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Can Sunlight, Heat, And Humidity Really Slow The Spread Of Coronavirus?


An empty playground is closed and wrapped in yellow caution tape during the Covid-19 coronavirus pandemic in Canada in March 2020. TamasV/Shutterstock

Editorial update as of April 27, 2020: Following the publication of this story, IFLScience received a response from the Department of Homeland Security linking to a newly published fact sheet and containing the following statement: 

"The Department of Homeland Security Science and Technology Directorate continues to study the impacts of environmental conditions on the coronavirus that causes COVID-19. While the results are still undergoing a rigorous scientific review, we felt it important to share information on the emerging trends that are being identified in our tests."

A leading health expert said at a White House coronavirus task force briefing that preliminary scientific evidence that has yet to be released suggests that heat and sunlight may help slow the spread of SARS-CoV-2, the novel coronavirus responsible for the respiratory disease Covid-19.

“If you look at the coronavirus as a chain with many links, what we’ve done through our study is we’ve identified some of the weak links in that chain, that the virus – the transmission of the virus depends upon,” said William Bryan of the Science and Technology Directorate at the Department of Homeland Security, noting that heat, humidity, sunlight, solar light, and UV rays may be the “weakness in that chain.”


Researchers at the National Biodefense Analysis and Countermeasures Center, a part of Homeland Security, have reportedly been studying the biology of the virus since February. IFLScience contacted the center to confirm the findings and request a copy of the preliminary study but has not received a response at the time of publication. Information about the study was not readily available on the DHS weekly Covid-19 update nor its response page. Though he did not provide a specific timeline as to when the findings will be made available, Bryan said there is evidence that UV rays can “kill” the virus when it comes into direct contact.

“Our most striking observation to date is the powerful effect that solar light appears to have on killing the virus – both surfaces and in the air.  We’ve seen a similar effect with both temperature and humidity as well, where increasing the temperature and humidity or both are generally less favorable to the virus,” he said.

There are three common forms of UV radiation in sunlight, which the US Food and Drug Administration notes produce UVA, UVB, and UVC – the latter being the “the most harmful one” due to its ability to destroy genetic material, virologist Juan Leon told NPR. The World Health Organization suggests that some UV rays may be good candidates for destroying some pathogens and similarly, research on SARS-CoV, a coronavirus closely related to SARS-CoV-2, found that UVC rays hold promise in destroying the virus.

Even so, the findings still don’t mean that coronavirus may simply go away during the warmer months.


“It would be irresponsible for us to say that we feel that the summer is just going to totally kill the virus and that if it’s a free-for-all and that people ignore those guidelines. That is not the case,” said Bryan.

A growing body of evidence confirms a link between higher temperatures and decreased transmission rates of coronavirus, but experts warn that this connection is not to be confused with whether heat or UV rays destroy the virus. As seen in the past, flu and cold viruses peak in colder months when people tend to gather inside in close quarters. By comparison, rates tend to decrease in the summer as schools and universities let out and people spend more time outside. The findings also don’t explain why warmer states and countries, such as Louisiana and parts of South America, continue to see the spread of the virus

“This is just another – another tool in our tool belt, right? Another – another weapon in the fight that we can add to it and, in the summer, we know that summer-like conditions are going to create an environment where the transmission can be decreased. And that’s an opportunity for us to get ahead,” said Bryan, adding that the new information should be a factor in the decision-making processes that governors should take into account when considering whether to reopen their states. However, the findings don’t take away from other social distance guidance policies set forth by the Centers for Disease Control and Prevention.


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