President Donald Trump has alleged that warmer weather could help slow the spread of the coronavirus, and a top official at the Department of Homeland Security repeated that claim this week. But many experts say there's little evidence that hot, sunny days actually make beaches safer.
Although sunlight can help kill coronaviruses on surfaces, it doesn't work fast enough to help beachgoers, said Andrea Armani, a professor of chemical engineering and materials science at the University of Southern California.
"It doesn't make it safe to go out to the beach because the time it takes for the sun to kill the virus is exceptionally long," Armani said.
A coronavirus' outer layer weakens as the temperature rises, Armani said. So when ultraviolet light from the sun heats up surfaces where the virus lands, its survival period will be somewhat shorter. (How much shorter depends on multiple factors, she added.)
But the sun won't make much difference in spots where shade or clouds reduce the optical intensity of sunlight. Nor will sunshine prevent the virus from spreading from person to person through droplets of saliva or mucus in the air.
Karin Michels, chair of the epidemiology department at the University of California, Los Angeles' Fielding School of Public Health, said there was "no good data" to support the idea that the UV in sunlight would make any difference in the coronavirus infection rate.
The risk, she said, depends more on how many people show up to the beach and whether they can practice proper physical distancing.
Judging from the photos of people flocking to Orange County beaches during last weekend's heat wave, that hasn't seemed to be the case, she said.
"People don't stay apart enough," she said, "and so that is also an argument to be a little concerned."
Dr. Amesh Adalja, an infectious-disease specialist at Johns Hopkins University, agreed that much of the risk hinged on human behavior.
"While it is true that the UV radiation contained in sunlight is detrimental to the (virus') viability on surfaces, many beach activities would give the virus an opportunity to pass _ not from surfaces _ but from person to person," he said in an email.
Armani also pointed out that people are less likely to have opportunities to wash their hands at the beach, and are probably less likely to wear face masks.
"A lot of those standards are going to be harder to maintain at the beach," she said.
The ultraviolet radiation in sunlight that reaches the Earth's surface, which includes the types known as UVA and UVB, should not be confused with a higher-energy form known as UVC.
UVC can be used to quickly kill pathogens on surfaces in controlled settings such as hospitals, but it's extremely dangerous and can potentially burn a person's skin or even blind them, according to Jim Malley, an environmental engineer at the University of New Hampshire.
