Sunlight, humidity ‘unlikely’ impacts on how COVID-19 spreads, Homeland Security study finds

Certain environmental conditions like heat and humidity do not appear to significantly impact how COVID-19 transmits, according to a recent study by the U.S. Department of Homeland Security.

Researchers sought to determine if environmental conditions like humidity and sunlight impacted the way variants of the novel coronavirus spread but found that these conditions did not affect how the virus survives. 

"There is still a lot for scientists to learn about SARS-CoV-2, the variants that have emerged, and what contributes to their transmission in the community," Homeland Security Science and Technology Directorate (S&T) researcher Dr. Lloyd Hough said in a press release. "But this research shows that the stability of these variants in the environment is about the same and that the risk assessments and tools that S&T produced early in the pandemic are still applicable."

Researchers tested isolated variant samples inside a rotating chamber where they exposed the samples to environmental conditions like simulated sunlight and humidity. 

 "The researchers determined that while certain variants may spread faster or be more lethal, they survive similarly in the environment, and therefore differences in transmissibility are likely not due to differences in aerosol stability," according to a release by Homeland Security. 

During the onset of the COVID-19 pandemic, many unfounded theories on how to treat the coronavirus spread like the disease itself, creating an outbreak of misinformation. 

At one press conference, former President Donald Trump hinted at unfounded claims that sunlight, heat and humidity can destroy the virus.

"I hope people enjoy the sun. And if it has an impact, that’s great...and if heat is good, and if sunlight is good, that’s a great thing as far as I’m concerned," he said.

Sunlight is known for improving mood and outdoor exercise is recommended during the pandemic’s social isolation, but there has never been any proof it will make the pandemic go away. 

Ultraviolet light is used for disinfecting masks and other medical equipment, but it hasn't been proven to be safe or effective for use on people to try to eliminate a virus, said Dr. Rais Vohra, an emergency medicine doctor at the Fresno branch of the University of California, San Francisco.

"For inanimate objects, it does make sense," but exposing yourself to ultraviolet light outside or from other sources can raise the risk of skin cancer, he said.

Dr. Michael Ryan, the World Health Organization’s emergencies chief, said in March 2020 that, "it’s a false hope to say, ‘yes, it will just disappear in the summertime like influenza.’"

Trump said early in the outbreak he expected the pandemic to end with the warmer weather of April 2020.

Previous research also indicates that some environmental factors do contribute to the spread of COVID-19. According to  Dr. William Hanage, an epidemiologist professor at Harvard University, the risk of transmission of the novel coronavirus is higher indoors.

As people cranked up their air conditioners amid record-breaking temperatures last summer, Edward Nardell, a professor at Harvard Medical School and an infectious disease expert, suggested in July that air conditioning use across the southern U.S. may be a factor in spiking COVID-19 cases.

"It is, but not necessarily for the ways you’re thinking," Nardell pointed out. "Because of air conditioning and excessive heat, people are indoors and re-breathing each other’s air."

RELATED: Air conditioners could be aiding the spread of COVID-19 indoors, epidemiologists say

Nardell presented data ( that surveyed places in countries that had the biggest increases in COVID-19 over time.

"The places that had the need for most air conditioning, have had the biggest increases in COVID," Nardell said. But Nardell noted that the data could have other correlates, and did not necessarily imply causation.

A study by the University of Maryland (UMD) showed that some air conditioning units could spread the virus, according to Don Milton, a professor of environmental health at UMD. His findings, which were published by the university’s school of public health, suggested that air conditioning can blow around infected droplets hanging in the air.

"Outbreaks — where you have a bunch of people infected all at once like that — are almost exclusively occurring indoors in poorly ventilated environments," Milton explained.

This story was reported in Los Angeles. Stephanie Weaver and The Associated Press contributed to this story.