People enjoy warm and spring-like weather during the coronavirus outbreak in Stockholm, Sweden, on April 22, 2020.
  • Direct sunlight could kill the coronavirus in minutes, the Department of Homeland Security suggested on Thursday.
  • President Trump surmised that sunlight could be used on the human body - a claim that one epidemiologist called "exceedingly dangerous."
  • Experts say that sunlight won't protect you from COVID-19, and exposing yourself to UV light can be dangerous.
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Direct sunlight could rapidly kill the coronavirus on surfaces like stainless steel or in droplets that hang in the air. But it probably won't protect your body.

Research at the Department of Homeland Security (DHS) found that direct sunlight can cut the amount of live virus on a nonporous surface in half in just two minutes, according to William Bryan, who leads science and technology at the department.

"Our most striking observation to date is the powerful effect that solar light appears to have on killing the virus, both surfaces and in the air," Bryan said in Thursday's White House coronavirus task force briefing.

Following those comments, President Donald Trump speculated about using sunlight on the human body as a protective measure against COVID-19.

"So, supposing we hit the body with a tremendous, whether it's ultraviolet or just very powerful light, and I think you said, that hasn't been checked but you're gonna test it," he said, turning to Bryan. "And then I said, supposing it brought the light inside the body, which you can either do either through the skin or some other way, and I think you said you're gonna test that too, sounds interesting."

But experts say people shouldn't rely on sunlight as a disinfectant, and definitely not as a treatment or preventive measure for your body.

"His comments are exceedingly dangerous and not supported by medicine or science, and have troubling ramifications," Dr. Saskia Popescu, an infection-prevention epidemiologist who runs SvPrevention consulting, told Business Insider in an email.

Hospitals use artificial UV light to disinfect objects, but it's dangerous to humans

Sunlight emits two types of UV radiation: UVA and UVB. A third type, called UVC, doesn't make it through Earth's atmosphere.

Hospitals, however, use UVC light to disinfect some surfaces, since it's powerful radiation that can kill microorganisms that might survive chemical treatment.

"In healthcare, UV disinfection is not a substitute for cleaning and disinfection, but rather a supplement to help cover more areas," Popescu said.

Exposing yourself to that artificial disinfecting light can be extremely dangerous, however.

"UVC is really nasty stuff - you shouldn't be exposed to it," Dan Arnold, who works in sales at the company UV Light Technology, told the BBC. "It can take hours to get sunburn from UVB, but with UVC it takes seconds. If your eyes are exposed … you know that gritty feeling you get if you look at the sun? It's like that times 10, just after a few seconds."

Sunlight won't protect your body from COVID-19

The sunlight you get outside, though less powerful than UVC light, can create an inhospitable environment for some viruses on certain surfaces.

However, Popescu said, "even if two people are out in the sun in close contact, if you cough on the other person or on their hands and they touch their face, the sun doesn't play a part in that."

Other experts say there is no evidence that UV rays, either from the sun or from an artificial source, can prevent coronavirus infection.

"Going out in the sun or exposing yourself to these high-intensity UV lamps is not going to protect you from COVID-19," Dr. Megan Ranney, an emergency physician at Rhode Island Hospital and an associate professor at Brown University, told CNN. "I don't want people to think that this is another miracle cure."

The World Health Organization has also warned against it.

"Exposing yourself to the sun or to temperatures higher than 25C degrees DOES NOT prevent the coronavirus disease," the WHO's myth busters webpage reads.

The page also cautions that "UV lamps should not be used to sterilize hands or other areas of skin as UV radiation can cause skin irritation."

Sunlight is also not helpful if you're already infected, and it's unclear what Trump meant by getting it "inside the body."

"UV irradiation and high heat are known to kill virus particles on surfaces," Dr. Penny Ward, a professor of pharmaceutical medicine at Kings College London, told the BBC. "Neither sitting in the sun, nor heating, will kill a virus replicating in an individual patient's internal organs."

'I wouldn't rely on it'

"Take a playground equipment, for example," Bryan said at Thursday's briefing. "The UV rays hitting a piece of playground equipment will kill the virus when it hits on the playground equipment, but underneath where the sun does not hit yet, if someone touched that on their hands, it could still be there. It has to be in direct light of the UV rays."

But sunlight is also unlikely to be helpful for killing the virus in everyday life.

"I wouldn't rely on it. The surfaces we worry about are those high-touch ones, like doorknobs, counters, etc. - those are indoors," Popescu said.

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