A team of scientists is calling for more research into how sunlight deactivates SARS-CoV-2 after finding a clear discrepancy between recent theory and experimental results.
UC Santa Barbara mechanical engineer Paolo Luzzatto-Fegiz and colleagues observed that the virus was inactivated in experiments as much as eight times faster than predicted by the latest theoretical model.
“The theory assumes that inactivation works by UVB hitting the RNA of the virus and damaging it,” Luzzatto-Fegiz explained.
But the discrepancy suggests that something more than that is happening, and figuring out what it is can be helpful in controlling the virus.
Some nucleic acid bases in DNA and RNA easily absorb UV light or the ultraviolet portion of the spectrum, making them able to bind in ways that are difficult to repair.
However, not all UV lights are the same. Longer UV waves, called UVA, do not have enough energy to cause problems. Medium-range UVB waves in sunlight are primarily responsible for killing microbes and compromising one’s own cells.
Shortwave UVC radiation has been shown to be effective against viruses such as SARS-CoV-2, although it is still safely wrapped in human fluids.
But this type of UV does not usually come into contact with the earth’s surface due to the ozone layer.
“UVC is great for hospitals,” said Oregon State University co-author and toxicologist Julie McMurry. “But in other environments, like kitchens or subways, UVC would affect the particles to cause harmful ozone.”
In July 2020, an experimental study tested the effects of UV light on SARS-CoV-2 in simulated saliva. The virus was found to be inactivated when exposed to simulated sunlight for 10 to 20 minutes.
“Natural sunlight can be effective as a disinfectant for contaminated non-porous materials,” Wood and colleagues concluded in the article.
Luzzatto-Feigiz and team compared these results to a theory of how sunlight achieved this, which was published only a month later and found that the math did not add up.
This study showed that SARS-CoV-2 virus is three times more sensitive to UV in the sun than influenza A, with 90 percent of coronavirus particles being inactivated for half an hour after half an hour of exposure to sunlight in summer.
For comparison: in winter, infectious particles can remain intact for several days.
Environmental calculations performed by a separate group of researchers found that RNA virus molecules are photochemically damaged directly by light rays.
This is more strongly achieved with shorter light wavelengths such as UVC and UVB. Since UVC does not reach the earth’s surface, the calculations of exposure to light in the environment were based on the medium-wave UVB part of the UV spectrum.
“Experimentally observed inactivation in simulated saliva is more than eight times faster than one would expect from theory,” Luzzatto-Feigiz and colleagues wrote.
“So scientists don’t know yet what’s going on,” Luzzatto-Fegiz said.
The researchers suspect that it is possible that UVA, instead of directly affecting RNA, works with molecules in the test medium (simulated saliva) in a way that accelerates virus inactivation.
Something similar is observed in wastewater treatment – where UVA reacts with other substances to create molecules that damage viruses.
If UVA can be harnessed to combat SARS-CoV-2, inexpensive and energy-efficient wavelength-specific light sources can be useful to increase air filtration systems with a relatively low risk to human health.
“Our analysis suggests the need for additional experiments to separately test the effects of specific light wavelengths and mean composition,” concludes Luzzatto-Fegiz.
Because this virus is able to stay in the air for extended periods of time, the safest way to avoid it in countries where it is rampant is still social distancing and wearing masks where distancing is not possible. But it’s nice to know that sunlight may help us in the warmer months.
Their analysis was published in Ljubljana Journal of Infectious Diseases.