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Mugs of hot cocoa, falling leaves, and—for those in northern climates—an approaching end to warm evenings outside with friends, 6 feet apart but still together.
It’s autumn, and the coronavirus has even changed how we think about the changing seasons. We know the virus travels through the air, and as Harvard School of Public Health professor Roger Shapiro told The Hill in May, “It definitely spreads more indoors than outdoors. … The virus droplets disperse so rapidly in the wind that they become a nonfactor if you’re not really very close to someone outdoors.”
So staying outside makes sense while the weather permits. But what about when it doesn’t? Indoors, things get more complicated with that 6-foot rule: Research suggests the virus spreads most efficiently in poorly ventilated spaces. At the website The Conversation, several engineering professors described this by pointing to how the smoke from one cigarette travels to every corner of an indoor room—and sticks around once there. Masks help prevent many virus particles from getting into the air, but how can we further reduce the risk of indoor transmission?
The simplest solution is the way we’ve all cleared the air after burning dinner: Just open a window, or better yet, several windows. A box fan in a window would speed up air exchange further. To consider airflow more precisely in settings like schools and churches, a carbon dioxide meter lets us quantify how we’re doing: People breathe out carbon dioxide, and it builds up when air stagnates. Conversely, when ventilation improves, carbon dioxide levels go back toward the near-zero level in outdoor air. (Note that carbon dioxide is the stuff we exhale, while carbon monoxide is the odorless, poisonous stuff in car exhaust.)
When falling temperatures make open windows impractical, air purifiers can still help remove the virus from the air. Their effectiveness does vary, and the simple, charcoal-based filters that take odors out of a room won’t remove meaningful amounts of virus from the air. HEPA filters, those old friends of hay fever sufferers, will. They won’t remove every last bit of virus, but they can reduce its load substantially. Other filter technologies exist but aren’t normally used in residential homes. For commercial settings, a powerful UV-C light system—installed inside an air duct or otherwise aimed away from eyes—can also help disinfect air.
Where a budget allows it, the best solution combines warmth with fresh air: A “heat recovery ventilator” exchanges stale air for fresh air from outside, and it also recycles the heat, using the stale air to warm the colder air coming in. Heat recovery ventilators aren’t cheap, but they work. My own church installed one several years ago, and testing with a carbon dioxide meter revealed that the system refreshed the air inside the church much better than opening the doors would have. They’re also more economical than leaving windows open in the winter.
Apart from the amount of virus in the air, one other factor affects potential viral exposure indoors: the amount of time you spend breathing that air. When all else fails, Mom’s time-honored advice for visiting friends still applies—don’t stick around until they wish you’d leave.