President Trump during a White House press briefing Thursday touted recent studies indicating that heat and sunlight can slow the spread of the novel coronavirus. (Michael Reynolds/EPA/Bloomberg)
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President Trump and Vice President Pence showcased emerging laboratory evidence on Thursday that suggests the spread of the novel coronavirus may ebb during the summer months, owing to how the virus interacts with ultraviolet light as well as heat and humidity.

At the daily press briefing of the White House coronavirus task force, William N. Bryan, the acting undersecretary for science and technology at the Homeland Security Department, detailed recent lab studies carried out by the agency at the U.S. Army’s high-level biosecurity laboratory at Fort Detrick, Md.

The results, which have not been peer reviewed but were briefed to the press and on live television via slides, largely match other laboratory studies and the suspicions of some researchers by showing that the novel coronavirus, like many other viruses, does not survive as long on certain surfaces and in the air when exposed to high amounts of ultraviolet light and warm and humid conditions.


William N. Bryan, the acting undersecretary for science and technology at the Homeland Security Department, details recent lab studies carried out at Fort Detrick, Md. (Drew Angerer/Getty Images)

The laboratory results show that increases in temperature, humidity and sunlight all can speed up how fast the virus is destroyed, based on measurements of its half-life when exposed to these elements.

The half-life is a measurement of the time it takes for a given amount of the virus to become reduced by half.

“So if we look at an 18-hour half-life, what you’re basically saying is that every 18 hours, the life of the virus is cut in half. So if you start with a thousand particles of the virus, in 18 hours, you’re down to 500, in 18 hours after that, you’re down to 250, and so on and so forth,” Bryan explained at the news conference.

A slide shared by Bryan revealed the half-life of the virus, in the absence of sunlight (indoors), lowers from 18 hours to one hour when the temperature rises from around room temperature (70 to 75 degrees) to 95 degrees and the humidity increases from 20 percent to 80 percent.


A slide from Bryan’s presentation details results from recent lab experiments of coronavirus survival under different environmental conditions.

This finding applied to the virus in contact with nonporous surfaces such as door handles. Adding in sunlight, the virus’s half-life decreases from six hours to two minutes at temperatures from 70 to 75 degrees and humidity of 80 percent. “That’s how much of an impact UV rays has on the virus,” Bryan said.

The laboratory experiment also tested how the virus decays when exposed to various elements while suspended in the air. When the airborne virus at temperatures between 70 and 75 degrees is exposed to sunlight, its half-life decreases from around 60 minutes before exposure to 1.5 minutes after.

Bryan summarized: “Within the conditions we’ve tested to date, the virus in droplets of saliva survives best in indoors and dry conditions. … The virus dies quickest in the presence of direct sunlight.”

Bryan said the results of these experiments can “support practical decision-making” to reduce the risk of infection.

“Increasing the temperature and humidity of potentially contaminated indoor spaces appears to reduce the stability of the virus,” he said. “And extra care may be warranted for dry environments that do not have exposure to solar light.”

A slide presented by Bryan also recommended moving activities outside.


A slide from Bryan’s presentation provides recommendations for how to reduce the risk of spreading the novel coronavirus based on environmental conditions.

The study was conducted under idealized conditions and in a controlled setting. Bryan said that in the real world, the virus on a playground surface exposed to direct sunlight would die quickly, but the virus could survive longer in shaded areas.

If the summer months reduce the transmission rates of the virus, that would help officials’ efforts to squelch its spread without resorting to drastic mitigation measures, such as stay-at-home orders, that have had massive economic repercussions.

“It would be irresponsible for us to say summer will kill the virus,” Bryan said, calling summer conditions “another tool in toolbox” to use against the virus.

Outside experts contacted by the Washington Post said the results suggest that the coronavirus behaves similarly to the flu in that it is most efficient at spreading during the colder months, when people are indoors and the air is cold and dry.

“If true, these results would point to preferred transmission in winter indoors (where we spend 90 percent of our time) when the air is dry. It suggests that in summer we could see some drop in virus activity in the Northern Hemisphere (again indoor humidity — both [relative humidity] and [absolute humidity] — is higher in summer),” said Jeffrey Shaman, a leading infectious disease expert at Columbia University, in an email.

Shaman cautioned that these results have not been published in a peer reviewed journal, and should be treated with some skepticism.

Likewise, David Heyman, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine and current chair of the World Health Organization’s advisory committee on infectious hazards, said it’s difficult to evaluate the results without them being published, and that some mitigation measures should be maintained even if they’re correct.

“This is another laboratory study but the interest is what happens in humans — the virus continues to circulate in Singapore in hot and humid conditions, and the precautionary measures in place (keeping surfaces clean and washing hands) that consider surfaces contaminated and a potential source of self-infection by hands touching the face should remain until proven otherwise, because this is [the] best practice in public health,” he said via email.

Trump said the results vindicate him for his much-criticized suggestions that the virus would ebb with the arrival of warm weather. “I just threw it out as a suggestion, but it seems like that’s the case, because when it’s on a surface, that would last for a long time. When that surface is outside, it goes very quickly. It dies very quickly with the sun,” Trump said.

When he made those comments in February, the virus was spreading quickly, going largely undetected in many communities, and the administration was far behind in distributing functioning test kits as well as supplies for hospitals that would soon see a flood of new cases.

“[Y]ou know, a lot of people think that [the virus] goes away in April with the heat — as the heat comes in. Typically, that will go away in April,” Trump told a gathering of governors at the White House on Feb. 10.

Trump said he’s not giving people advice to go outside to stay free from the virus, but said, “I hope people enjoy the sun, and if it has an impact that’s great.”

The caveats

The weather is no panacea when it comes to the coronavirus pandemic, considering that warm states, such as Georgia and Florida, already are seeing significant outbreaks, as are warm and humid countries, including Singapore. Even if the virus were to wane during the summer, a dreaded second wave would still be likely in the fall, as has happened with past pandemic flu outbreaks.

Earlier this month, a panel convened by the National Academies of Sciences reported to the White House that the pandemic is unlikely to wane substantially with the arrival of summer, though there are many uncertainties remaining. The new lab study, which is directed toward the same NAS group and the White House science adviser, may help reduce some of those uncertainties.

That report pointed to shortcomings in the studies published so far that trace the spread of the coronavirus and connect the pattern of spread to temperature and humidity, stating they “should be interpreted with caution.”

For example, one such study published in March by MIT researchers found 90 percent of the coronavirus transmissions so far have occurred within a specific temperature (37 to 63 degrees) and absolute humidity range. For areas outside this zone, the virus is still spreading, but more slowly.

The NAS report states: “There is some evidence to suggest that SARS-CoV-2 may transmit less efficiently in environments with higher ambient temperature and humidity; however, given the lack of host immunity globally, this reduction in transmission efficiency may not lead to a significant reduction in disease spread” without mitigation measures, such as social distancing.

At the news conference, Trump ventured far beyond the lab study by speculating about using the virus’s vulnerability to heat and ultraviolet light to treat patients. “Supposing you brought the light inside the body, which you can do either through the skin or in some other way,” Trump said to Bryan, who is not a medical doctor. He suggested to Bryan that he test it.

Colby Itkowitz contributed to this report.