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Once it was known that COVID spread mainly through the air, most experts thought it did so via droplets. This would have meant that anyone more than a few feet away, or sharing the same air space just a few seconds later, would've been safe. But further research showed that aerosols also contribute significantly to transmission. They're capable of containing quite a bit of virus, and because they weigh much less, they can travel farther and linger in the air longer than droplets. And for a time, at least, the virus was evolving to spread even more via aerosols-- people with the Alpha variant exhale about eighteen times more virus in aerosols than people with the original virus do.23 Part of the reason aerosols were underestimated is that, because they're so small, they usually dry out quickly, which makes the virus particle inactive. One study used a computer simulation to show that COVID viruses--particularly the delta and omicron variants--have an electrical charge that attracts substances from the lungs that slow down the process of drying out aerosols.24 We need to study transmission dynamics a lot more so that next time we understand quickly how transmission is taking place. Depending on the conditions in the room--temperature, airflow, humidity, and so on-- aerosols may stay in the air for as long as a minute, and they can travel several feet. It's not clear yet what share of cases is caused by aerosol transmission, but it could be more than half. What does this all add up to? Airflow and ventilation matter, probably quite a bit. You should install high-quality air filters to remove aerosols if you can, and if that's not possible, there's also a simpler and cheaper option: opening the windows. In a study done in Georgia, schools that opened doors or windows and used fans to dilute airborne particles had 30 percent fewer COVID cases than those that didn't. Schools that also installed air filters had 50 percent fewer cases.