A year into the COVID-19 pandemic, our understanding of how and why the coronavirus spreads continues to change, especially as new variants emerge around the world. While you may have spent much of last March wiping down your groceries, research has since shown that the coronavirus that causes COVID-19 is primarily spread through respiratory droplets, not surface transmission. Like many respiratory viruses, the coronavirus spreads more effectively in the winter, with studies showing that prolonged, indoor contact in settings with poor ventilation are more likely to cause transmission. But what do we know about how the virus spreads outdoors in winter? Michael Brauer, a professor at the University of British Columbia School of Population and Public Health, said it's a difficult, dynamic question to answer — one that would need to take into account temperature, humidity, wind direction and more. "The physics is pretty complicated," he said. "When transmission does happen outdoors, it's very likely that it's people not wearing masks and standing close together." What are droplets? Brauer explained when you exhale you're humidifying the air around you, releasing breath at body temperature, 37 C. Your breath is saturated with water and if you're carrying the coronavirus — whether you're symptomatic or not — you're exhaling microscopic virus particles coated in a layer of water. What makes masks an effective tool against transmission is that they block these droplets from entering the air in the first place — though they're less effective at protecting you from someone else's droplets. Why does humidity make a difference? Once droplets exit your body, their size changes depending on the conditions of the air around you, Brauer says. Humidity combined with temperature affects the way droplets react, with droplets becoming smaller the less humid the conditions. A smaller droplet is likelier to hang in the air for a longer amount of time, while a larger one will sink to the ground, making it less likely to reach another person. "There are no hard and fast rules. Everything is very dynamic. There's wind direction [for example]. But in general, the smaller those droplets are — so they tend to evaporate or dry out — they'll remain in the air longer," he said. That's why it's possible that varying winter conditions in some parts of Canada may affect how the virus spreads outdoors, he added. "In the east when it's cold and very dry, then you do get in a situation where those droplets could become smaller and could remain airborne for longer. I'm not convinced that would be the case here in Vancouver. Those droplets could basically remain the same size, which means they're not going to last in the air for that long," he said. Brauer said there's also some evidence to suggest that if a droplet is on a surface, it won't be as infectious in warm temperatures or high humidity. Still, he said a recent study shows that out of 7,000 cases of coronavirus transmission, just five occurred outdoors. The vast majority of transmission happens indoors, which is why respiratory viruses spread more rapidly in the winter, when people are less likely to be gathering outside. What about ventilation and movement? The role that ventilation and wind play is much better understood. "A two-metre distance is generally protective. If you're two metres away and it's very, very calm air, that's less protective than if you're two metres away if there's a lot of wind," said Brauer. "If you're within two metres and you have a gust of wind, you may be perfectly safe." Exercise also creates its own ventilation, making it safer to have a conversation while moving than when standing still. "When people are moving, that's going to be safer in general, just because you're getting more air flow — and so that's just like having ventilation, you're creating your own micro-wind," said Brauer. Are outdoor activities safe? Brauer said another factor to take into account is that the combination of exercise and cold temperatures make people more likely to cough, which expels more droplets. Heavy breathing caused by exercise also makes you more likely to exhale and inhale droplets. But he said no one should be discouraged from exercising outdoors — most outdoor activities are safe, and it's usually the events leading up to exercise that are more likely to allow for transmission. "Certainly, activities where you're close to people — that's where you need to be concerned. Skiing itself wouldn't be a particular concern, but waiting in line might be," he said. "Even when you're outdoors and together with people, keeping that two-metre distance is good advice. But I don't think it's absolutely necessary to wear a mask every moment you're outside if you're not in close contact with people."