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The ads are everywhere - germicidal ultraviolet lights that can disinfect surfaces and perhaps help stop transmission of coronavirus.

But how well do they work? Maybe not as well as people may think, the Food and Drug Administration says.

Spotty, limited use

Lamps that use ultraviolet light to kill germs can inactivate coronavirus, but they are not always safe and it’s not clear how good a job they do at killing the virus, the FDA says on a newly posted advisory.

Ultraviolet light is part of the electromagnetic spectrum emitted by the sun and can be produced by light bulbs, also. UVC rays are absorbed by the ozone in the Earth’s atmosphere ozone, but rays of UVA and UVB do reach the Earth’s surface and can cause sunburn and skin cancer if people get too much.

The FDA said UVC wavelengths are better than UVA and UVB light at destroying viruses, but UVC lamps have their limits.

“The effectiveness of UVC lamps in inactivating the SARS-CoV-2 virus is unknown because there is limited published data about the wavelength, dose, and duration of UVC radiation required to inactivate the SARS-CoV-2 virus,” the FDA said in a newly posted statement.

Plus the lamps only work in limited circumstances, which don’t mimic many real life situations.

“It is important to recognize that, generally, UVC cannot inactivate a virus or bacterium if it is not directly exposed to UVC. In other words, the virus or bacterium will not be inactivated if it is covered by dust or soil, embedded in porous surface or on the underside of a surface,” the FDA said.

There are real dangers

There’s been an explosion of products on offer to fight coronavirus, including germicidal lamps. The World Health Organization warns against trying to use them to disinfect human skin, including the hands.

If a light is going to inactivate a virus, it takes both time and intensity to do so. A quick flash of weak light is not going to do any harm to a virus, and if it is strong enough to take apart a virus, it could damage human skin and especially tender eyes.

“Direct exposure of skin and eyes to UVC radiation from some UVC lamps may cause painful eye injury and burn-like skin reactions,” the FDA cautions. “Never look directly at a UVC lamp source, even briefly.”

In addition, some UVC lamps generate ozone, which can irritate airways. “UVC can degrade certain materials, such as plastic, polymers, and dyed textile,” the FDA added.

“Some UVC lamps contain mercury. Because mercury is toxic even in small amounts, extreme caution is needed in cleaning a lamp that has broken and in disposing of the lamp.”

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Pulsed xenon lamps can be used to disinfect hospital rooms, but the strong light means they are used when people are not in the room, the FDA said.

LED lamps can produce UV radiation but they don’t cover much area, making them “less effective for germicidal applications,” the FDA said.

CNN’s Sandee LaMotte contributed to this story.