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How to Assess Your COVID Risk Before Going Out

Here’s what an engineer has to say about risk assessment.
FACT CHECKED BY Emilia Paluszek

Conflicting advice from public health officials has been a frustrating (if understandable) aspect of the COVID-19 pandemic since day one, so it's important to understand risk assessment in daily life. "Since we are in the 'assess your own risk' phase of this pandemic, it would be helpful if public health officials would give a master class on assessing risk. Looks like academics and engineers need to do it instead," says professional engineer Joseph Fox, P.Eng., M.A.Sc. "To assess the risk of a space, first you need to know how you get infected and what contributes to it." Here is how to assess your COVID-19 risk, according to an expert. Read on to find out more—and to ensure your health and the health of others, don't miss Already Had COVID? These Symptoms May "Never Go Away".


Primary Way of Getting Infected

Waiter coughing into elbow while serving customers in a restaurant.

"The primary way you get infected with COVID is by inhaling an infectious dose over a certain period of time," says Fox. "The dose is defined by 4 factors: Dose = Virus concentration x time x breathing rate x respiratory tract deposition rate. Dose is related to the probability of infection. You can still get infected with a low dose (like outdoors), it's just lower probability. That's why it's problematic to say something is 'safe'. Lowering the dose reduces risk of infection, but doesn't eliminate risk."


Time Exposed to Virus

Women with face masks down

"Time: The longer you spend exposed to a certain concentration, the higher the risk," says Fox. "For activities that don't require spending extended periods of time, like going to a store, minimizing time is an effective measure to reduce risk."


Breathing Rate

People wearing surgical mask sitting in subway in Shanghai

"Breathing rate: For some reason the 'hold your breath' method hasn't been promoted enough. When breathing rate is 0, you don't get infected through inhalation," jokes Fox. "The faster you are breathing, the more air you inhale over a period of time. Putting time and breathing rate together, the risk is related to the number of breaths you've taken. More breaths means higher risk."


Deposition Rate


"Deposition rate: Many of the virus particles you inhale do not stay inside you, you just breathe them back out," Fox says. "Only the ones that deposit inside you can infect you. However, I don't think this is relevant for assessing risk. One possible exception is you have innate immunity and it can remove particles that have deposited instead of them infecting you. Exposure to low humidity inhibits this and increases the risk from deposition. Run a humidifier when it's dry inside."


Virus Concentration

Young sick woman laying in her bed.

"Virus concentration: The final and most complicated factor," Fox explains. "The higher the concentration you inhale, the fewer breaths required to reach an infectious dose. So how do you assess virus concentration? Range – initially when you exhale, there is a jet of concentrated air in front of you (think of a smoker breathing out or smelling someone's breath). The virus concentration is highest at short range from the source. That's why infection risk is highest in close contact. However, the majority of air you breathe in at short range isn't actually from that person's breath. Most of the air is still from the room, so low virus concentrations in the rest of the room make a difference – even at short range.After the short range jet, the virus laden aerosols in the air then diffuse into the room. There are many factors that affect the final virus concentration and they're important for assessing risk."


How to Stay Safe Out There


Follow the public health fundamentals and help end this pandemic, no matter where you live—get vaccinated or boosted ASAP; if you live in an area with low vaccination rates, wear an N95 face mask, don't travel, social distance, avoid large crowds, don't go indoors with people you're not sheltering with (especially in bars), practice good hand hygiene, and to protect your life and the lives of others, don't visit any of these 35 Places You're Most Likely to Catch COVID.

Ferozan Mast
Ferozan Mast is a science, health and wellness writer with a passion for making science and research-backed information accessible to a general audience. Read more about Ferozan