Joseph G. Allen is an associate professor and director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health. He co-wrote “Healthy Buildings: How Indoor Spaces Drive Performance and Productivity.”

About a year ago, in the initial stages of the pandemic, there were earnest but totally misguided efforts to protect people by closing parks. Park benches taped off. Tennis nets taken down. Playgrounds closed.

A year later, we’re doing it once more with sports. It’s deja vu all over again.

Take the latest guidance from the Centers for Disease Control and Prevention, which says that for schools in communities with the highest levels of spread — which covers nearly half of the country today — there should be a blanket ban on sports. Not even outdoor sports. Another 30 percent of schools can hold outdoor sports, but the CDC requires six feet of distance or more.

Let’s start with an inconvertible fact: Being outdoors is very low risk. This holds for kids and adults. A new study, out in preprint but not fully peer reviewed, from the University of Wisconsin, which followed nearly 1,000 schools and more than 150,000 athletes, found that outdoor sports had half the rate of new cases as indoor sports.

There are two protective benefits that outdoor sports offer athletes. The first is unlimited ventilation. Any kind of wind (or movement by the players) would quickly disperse aerosols from an infectious person before they even reach another athletes’ mask.

Another benefit doesn’t get much attention, but it should: ultraviolet radiation from natural sunlight. Without sunlight, it can take more than four hours for 90 percent of the novel coronavirus to inactivate. Outdoors in the sunlight typical of early fall and late winter? Nineteen minutes. In summer sun, just eight minutes.

Instead of blanket bans on sports, we should be encouraging people to play sports regardless of community spread. Some are obvious right off the bat. Softball and baseball? Extremely low risk. They keep people separate, by design!

Tennis? Virtually zero risk. You’re never closer than six feet except for fleeting moments, and there is no risk of transmission through a tennis ball. And how about track and field? Golf? Swimming? Cheerleading and dance? All doable.

How about the seemingly riskier sports with close contact, such as lacrosse, field hockey and soccer? A study of rugby players — hard to imagine anything more close contact that that — reported that positive cases were most likely traced to social interactions outside of the match rather than in-match transmission (no masks).

So this must mean indoor sports for kids are a no-no? Well, no.

There are some activities that will obviously not work during a pandemic. Hopefully, it does not require a scientist to explain why wrestling is not recommended.

But school gyms with a lot of volume and high ceilings means aerosols have more room to spread out and dissipate. Even better, the ventilation rates for these places are designed for many spectators — sometimes hundreds. So if audiences are kept small and ventilation systems run at full design capacity, there is already extra ventilation built into the system. Basketball and volleyball are certainly higher risk than outdoor sports, but higher risk doesn’t mean high risk.

To be clear, masks should be worn for all sports with close contact. That University of Wisconsin study also found that mask use was associated with decreased covid-19 incidence for indoor sports such as volleyball and basketball, and even showed signs of this benefit for outdoor sports such as football.

Now let’s address the predictable critiques that are sure to come. What about the bus! The locker room! The outbreaks we have seen with sports!

Here we go: 1) Opening the windows on a bus can serve up 20 to 40 air changes per hour — two to three times more than a hospital airborne infection isolation room. 2) Close the locker rooms, or make sure they are well-ventilated or have portable air cleaners, and spend little time in them. Use them for changing when athletes need privacy, not for putting on sports gear, which can be done outside. 3) The outbreaks we saw with sports are indoor sports, such as ice hockey, and only occurred when people weren’t wearing masks. A Yale study found the probability of infection for ice hockey players was 1.5 to 2.5 times lower for those wearing masks.

By the end of this month, we’ll have enough vaccine doses for 130 million adults. The highest-risk adults should all have had the opportunity to get a shot. Once vaccinated, adults can and should go back to sports without restrictions.

In the meantime, kids should be allowed to play sports now. Children are suffering immensely from the mental health strain of the past year. Mental health visits for kids are skyrocketing, and there are reports of increases in teens seeking help around suicide and suicide ideation. The risk of death from covid-19 for those under 14 is 1 in 1 million. For those 15 to 24, it’s 1 in 100,000. The risk of death from suicide for both age groups is 10 times higher. Physical activity and socialization from sports are key to addressing the mental health crisis.

The playbook for sports is simple: Play outdoors. Wear a mask for close contact sports. Hold off on close-contact indoor sports for adults until they’re vaccinated. De-densify gyms for indoor sports. Limit time in locker rooms. Use hand sanitizer before and after the practice or game. Play ball!

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