic screens may do more harm than good, because they could block air circulation and proper ventilation. As Professor Jiminez notes, this guidance should not be taken lightly and should be acted on quickly. Architects and engineers should act quickly as well with all their planning for people to go back to the office; the priorities just changed. Key issues now are opening windows, lots of fresh air, no recirculation without really good filtration. The CDC also notes that all that scrubbing and spraying may not be as important as was thought: "Spread from touching surfaces is not thought to be a common way that COVID-19 spreads." The consensus now is that the virus is in the air. Dilute the air and you dilute the virus.