Question: A friend and I had a lengthy conversation this morning about safety in restaurants from air circulation: Even if restaurants employ good social distance practices, aren’t we at-risk from air conditioning/air circulation within the restaurant? And, isn’t this analogous to the days when we had “no smoking” sections in restaurants but all were exposed to smoke anyway? Finally, I wonder about outdoor dining and exposure to infected patrons via air currents from wind?

Answer: So many of us are trying to figure out our “new normal.” When it comes to risk of infection, put simply it’s about how much of the virus you’re exposed to (viral load) and how long your exposure is. Most outdoor activities will be safer than indoor activities because the sun, wind, and sheer openness will cause the virus to dilute far more quickly, which will thereby reduce both amount of virus you could be exposed to and the amount of time you could be exposed. The New York Times has had a couple of articles in the last few days that have provided a nice overview of risks and considerations when it comes to outdoor activities and restaurant eating (both indoor and al fresco). And for a bit more on the research on indoor/outdoor risks, see our Q&A of 5/12. I would write more, but those Times articles really do a fantastic job! Now, when it comes to amount of virus and amount of time thresholds for transmission, we still don’t know for sure (that’s our common refrain!). In the absence of direct evidence, a team researchers recently wrote a paper in The BMJ on the role of viral load in home-based transmission dynamics, which gives a nice overview of the state of evidence. The researchers state, “Although the infecting dose from a combination of droplets and environmental contamination cannot be easily measured, high quality experiments under controlled conditions in animal models can provide indirect evidence. We are not aware of infecting dose experiments with animal models of covid-19, but animal models of other viral infections show that variation in the infecting dose determines how many animals get infected and how severe the illness is.” And when it comes to length of exposure, CDC says that while data are insufficient to precisely define “prolonged exposure,” “15 min of close exposure can be used as an operational definition.”