Preliminary results from government lab experiments show that the coronavirus does not survive long in high temperatures and high humidity, and is quickly destroyed by sunlight, providing evidence from controlled tests of what scientists believed — but had not yet proved — to be true.

A briefing on the preliminary results, marked for official use only and obtained by Yahoo News, offers hope that summertime may offer conditions less hospitable for the virus, though experts caution it will by no means eliminate, or even necessarily decrease, new cases of COVID-19, the disease caused by the coronavirus. The results, however, do add an important piece of knowledge that the White House’s science advisers have been seeking as they scramble to respond to the spreading pandemic.

The study found that the risk of “transmission from surfaces outdoors is lower during daylight” and under higher temperature and humidity conditions. “Sunlight destroys the virus quickly,” reads the briefing.

While that may provide some good news about the outlook for outdoor activities, the Department of Homeland Security briefing on the results cautions that enclosed areas with low humidity, such as airplane cabins, “may require additional care to minimize risk of transmission.”

In a statement to Yahoo News, the DHS declined to answer questions about the findings and strongly cautioned against drawing any conclusions based on unpublished data.

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“The department is dedicated to the fight against COVID-19, and the health and safety of the American people is its top priority. As policy, the department does not comment on allegedly leaked documents,” the DHS said in a statement. “It would be irresponsible to speculate, draw conclusions, or to inadvertently try to influence the public based upon a document that has not yet been peer-reviewed or subjected to the rigorous scientific validation approach.”

The results are contained in a briefing by the DHS science and technology directorate, which describes experiments conducted by the National Biodefense Analysis and Countermeasures Center, a lab created after the 9/11 terrorist attacks to address biological threats.

While the DHS describes the results as preliminary, they may eventually make their way into specific recommendations. “Outdoor daytime environments are lower risk for transmission,” the briefing states.

Simulated sunlight “rapidly killed the virus in aerosols,” the briefing says, while without that treatment, “no significant loss of virus was detected in 60 minutes.”

The tests were performed on viral particles suspended in saliva. They were done indoors in environments meant to mimic various weather conditions.

While the results of these tests have not been previously made public, Harvey Fineberg, head of the National Academies Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats, broadly described plans to conduct the experiments in an April 7 letter to the White House.

In that letter, addressed to President Trump’s top science adviser Kelvin Droegemeier, Fineberg wrote that the DHS lab “is well suited for the kinds of studies they have planned, and the scope and relevance are noteworthy. In particular, they plan to create simulated infected body fluids, including saliva and lower respiratory secretions.”

Droegemeier’s office did not respond to a request for comment on whether it has received the latest results from the DHS. The National Academies also did not respond to a request for comment.

While the lab results are new, scientists for many weeks have predicted, based on available data on the disease’s spread, that warmer, wetter climates would be less hospitable to the spread of the coronavirus. An early analysis by scientists observed that the virus was spreading more slowly in countries with warmer climates.

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