A sensible way to estimate the risk of everyday activities.

A.M. Carter
6 min readMay 21, 2020

States are reopening, but the choices are yours.

(photo of the author)

**I’m writing this article because I have some experience interpreting medical literature and applying it to my own life. I’m here to (hopefully) help you do the same.**

People are feeling confused, and deeply frustrated. I get it. I feel pretty exasperated myself. Half of my friends seem to think that they need to wipe their groceries down with disinfectant*, and the other half are agitating for bars and nightclubs to reopen.

I think the confusion is causing unnecessary suffering and conflict over what constitutes a reasonable balance of safety and liberty.

Here’s what you need to know:

The risk of SARS-CoV-2 infection is a function of intensity of exposure x time.

(This is true of nearly every viral infection.)

Viral infection occurs because a critical mass of active virions (virus reproductive particles) invade your tissues, and begin to reproduce there. The important term to note here is “critical mass.”

You will not experience COVID-19 infection unless you are exposed to enough active novel coronavirus to cause infection.

So how do you get enough?

  1. You spend a long time in an environment with a low concentration of active virus. (Example: You work 8 hours a day in a busy grocery store.)
  2. You spend a short time in an environment with a high concentration of active virus. (Example: Someone coughs directly into your face for about 5 seconds.)

Let’s put this into perspective for our daily activities.

High-risk activity =

  • longer period of time (1 hour or more)
  • indoors
  • talking/singing/exercising/breathing deeply
  • multiple other humans from outside your household

Why are these activities high risk?

Well, it’s because the particles that come out of other people’s mouths and noses are the primary vector of infection, and the more time you spend marinating in other people’s vapors, the more likely you are to become infected. (Imagine Blanche Devereaux from The Golden Girls saying “vaay-pahs.” It made me feel better.)

Social distancing (standing 6 feet away from people) inside becomes less and less effective —

a. The more time you spend inside,

b. As the number of people inside with you increases,

c. As the number of people wearing masks decreases.

This is why churches have been outbreak hotspots in many communities. This is why indoor seating in restaurants, or visits to the gym, are high-risk activities (people breathing/eating/talking, for hours at a time, inside, without masks).

A single patron at a restaurant in Wuhan infected people seated across the room, because the ventilation system carried enough virions over the course of their meal to cause infection, well over 6 feet away.

At a choir practice of 60 people in Washington state, more than half became infected, even though they stood 6 feet apart, because 2.5 hours of singing indoors basically enveloped the room in a cloud of virus.

This is also why sanitizing surfaces has limited effectiveness.

If you’re washing your hands constantly and not touching your face, you’re protected from transmission from surfaces. The virus doesn’t permeate your hands, it gets in through your mouth/nose/eyes.

You can control your hand-washing and face-touching behaviors.

What you can’t control is the air you breathe.


Low-risk activity =

  • shorter period of time
  • outdoors
  • 6+ feet of space between you and others
  • fewer humans from outside your household

Uncrowded parks and beaches, or small backyard-only gatherings — with STRICT social distancing, so everyone maintains their own airspace at all times — have not been known to cause a single case. Restaurant seating outdoors at staggered tables is lower-risk, for example.

It’s worth noting here that time is a double-edged sword.

Time can be a good thing:

If the novel coronavirus hangs around outside a human body long enough, it decays past the point of being infectious. Virus death is accelerated by sunlight, outside air/temperatures/weather, but it will occur regardless, once it’s been outside a host for too long.

Time can be a bad thing:

If you hang out in a small room with a low level of active virus for too long, you can acquire COVID-19, even though the concentration of virus is low.

In practical terms, novel coronavirus is not going to survive in the air at the beach for very long. Sun, wind, salt and air temperature are all conspiring against it. In contrast, the virus may live substantially longer in the air in a windowless, air-conditioned room.

Why are masks important?

Masks outside are not necessary, as long as you can maintain at least 6 feet of space between you and other people at all times. If you live in a densely populated area and you pass a lot of people on the sidewalk, wearing a mask outside makes sense.

Masks inside, however, are vital for all of us.

Mask-wearing (meaning, everyone in a mask, not just some of us), serves two purposes:

  1. It blocks droplets from the wearer’s nose and mouth from being blown into the air or onto surfaces.
  2. It slows down the build-up of active virions in an environment.

#2 is important. People walking around breathing without masks shed more virus into the room, faster. People with masks shed less virus, slower.

Mask-wearing decreases the amount of virus and the rate of virus accumulation in the air, which makes it hard for viral loads to build up to high-risk levels before the virions start to die.

When you wear a mask at the grocery store, you’re protecting:

  • The people you may pass closely by while shopping,
  • The grocery store staff who have to be in that store all day.

Essential workers breathe the air inside their workplaces ALL DAY.

That’s a big increase in the time variable of the infection equation, which means we all need to do our part to help decrease the concentration of virus in the stores and offices we visit, in order to help keep the people who work there safe.

Masks decrease the intensity of exposure.

Even basic cloth masks can help inhibit your virions from spilling out into the air around you.

Basic cloth masks with filters of some kind can also block some virions from being breathed in by you.

What about everything else?

Haircuts indoors with masks worn by both stylist and customer, a doctor’s visit, running errands — these are in the middle somewhere between a day at the park and a crowded choir practice. There’s some risk, because of the intensity of exposure (people close together, indoor spaces), so make your choices accordingly.

More importantly, realize that if you’ve made a risky choice recently, you need to protect the other people with whom you come in contact — by wearing a mask inside, washing your hands, and maintaining space between you and others.

If you feel ill, stay at home until you feel better (do not leave your home if you have a fever, period, unless you’re going to the doctor).

If you are immunocompromised (I’ve been there), you will have a higher risk than most for all activities outside your home, so be exceptionally careful.

Viral infection is about the concentrations of virus you’re exposed to, and the length of time you’re exposed to them.

It’s a weird new world. It’s clear that we can’t stay at home forever, but it’s also clear that the danger of infection isn’t going to be gone anytime soon.

Let’s make informed decisions. I believe we can do it. ❤

*People wiping down groceries with disinfectant is, at best, a misuse of time, and, at worst, a misunderstanding of risk. There is no evidence that we need to do that. Wash your hands thoroughly after you put your groceries away. Wipe down your kitchen counter. Give yourself a break :-)

Author’s Note: I’d like to emphasize that I did not come up with this information on my own. I’m not a medical expert, I’m a journalist. This article is an attempt to synthesize many scientific sources into coherent, actionable information that is meaningful for regular people in our everyday lives. I drew heavily from the exceptional work written by assistant professor in immunology Dr. Erin Bromage at the University of Massachusetts at Dartmouth. Do yourself a favor and read this article for more detailed information:

Also consider reading this article, that explains more about viral load, and what constitutes an “infectious dose” for COVID-19 :



A.M. Carter

A.M. Carter earned a bachelor’s degree in journalism from the University of Florida. She writes about philosophy, science, politics and current events.