In recent years, several studies on different viruses, including the SARS-CoV-1, have demonstrated that their survival in a certain environment depends on multiple factors, including temperature, humidity, sunlight, and the material in which the virus is suspended. To date, amongst the increasing numbers of studies on SARS-Cov-2 due to the ongoing pandemic, there are some focused at the understanding of the virus behavior and resistance on different surfaces, and in different indoor environments. For example, it has been reported that SARS-CoV-2 could persist for several days on non-porous surfaces in indoor conditions of 23 ⁰C, and 40% of relative humidity, or at 22 ⁰C and 65% relative humidity.
Just recently, first evidences for a possible activity of sunlight on SARS-CoV-2 inactivation have been published in The Journal of Infectious Diseases. The experiments were conducted through a simulation of sunlight conditions reflecting parameters of midday in summer solstice and in winter solstice, at specified latitudes. Furthermore, the experiments used an ultraviolet B source comparable to the UVB outdoor levels, and a simulated matrix of saliva dried on a non-porous surface. Under the conditions of the midday of summer solstice, the results showed a 90% of viral inactivation every 6.8 minutes in simulated saliva dried on a surface. For the conditions related to the winter solstice the viral inactivation was reported as 90% every 14.8 minutes.
The data collected from the researchers suggest that the viral transmission may also be significantly reduced upon exposure to direct sunlight in an outdoor environment, although the percentage of viral inactivation would depend on the UVB exposure levels, which in turn will be affected by the specific time of the year and the local weather conditions. Different from the controlled experimental conditions, a certain degree of day-to-day variability is then expected in the persistence of SARS-CoV-2 on non-porous surfaces in the outdoors.
In the proposed study, it was also reported that no virus inactivation was observed in the darkness over a 60-minute test duration. The study also reported that the nature of the matrix in which the SARS-CoV-2 is resuspended affects its decay: for instance, the inactivation was two times faster in simulated saliva than in culture media. This is possibly due to the presence in the media of components which protect the virus from the sunlight, coherently with what was reported for SARS-CoV-1, in which the presence of albumin in the medium conferred some protection to the virus from the photoinactivation.
Although additional parameters should be taken into consideration to assess the risk of exposure to the virus in an outdoor environment, such as the viral load present on the surface, the transfer efficiency of the virus from different surfaces upon contact and the amount of viruses needed to cause infections. Moreover, this study demonstrates for the first time the possibility that sunlight may decrease the exposure risk in the outdoors with respect to the indoors, by inactivating SARS-CoV-2 in a faster manner from a surface, and may be an effective disinfectant for contaminated, non-porous material.