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As the global COVID-19 pandemic continues, new ways of battling SARS-CoV-2, the virus that causes the disease, are needed, especially in areas where vaccine distribution is low. Sunlight is an inexpensive and widely-available tool for disinfection, but may require too much time to be effective. Researchers have discovered a discrepancy between the expected and actual rates of ultraviolet (UV) sterilization for the SARS-CoV-2 virus, although the reason for this discrepancy is unknown.

Sunlight contains a spectrum of wavelengths of UV radiation including UVA and UVB light, which directly damage viral RNA and inactivate viruses. The time it takes for sunlight to sterilize a given virus depends on the size of the viral genome and the wavelength of light. In a 2005 report from the US Army, researchers created an equation for calculating the sunlight sterilization times for various viruses in a variety of geographic locations.

The same group of researchers used their model to estimate the expected time to 90 percent sterilization of the SARS-CoV-2 virus for various locations within and outside the US during different seasons. A second group of researchers from the National Biodefense Institute conducted an experimental study in which they measured the [90 percent sterilization time] of the SARS-CoV-2 virus (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7313905/) suspended in simulated saliva or laboratory media and dried on stainless steel using artificial UVB light.

When compared to the estimates produced by the first group of researchers, actual time to sterilization for SARS-CoV-2 virus in simulated saliva was eight times faster than expected. When exposed to UV light at a level one would expect at a latitude of 40 degrees north at noontime during the summer, the 90 percent sterilization time was 6.8 minutes. Sterilization time for the virus in laboratory media was three times faster than expected at 14.3 minutes.

Both groups of researchers concluded that sunlight could be an important tool in fighting the spread of the SARS-CoV-2 virus, but factors like latitude, season, and weather all affect the time required for sterilization. They cautioned that low sunlight exposure in many northern cities during the winter months may lengthen the 90 percent sterilization time. Further research is needed to understand why sunlight sterilization is more effective against the SARS-CoV-2 virus than originally estimated.

Link to full study.

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