In 2020, researchers discovered a vulnerability of the COVID-19 coronavirus to ultraviolet (UV) light — raising hopes of an additional precautionary countermeasure against the virus, according to a study published in the American Journal of Infection Control.
However, public confusion abounds surrounding how and where UV light should be used to disinfect surfaces — which aren't even a primary source of most COVID-19 infections. But UV light can cause serious harm to human organs, as cases surged in later 2020 of people developing UV-photokeratitis — damage to the eye after overexposure to ultraviolet light, according to another study published in the journal Ocular Immunology and Inflammation.
"Of all the spectrums [of ultraviolet light], UVC is more toxic for the microbes," said Doctor Guillermo Amescua, an ophthalmologist of the University of Miami, Florida in a telephone interview with Interesting Engineering. But "it's also more toxic for the outer surface of the cornea," in the human eye.
The danger is real, but the question remains: should we use UV light to kill the coronavirus?
UVC light can kill the coronavirus, but damages eyes
Scientists have long known the capacity of UV light to kill many germs since the 19th century — when the shorter wavelengths emitted by the sun were discovered, which are now called ultraviolet-C rays, or UVC light. UVC rays don't naturally reach the surface of the Earth, which means microbes never had the chance to evolve and adapt a sufficient defense against them. Neither have we.
"To protect the crystalline lens of the eye and the retina — the cornea has a mechanism to filter 99% of the UV that goes into the eye" — a roughly 50-micron layer called the cornea epithelium that protects the cornea from bacteria-ridden fluids. "When you lose the corneal epithelium, there's a chance that the microbes that live on the surface — part of the human microbiome — can enter the eye, and develop keratitis," explained Amescua.
When the news broke that UVC-emitting lamps could kill the coronavirus "managers wanted to protect employees" while they were in the office, said Amescua. And "as long as no one's there, it's no problem." But in the second study above, seven patients developed acute pain in their eyes after exposure to UV-emitting lamps. Luckily, five "of six patients endorsed complete resolution of symptoms within 2 to 3 days," according to the study. "Inadvertent exposure to suprathreshold levels of UV light can unfortunately cause damage to the ocular surface."
In a healthy eye, the cornea absorbs nearly all UVC rays. "If you look at the cornea — it has five layers," and a deep layer roughly 10 to 20 microns has cells "that are constantly pumping fluid out of the cornea — so the cornea is constantly dehydrated," said Amescua. To protect the cornea epithelial cells in broad daylight, most people wear sunglasses. "If you go skiing and you don't have any protection for UV light, you will accumulate a lot of UV toxicity, and the cornea will suffer."
"Cornea epithelial cells will die, and you may have multiple spots of epithelial inflammation." This is an area of the eye with "millions of nerve endings," which is why "when you scratch your cornea, it's extremely painful," Amescua said. This is why it's important to minimize your exposure to UV light. And "it's not just the eyes, it's the skin. When someone goes skiing, you have a less dense atmosphere, and the UV rays are penetrating easier, so you have to protect your skin and eyes." The classic case of someone developing photokeratitis is someone who went skiing without eye protection, or to a long day at the beach, where the water reflects for a long time. Either can create toxicity of the cornea.
Cases of photokeratitis from ultraviolet light have dropped
It's no wonder, then, that bringing the kind of UV light damage typically found on a bright beach or ski slope into a place of work, the eyes will suffer. "If you're staying in that room with UVC on — there's been cases in restaurants — one case it was left on and many customers were affected while they were eating," explained Amescua. UVC might be the most effective at killing the coronavirus, but it also has the most toxicity for the cornea.
The diagnosis of photokeratitis is fairly simple. "We use the yellowish dye, and that dye stains unhealth epithelium, so when you put that in you'll stain sick epithelium," said Amescua. Everyone should avoid being exposed to UVC light, but people with dry eyes or other ocular conditions are especially prone to corneal damage. "Dry eyes are more common in females and post-menopausal females, but the epithelium in male and female — they both have the same threshold for toxicity for UVC."
There was a significant spike in photokeratitis patients from UVC-emitting lamps in September and October of 2020. But Amescua hasn't heard of any additional cases so far in 2021. "I think people are being more careful," he said. This reduction in cases could be linked to the reduced push to sanitize surfaces — since surface contact has comprised a small minority of COVID-19 coronavirus infections. But if the drive to sanitize indoor surfaces is irresistible, there is no reason — except hatred of eyesight — to be around while the UVC lamps are switched "on."