Does UV Light Kill Coronavirus?
One type of ultraviolet light—UVC—may help kill viruses and reduce the transmission of Covid-19, but a UV light expert explains the potential risks, like skin irritation and eye damage.
Ultraviolet (UV) light has become a focal point of interest when it comes to reducing the spread of Covid-19. Just ask Google, which will return 13.3 million results if you search for “Can UV light kill coronavirus?”
So can it kill the virus that causes Covid-19? The answer is probably yes, but there are quite a few things you need to know before buying a device or relying on the sun to kill germs—including the fact that UV-emitting devices can be dangerous and ineffective if used incorrectly.
“UV is a very effective tool in the toolbox, and one of the multiple barriers to disease transmission that, when done correctly, has been effective for 100 years or more,” says James Malley, a 30-year UV expert, and professor of civil and environmental engineering at the University of New Hampshire’s College of Engineering and Physical Sciences.
In other words, the right kind of UV rays have been proven to kill a variety of pathogens, including bacteria, viruses, and other coronaviruses, like Middle Eastern respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome-related coronavirus (SARS)—two dangerous viruses related to the one that causes Covid-19. That said, science has not yet demonstrated that UV light kills SARS-CoV-2, the Covid-19-causing virus. (This is what SARS, MERS, and Covid-19 have in common.)
Malley also warns that there are lots of worrisome myths floating around about UV light, like President Trump’s theory that sunlight or ultraviolet light might work to wipe out Covid-19 inside the human body. (For the record, that’s dangerous—never apply a UV-emitting gadget to the body, particularly the eyes, as it’s for surfaces only). And Malley notes that there are some gadgets that promise to clean your phone and masks of the contagion, but most likely do nothing (more on that later).
This is what we currently know about UV light, its effectiveness in killing the virus that causes Covid-19, and how it can affect human skin.
There are different types of UV light
There are various kinds of UV light, all originating from the sun (or from UV light-emitting devices.). Only one type of UV light is likely to be effective in eradicating coronavirus—UVC.
If UVA and UVB sound familiar, it’s because they’re the culprit behind painful sunburns and premature aging. They have either no or negligible disinfection effectiveness, respectively, says Malley. UVC, which is largely absorbed by the earth’s ozone and other elements, is the true germ killer. It’s the same stuff the Metropolitan Transportation Authority in New York City is using to blast empty subway cars nightly to safeguard against Covid-19 transmission. (This is how to safely avoid coronavirus on public transit.)
UV light kills germs by damaging DNA or other genetic material, which also makes it a potential carcinogen when applied to the human body. While we are familiar with the damaging effects of UVA and UVB from sunlight (including skin cancer, cataracts, immune suppression, and more), UVC exposure is more dangerous and largely from man-made devices alone because it’s normally blocked by the earth’s atmosphere.
“UVC devices need to be [used in] fully enclosed [areas], or we need to vacate the area being dosed, or we need to have trained operations persons using the equipment in full protection including protection of all skin surfaces and eye protection,” says Malley. “So, using it on the subway cars along with the normal wet cleaning and dry cleaning can be a very effective approach to reducing viral load and to reducing the risk of disease transmission.”
Does UV light hand sanitizer work?
You may have seen products sold as UV light hand sanitizers to disinfect and kill germs on the hands. The World Health Organization (WHO) warns people against using UV lamps to disinfect hands or other areas of the body due to skin health concerns, like skin irritation and eye damage.
Instead, they suggest using an alcohol-based hand rub or washing your hands with soap and water to remove the virus that causes Covid-19. (This is what happens if you don’t wash your hands.) Echoing the WHO’s recommendations, Malley says, don’t even think about using UV light as a hand sanitizer of sorts.
“You should never, ever dose your skin, your eyes, or your pet [with UV light]. Really, you shouldn’t dose living tissue with ultraviolet of any kind, unless you’re in a medical setting, with a trained medical professional that’s using this treatment, and who knows exactly what they’re doing,” he says. “Because almost all UV—germicidal UV that we care about—will damage the skin and eyes of humans.” (Here are some sources of UV light you may be ignoring.)
While you should take extra care when handling UV exposure of any sort, the UV light most effective at killing coronavirus, UVC, is also most effective at injuring living tissue.
“That, in essence, is why it is a good disinfectant. Basically, if something does a good job at inactivating bacteria and viruses, it is working on the same building blocks of life—DNA, RNA, proteins, and lipids—that human cells are composed of too.” And the results can be devastating.
“The consensus is that UVC will damage human skin and human eyes particularly the cornea,” says Malley. “Continued or repeated exposures can lead to permanent eye damage or blindness and can lead to skin cancer, and the most concerning of those would be melanoma.”
How good are UV disinfecting devices?
During Covid-19, there has been an increase in relatively inexpensive UV light-emitting gizmos that claim to eradicate the novel coronavirus from your cell phone and surgical masks. But are they effective at killing coronavirus?
