Some of you may remember the “tragedy of the commons” from a stray econ class you took in school. There’s a pasture open to all. Too many farmers decide to bring their cattle to graze in it, and soon there isn’t enough grass left for anyone.

These days, we are the cattle. Hundreds of millions of Americans are being asked to stay inside to prevent the spread of COVID-19. But in many parts of the country, residents are still allowed to go outdoors for exercise and fresh air. Unfortunately, when everyone decides to get some fresh air in the same place at the same time, crowds start to form. This has led to tighter restrictions — many beaches and parks in California are now closed because of crowds — and plenty of social media shaming.

But it’s the outdoors! There’s fresh air! Isn’t that a better place to be with a crowd than, say, a movie theater?

“The higher number of people close together, the higher the likelihood that you’ll get something, no matter if you’re indoors or outdoors,” said Dr. Kartik Patel, a primary care doctor with UCHealth in Colorado.

Oh.

The Centers for Disease Control and Prevention as well as many public health officials recommend keeping a distance of at least 6 feet from other people when you leave your home, including visiting public outdoor spaces. (And one MIT professor has wondered whether 6 feet is far enough.) While that seems fairly straightforward on paper, in practice it can get confusing quickly. If you’re jogging down the street and start approaching a neighbor walking in the opposite direction, for example, should you cross to the other side?

“I’ve been crossing the street,” said Carolyn Cannuscio, a social epidemiologist at the University of Pennsylvania’s Center for Public Health Initiatives. “I know that offends people sometimes. Someone called me out the other day and said, ‘I’m not going to give you the virus!’ I said, ‘I’m sorry, I’m an epidemiologist.’”

Other experts I spoke to felt the risk of merely passing by someone on the street is minimal, but that the more contact you come into with people — no matter where it is — the higher the risk. We know one way the virus spreads is through respiratory droplets that are expelled when an infected person coughs or sneezes.

“[This] can also include laughing and singing,” Patel said. “All of these actions are essentially expelling much of our lung volume when we do them. In the same context, heavy breathing while exercising would do the same as the aforementioned and, thus, could also cause the spread of a virus. There is no specific study that shows all of this; however, it is extrapolated from numerous studies on virology and respirology.”

While Cannuscio said being outdoors in its well-ventilated glory can help reduce the risk of transmission generally, that benefit starts to erode as more people congregate.

Even if an outdoor space technically has enough area for each person using it to have a 6-foot buffer around them, people have a habit of encroaching on those boundaries.





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Let’s use the Great Lawn in New York City’s Central Park as an example. The Great Lawn spans 55 acres, which is a little under 2.4 million square feet. If you drew a circle around yourself with a radius of 3 feet, that circle would have an area of about 28 square feet. With that math, as long as no one’s circle overlapped, around 80,000 people could safely use Central Park’s Great Lawn at the same time. But, obviously, people have a habit of not sitting still. Tens of thousands of people aren’t going to stay 6 feet apart for the entirety of their stay in the park, and navigating around all those people while trying to maintain that buffer would be extremely difficult, if not impossible.

But exercise and fresh air are still recommended. The CDC endorses exercise as a way to manage the stress and anxiety caused by our current social disruption, and some studies have shown that regular exercise can bolster your immune system.

No matter where you are, the key is to avoid other people. This could mean being flexible in your plans (if one trail is busy, find another), going out in the early morning or late evening, or deciding to work out at home more often.

“There’s always going to be some risk in everything we do,” said David Nieman, a biology professor who researches immune function at Appalachian State University. “Our goal is to minimize the risk so that we can experience the benefits.”

CORRECTION (April 1, 2020, 5:28 p.m.): An earlier version of this article misstated the number of people who could be on Central Park’s Great Lawn while remaining at least 6 feet apart. People could maintain a safe distance if everyone stood in a circle with a radius of 3 feet, not 6 feet, assuming no one’s circles overlapped. As a result, around 80,000 people could fit on the Great Lawn, not 21,000.