Can artificial sunlight kill the COVID-19 causing virus?
Researchers believe that simulated sunlight — which has UVB radiation in the range of 280-315 nm — can inactivate the COVID-19 causing virus on surfaces.
So far, we know that SARS-CoV-2, the causative agent of COVID-19, can stay viable for various durations on surfaces. Fomites (contaminated surfaces) are being considered a potential source of infection spread and cleaning and disinfecting high-touch surfaces is being suggested as one of the preventive measures against the disease.
A previous study had suggested that UVC in the range of 254 nm can be used to kill the virus.
Now, a group of researchers in the National Biodefense Analysis and Countermeasures Center, Operated by BNBI for the US Department of Homeland Security say that simulated sunlight — which has UVB radiation in the range of 280-315 nm — can quickly inactivate the COVID-19 causing virus on surfaces.
For the study, the research team infected a cell culture in the lab with the SARS-CoV-2 virus at a temperature of 37°C and 5% CO2. After a significant amount of viral titer was obtained, the virus was suspended in an artificial saliva solution or culture media and five droplets of 5mcL of the suspension were deposited on the surface of 19mm stainless steel coupons (rectangular plates of metal).
The droplets were let to dry for about half an hour at about 24°C and 29-30% relative humidity, then put in a chamber with a quartz window for allowing in simulated (artificial) sunlight.
Various filters and power supplies were used to control the intensity of the sunlight and the coupons were exposed to various durations, ranging from 2 to 18 minutes in the light. The temperature and relative humidity in the chamber were maintained within a very narrow range of 16-24°C and 14-24% respectively.
Results from the study
The study results showed that about 90% of the virus was inactivated every 6.8 minutes in the artificial saliva droplets and every 14.3 minutes in the culture media when they were exposed to simulated sunlight that corresponded to the summer solstice at a 40-degree latitude above sea level on a clear day (without clouds). Summer solstice or midsummer is the day in the year when one of the poles of the earth has a maximum tilt towards the sun. It is the longest day of the year.
Even though significant inactivation occurred at lower sunlight levels too but it was at a much slower rate. The inactivation time was 14.3 minutes for stimulated sunlight corresponding to the winter solstice at 40-degree latitude. Winter solstice is the shortest day of the year and occurs when a pole of the earth has its maximum tilt away from the sun.
Things to consider
Until now we did not have any proof of the effectiveness of sunlight on SARS-CoV-2. It was even mentioned as one of the myths in the WHO myth busters.
This study indicated that outdoor transmission through fomites may be less of a problem than indoor transmission since the virus may be inactivated by sunlight. However, the researchers pointed out that local weather conditions and cloud cover can have a role in the effectiveness of sunlight. So one day the fomites may have higher levels of the virus and on another day it may be less.
Also, the study only included 5mcL droplets, while it is known that the droplet size may vary and can determine the survival of the virus on a surface.
More studies are needed to understand the relation between droplet size and their persistence on various surfaces to fully understand the impact of sunlight exposure on viral inactivation.
For more information, read our article on How long can SARS-COV-2 survive on different surfaces
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