Many communities are opening up parks, stores, restaurants, hair salons and more. And of course, every school district in the world is struggling with the question of how to open schools safely. Very few places have done enough testing to know how widespread the coronavirus is in their communities, so the risk of infection remains real. However real, though, the risks of various activities are not equal and by applying what’s known about how coronavirus survives in different environments, we can rank those risks based on some pretty simple variables.
Before we look at those variables, you need to know this: in order to become infected with coronavirus, you have to breathe in more than a single viral particle. The exact number of SARS-CoV-2 viral particles needed to make you sick (aka the infectious dose) has not been determined, but comparison with similar viruses, and some clinical and epidemiological evidence, suggest that around 1000 infectious viral particles are needed. You could inhale 100 particles in a single breath, or you could accumulate 1000 particles over many breaths. (Of course, you could transfer 1000 particles to your eyes or mouth by touching a contaminated surface and then your face. Wash your hands! Don’t touch your face!)
If we start with the principle that you need to accumulate 1000 viral particles to get sick, your risk evaluation for any given activity boils down to just five factors:
- How big is the space you’re in and how many people are in it?
- What’s that space like?
- How long are you spending in that space?
- What are you doing in that space?
- How likely is it that there’s an infected person in that space?
Factor one is easy: the bigger the space, the lower the risk. Complications like ventilation aside, any virus exhaled by an infected person will be diluted in all that air. So, you will accumulate fewer viral particles per breath and be less likely to reach that infectious dose.
Factor two takes into account how long the virus is likely to survive in the air. Bright sunshine is hard on the virus. The virus stays viable longer in relatively cooler temperatures. If it’s windy or the room has lots of open windows, the virus will get diluted (thus, fewer infectious particles per breath).
Factor three is straightforward: the longer you’re in a space with virus in the air, the more likely you’ll accumulate an infectious dose.
Factor four is a little more complicated. And that’s because of physics. It matters how much virus is in each exhalation, and it matters whether that virus is contained in a tiny droplet of moisture (like a fine mist) or in big globs (like in spit—just don’t spit at all, OK; it’s gross). Between mist and spit there are other droplet sizes. Coughs and sneezes eject big droplets. Heavy breathing, like during a hard workout, ejects medium-sized ones. Shouting and singing eject smaller ones (but those droplets travel pretty far). Finally, soft talking or just quiet breathing eject the smallest droplets of all.
Finally, factor five. Obviously, there’s no risk of infection if there isn’t anyone carrying the virus sharing your air space. Figuring out how much weight to give this factor depends on the community you live in, and what the people in your community do. Are there lots of first responders like police and firefighters? Essential workers like grocery clerks? Do most people commute via public transportation? These groups are more likely to be infected.
OK. So now you have all the tools you need to compare risks, and try to reduce them for any given activity.
Next, in Part II, we’ll put this toolkit to use evaluating different scenarios. Have a scenario you want us to evaluate? Email Director of Teacher Support Lin Andrews at firstname.lastname@example.org.