The U.S. Centers for Disease Control and Prevention updated its guidance on Tuesday to emphasize that "outdoor visits and activities are safer than indoor activities." After a year in which many of us have learned to dutifully wear masks and look askance at anyone who does not, it's understandable that people remain fearful when they cross paths with the unmasked. So how do you make the right decision about when to wear a mask outside? Whether a mask is needed outdoors depends on the circumstances, including local public health rules and whether you and the people you're with are vaccinated. Brief encounters with an unmasked person passing you on the sidewalk or a hiking trail are very low risk, said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech and one of the world's leading experts on viral transmission. Viral particles quickly disperse in outdoor air, and the risk of inhaling aerosolized virus from a jogger or passers-by is negligible, she said. Even if a person coughs or sneezes outside as you walk by, the odds of you getting a large enough dose of virus to become infected remain low, she said. To understand just how low the risk of outdoor transmission is, researchers in Italy used mathematical models to calculate the amount of time it would take for a person to become infected outdoors in Milan. If a person avoided crowds, it would take, on average, 31.5 days of continuous outdoor exposure to inhale a dose of virus sufficient to transmit infection.