How do I decide what’s safe, Part I: Assembling your tool kit

NCSE Executive Director Ann Reid, who as a research biologist helped sequence the 1918 flu virus, provides ways to assess the risks involved as we begin to venture out into the world.

Person wearing a mask on a commuter train

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Everybody’s got a lot of questions: Is it safe to go back to school? How about the skate park? My friend’s basement? In this article, we’re going to build a toolkit for thinking about these questions and then in a follow-up article practice with our new tools.

We’ve discussed some of the science behind the risk of contracting COVID-19 from touching contaminated surfaces. Now, we’re going to talk about the risk of contracting COVID-19 through the air.

But first, a quiz: Which of these is more dangerous?

Shark and sunbathers

How about these?

Shark and deer

If you answered “shark” either time, think again.

No one has ever made a movie about someone who goes to the beach a lot, gets a few bad sunburns but otherwise just tans nicely, and then is diagnosed with melanoma at the age of 65. It just doesn’t seem very dramatic. Meanwhile, sharks have their own horror movie genre and a week dedicated to them on TV. In the real world, though, in any given year only about 10 people are killed by sharks around the world while about 60,000 people die from melanoma.

And when’s the last time you saw a killer deer movie? You’re probably more afraid of sharks than deer, but sharks are responsible for about one death per year in the United States, while deer are responsible for 200 (because of collisions with cars—no need to start worrying about actual killer deer).

All this to say that we humans are not so great at understanding relative risk. We would prefer a simple answer: safe or not safe.

But it turns out the question to ask isn’t “Is this safe or not?” Instead, the questions we should be asking are: “How safe is this relative to other activities?” and “Can we modify this activity to make it safer?” Developing some skill at thinking rationally about risk can reduce our anxiety, and, even more importantly, help us choose behaviors that minimize risks to us, our loved ones, and our whole communities. As we walk through the variables you need to think about, I want to acknowledge Erin Bromage’s recent blog post “The Risks—Know Them—Avoid Them.” It’s worth reading in full and most of what follows is based on his description of the clinical and epidemiological evidence about coronavirus, and his reasoning process about how to evaluate risk.

The questions we should be asking are: “How safe is this relative to other activities?” and “Can we modify this activity to make it safer?”

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:

  1. How big is the space you’re in and how many people are in it?
  2. What’s that space like?
  3. How long are you spending in that space?
  4. What are you doing in that space?
  5. 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

NCSE Executive Director Ann Reid
Short Bio

Ann Reid is a former Executive Director of NCSE.