The germicidal action of the sun exists because the star emits ultraviolet radiation. There are some types of these rays, divided by the wavelength with which they travel through the environment. Type C ultraviolet (UVC) is the most powerful and can eliminate more than 90% of the new coronavirus housed on surfaces in a few seconds, according to experiments carried out in several laboratories with special lamps.
Ultraviolet radiation acts on the genetic material of the virus, causing damage that prevents the microorganism from multiplying, making it unable to start an infection. But the UVC emitted by the sun is retained in the atmosphere, before reaching the ground. And just as well: this type of radiation is harmful to people, animals and plants that receive the rays directly for a longer period, and can cause burns to cancer.
Devices that use artificial UVC emitted by special lamps for disinfecting environments are already sold in Brazil and used in different parts of the world for the elimination of microorganisms in public transport and indoors. From the sun, ultraviolet rays of type A and B (UVA and UVB) reach the surface of the planet. UVB, which reaches the soil in less quantity, has some germicidal action, although with power thousands of times lower than that of UVC rays.
United States scientists carried out a laboratory simulation with lamps that mimic sunlight at sea level on a clear day. The researchers dropped the radiation on a saliva-like substance containing Sars-Cov-2 and a cell culture containing the virus.
According to the results of the experiment, 90% of the viruses were inactivated in about seven minutes in the false saliva and in approximately 14 minutes in the cell culture. The study was published in May in the scientific journal The Journal of Infectious Diseases, linked to the American Society of Infectious Diseases (IDSA). These results were obtained under ideal conditions, disregarding the presence of clouds or blockages caused by pollution, present in real situations.
Another article published earlier this month by United States researchers in the journal Photochemistry and Photobiology used theoretical calculations to estimate the deactivation of the new coronavirus by sunlight in different locations on the planet. On average, more than 90% of the virus would be inactivated within 34 minutes for most cities receiving midday sunlight in the summer. According to the researchers’ estimate, the deactivation of the virus in São Paulo on a winter day would take about 41 minutes, for a midday sun.
The authors of the text, two retired researchers, did not carry out any experiments with Sars-Cov-2 and used a model they developed more than ten years ago to calculate the inactivation of different viruses by sunlight. The model for the calculations uses data from the sensitivity to ultraviolet radiation presented by several viruses.
In the text, which uses a Wikipedia page as one of their references, which is unusual in scientific articles, the authors argue that social detachment and quarantine imposed by governments to minimize the transmission of the new coronavirus may have caused harm to the population by preventing greater exposure to the sun.
The scientists did not respond to questions sent by the report asking for details of the study, but Jose-Luis Sagripanti, one of the authors, sent a signed opinion article in which he questions the effects of the quarantine imposed by governments.
Brazilian researchers in the field who analyzed the article found a strong bias in the study. For Tania Mateus Yoshimura, postdoctoral fellow at Ipen (Institute of Energy and Nuclear Research) in the area of applying light to health, it is problematic to take this result to the letter.
“It is an estimate. To really know if the virus has that sensitivity to light, we need experiments. There is still no conclusive data on the effect of UVC, much less for the use of UVB”, says the researcher. Viral particles are in the air, usually wrapped in organic matter, such as saliva or nasal secretion, for example, that would act to protect the virus from receiving radiation directly on the genetic material, recalls Yoshimura. “The study was done with the assumption of a direct action on the genetic material”, he adds.
“Even if inactivation by the sun is possible, it does not help much to be outside and someone coughs or sneezes nearby; the virus will not be waiting half an hour in the air until it is inactivated”, says the scientist. In addition, she recalls that people who walk the streets do not stay under the sun all the time. “They will take public transport, enter indoors. In these cases, the germicidal action of the sun makes no difference,” he says.
For Caetano Padial Sabino, a doctoral student at USP and a researcher on the applications of light for health, the statement circulating on social networks that the risk of infection with the new coronavirus is lower on sunny days is dangerous and can cause health risks. “Brazil has a solar incidence rate that is among the highest in the world, and yet we are the epicenter of the pandemic,” says the scientist, who is also the founder of a company that produces ultraviolet equipment for disinfecting environments.
Chinese university researchers cross-referenced data on the spread of Sars-Cov-2, temperature and solar radiation from more than 200 cities in China. The results, published in April in the scientific journal European Respiratory Journal, indicate that solar radiation and temperature did not influence the rates of contagion.
“Our study does not support the hypothesis that high temperatures and levels of ultraviolet radiation can reduce the transmission of Covid-19. It is premature to count on the warmer climate to control the disease”, write the authors in the article. Although sunbathing can stimulate the production of vitamin D, which has a beneficial potential for the immune system, scientists rule out a supposed therapeutic function of the sun for cases of Covid-19, since radiation only acts on the surface above which affects and viruses multiply inside cells, inside the body. “Sunlight brings several benefits, including decontamination. But to conclude that it can reduce the contagion by the virus, it is necessary to analyze other variables”, concludes Yoshimura, from Inpe.