Sunlight appears to inactivate the coronavirus which causes COVID-19, according to a study.

Scientists found imitation sunlight “rapidly inactivated” SARS-CoV-2 on stainless steel coupons in a lab. The findings were published in The Journal of Infectious Diseases.

Past studies have shown that SARS-CoV-2 can linger in the right conditions on non-porous indoor surfaces for days, the authors explained in their paper. One widely-referenced study published in the New England Journal of Medicine showed the germ can live for up to three days on plastic and steel, compared with 24 hours on cardboard, four hours on copper, and up to three hours as an aerosol.

However, in updated guidance published earlier this month that has made headlines in the past day, the U.S. Centers for Disease Control and Prevention said that the virus isn’t thought to easily spread from touching surfaces or objects.

To explore whether SARS-CoV-2 can survive in outdoor-like conditions, the team used a device which simulates natural sunlight, including ultraviolet rays. They also controlled the temperature and humidity in a lab chamber. The virus was grown both in lab culture and in a liquid resembling human saliva. The samples were then dried on stainless steel coupons.

The contaminated coupons were stuck to a mounting strip and attached to the wall of the chamber, and exposed to light for different lengths of time, ranging from 2 – 18 minutes. Another set of virus-laden coupons were put in the chamber for up to 60 minutes in darkness as the control.

Ultraviolet B (UVB) rays were found to quickly inactivate SARS-CoV-2, according to the team. In conditions resembling midday sunlight on the longest day of the year at 40 degrees north latitude, 90 percent of the virus was inactivated every 6.8 minutes in the saliva.

Sunlight representing winter solstice in the same latitude inactivated the virus every 14.3 minutes in saliva. SARS-CoV-2 was inactivated at rates twofold greater in saliva than in the culture media for reasons that weren’t immediately clear. The virus on the coupons kept in the dark, meanwhile, barely changed.

The findings indicate that the virus’ ability to spread may be “significantly reduced” in outdoor conditions when exposed to direct sunlight, compared to indoor conditions, the authors wrote. “Additionally, these data provide evidence that natural sunlight may be effective as a disinfectant for contaminated non-porous materials,” they said.

But the team stressed that results could differ in real-world scenarios. They said: “While significant levels of viral inactivation were observed within minutes at all simulated sunlight levels investigated, it should be noted that the duration of time each day that outdoor UVB levels exceed those used in the present study is dependent not only on the time of year, but also on the local weather conditions, especially cloud cover.

“Thus, it is possible that significant day-to-day variability may exist in the persistence of SARS-CoV-2 on surfaces in outdoor environments.”

The team noted they used one size of virus droplet, and studies in the future could explore how this could affect deactivation. And while they tried their best to simulate saliva, it may differ to that of an infected person and skew the results.

To reveal the full risk of exposure to SARS-CoV-2 outdoors, more research is needed, looking at variables such as viral load on different surfaces, how easily the virus passes from objects, and the amount of virus needed to cause an infection, they said.

Emeritus Professor Ron Eccles of the U.K.’s Cardiff School of Biosciences, an expert on the nose and upper airways, told Newsweek: “The study is interesting and adds some new knowledge to our understanding of how UV light works on viruses out of doors.

“It has been known for many years that UV light inactivates viruses and bacteria and UV light has been used to help sterilize hospital operating theatres when not in use. The new knowledge is that UV light of the intensity seen on a sunny beach does inactivate viruses such as SARS COV-2 and this is important as it means we are safer outside than in any indoor public spaces.

“‘Stay at home’ should be replaced with ‘stay outdoors in the sunshine but use suncream and keep your distance from others as viruses can still spread from coughs and sneezes.'”

This week, five months into the COVID-19 pandemic, the total number of confirmed cases hit 5 million, according to Johns Hopkins University. A total of 333,001 people have died of COVID-19, and more than 1.9 million are known to have recovered. The U.S. is the country with the most known COVID-19 cases, as the graph by Statista shows.

covid countries may 20
This graphic shows the ten countries with the highest number of coronavirus cases as of May 20.
Statista

This article has been updated with comment from Professor Ronald Eccles.

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