Add humidity to the list:
after three months of data crunching, they found that the most powerful correlation between national numbers of daily new coronavirus cases and daily Covid-19 deaths was indoor relative humidity. Even controlling for dozens of other factors, the data showed that as indoor relative humidity went up during the summer months in the northern hemisphere, deaths plummeted. In the southern hemisphere, the opposite was true—as humidity fell during those nations’ winter months, deaths began to climb. “It’s so powerful, it’s crazy,” says [Stephanie] Taylor. That work has not yet been published. But Taylor believes it’s the strongest evidence yet that humidity needs to be as much a part of the conversation about containing Covid-19 as is discussion of ventilation, masks, and hand hygiene. “It’s hard to prioritize one intervention over another; we need all of them,” says Taylor. “Humidifiers aren’t a replacement for masks or social distancing or ventilation. But when you have more humidification, it enhances all these other things we’re already doing.” At higher humidities, respiratory particles grow faster and fall to the ground earlier, so there’s a better chance that staying 6 feet apart from infectious people really will dilute how many bits of their aerosolized virus you might happen to inhale. In a recent modeling study, Japanese researchers found that air with 30 percent relative humidity can carry more than twice the number of infectious aerosols, compared to air with relative humidity levels of 60 percent or higher. That also means masks are more likely to block more of the particles coming out of people’s noses and mouths, because they tend to be better at trapping bigger particles than smaller ones. And it means that air purifiers (even cheap, DIY ones) will filter out a larger proportion of potentially infectious particles.
so many good things here