Malley says they’re likely duds when it comes to germ-killing potential. Be suspicious of anything less than a few hundred dollars, he says. If it doesn’t bathe your personal belonging in light, that’s another tell. What UV cannot touch, it cannot kill.
Malley says you can spot an effective device by determining if it has “line-of-sight technology.” Inside his high tech lab, Malley says he and his students test devices with heat-mapping technology. But even if you’re not a scientist, your naked eye will serve you well.
“Is it really clear that the mask or the cell phone or the car keys that you’re going to put in this device are going to get UV light all around them? On all sides, top, bottom, left, right, front, back? If anything in your product device looks like it would be in shadow, if it looks like the UV lamps are just on the side and none of them come down directly on the whole top of that cell phone, it is not going to get disinfected,” he says.
Can you use UV light to disinfect a face mask?
The answer is a qualified yes. If you are looking to disinfect your surgical mask, however, make sure it has the right kind of UV light. “We like to talk about 2,000 millijoules per centimeter squared. About 2,000 in the right-designed device does a darn good job on N95s and KN95 [masks].” (These are the 10 stylish face masks to buy for work.)
Those devices can retail between $400 to $3,500, he says. Another clue lies in the devices’ fine print, sales pitch, or user manual. Look for the phrases “UV dose” and for detail on the type of UV lamp.
“If they do not talk about UV dose or they don’t talk about anything but [length of] time [to disinfect], that’s a red flag,” adds Malley.
Keep in mind that in high-risk health care settings, experts don’t recommend sterilizing and re-using masks if at all possible (and definitely not if soiled). There are specific, multi-step guidelines for sterilization and re-use of N95 or similar respirator masks, according to an April study in medRxiv, a site for preprint studies that are not yet peer-reviewed, and the CDC also says they should not be re-used unless there is a shortage.
For cloth face masks, the CDC recommends washing after each use in the washing machine with regular laundry detergent, and drying on the highest heat setting.
So, what if you have a legit UV device for your home that works? “We really urge people to either not be in the room at all, turn a timer on, or if you’re in there with your UV device, that you’re pushing it around on a cart, and you’re in full protection.”
Do UV light-emitting wands work?
Malley has seen a flood of UV light-emitting wands on the market but he cautions that they are difficult to use correctly.
In a study Malley co-authored with his graduate student, which was published in the magazine UVSolutions, the duo found that one would have to hold a UV light-emitting wand 0.2 inches away from an object’s surface for 10 seconds to kill 99.99 percent of germs (in this case a common type of laboratory-grown bacteria).
The average user won’t be so stringent. Malley and his student found that the wand, when held three inches above the surface for the same period, killed only 9 percent of surface bacteria and at six inches away, killed no bacteria.
“I look at the product literature, and they’re waving this wand around like Darth Vader,” he says. “I always discourage anything that depends on human beings. I like things that have a stationary stand.”
Can sunlight kill coronavirus?
As we mentioned, the sunlight that reaches the earth’s surface contains UVA and UVB and sunlight has not been shown to prevent transmission of the virus that causes Covid-19. “You can catch Covid-19, no matter how sunny or hot the weather is,” according to the World Health Organization. “Countries with hot weather have reported cases of Covid-19.”
While being outside is thought to be safer in some cases than being inside an enclosed room with other people, that’s because of a greater ability to social distance and a lower risk of inhaling the respiratory droplets that can circulate in rooms with poor ventilation.
That said, one study of simulated sunlight published in May in the Journal of Infectious Diseases found that UVB light exposure for 6.8 minutes inactivated SARS-CoV-2 in simulated saliva and 14.3 minutes inactivated the virus in culture media. However, more study is needed and the amount of light needed to replicate the strength in the study is “dependent not only on the time of year, but also on the local weather conditions, especially cloud cover,” the authors note.
Practice social distancing, wear a mask, and clean surfaces
Perhaps surprisingly, Malley says he doesn’t use UV light-emitting devices to disinfect his personal belongings inside his home, although he does use a UV light-emitting device to kill the pathogens in his well water.
“I do go to work every day, and it’s kind of funny. I have my mask. I have my inexpensive face shield. I’ve got my bottle of hand sanitizer. We try to do the normal wet cleaning, dry cleaning.” And he practices strict social distancing.
“I don’t go near anyone,” he adds, “but my dog.” (Here’s what doctors need you to know about coronavirus face masks.)
- James Malley, University of New Hampshire, Durham, New Hampshire
- The National Academies of Sciences Engineering Medicine: "Does ultraviolet (UV) light kill the coronavirus?"
- The New York Times: "Trump Muses About Light as Remedy, but Also Disinfectant, Which Is Dangerous"
- World Health Organization: "Coronavirus disease (COVID-19) advice for the public: Mythbusters"
- Journal of Infectious Diseases: "Simulated Sunlight Rapidly Inactivates SARS-CoV-2 on Surfaces"
- MedRxIV: Rapid evidence summary on SARS-CoV-2 survivorship and disinfection, and a reusable PPE protocol using a double-hit process
- UV Solutions: COVID-19 Pandemic: UV Air and Surface Disinfection