Sunshine might be an Achilles heel of SARS-CoV-2. It’s been known for some time that the coronavirus that causes COVID-19 is quickly destroyed on a surface that’s doused in simulated sunlight, but a team of scientists now argues the virus may be even more susceptible to ultraviolet radiation than previously appreciated.
In July 2020 an important study found that simulated sunlight “rapidly inactivates SARS-CoV-2” on surfaces. By their estimates, 90 percent of the SARS-CoV-2 virus was inactivated every 6.8 minutes in simulated saliva when exposed to simulated sunlight representative of a clear summer's day at sea level. The following month, another study produced a theoretical model that described the sunlight inactivation of SARS-CoV-2.
However, there is some discrepancy between these results, according to a team of researchers from UC Santa Barbara, Oregon State University, University of Manchester, and ETH Zurich writing in the Journal of Infectious Diseases. They explain that the lab experiments show sunlight inactivation that’s several times faster than predicted by theory. In fact, viruses were inactivated more than eight times faster in the experiments than would have been predicted by the theory.
To explain this gap, they argue that we need to look beyond Ultraviolet B (UVB), the higher energy ultraviolet light associated with skin burning, and start paying more attention to Ultraviolet A (UVA), the lower energy component of sunlight associated with skin aging.
"The theory assumes that inactivation works by having UV-B hit the RNA of the virus, damaging it,” Paolo Luzzatto-Fegiz, lead author from the Department of Mechanical Engineering at the University of California Santa Barbara, said in a statement.
"People think of UV-A as not having much of an effect, but it might be interacting with some of the molecules in the medium," added Luzzatto-Fegiz.
This all just conjecture for now. The researchers didn’t carry out any modeling or experiments for themselves, but simply highlight the peculiar discrepancy between the data and the theory.
"So, scientists don't yet know what's going on," Luzzatto-Fegiz added; "Our analysis points to the need for additional experiments to separately test the effects of specific light wavelengths and medium composition."
If they are on the money, it could be promising news. Some hospitals and other high-risk environments disinfect their air using the power of UVC, which has even higher energy than UVB. However, this wavelength is largely absorbed by the Earth’s ozone layer and does not reach the surface, meaning it must be artificially created.
“UV-C is great for hospitals," adds co-author Julie McMurry. "But in other environments -- for instance kitchens or subways -- UVC would interact with the particulates to produce harmful ozone.”
On the other hand, UVA is safe and easy to generate with inexpensive LED bulbs that are many times more potent than natural sunlight. If UVA actually is the missing piece of the puzzle, then it could be easily implemented into air filtration systems and disinfecting methods to slow the spread of COVID-19 in high-risk spaces.