Abstract
Solar UV-C photons do not reach Earth’s surface, but are known to be endowed with germicidal properties that are also effective on viruses. The effect of softer UV-B and UV-A photons, which copiously reach the Earth’s surface, on viruses are instead little studied, particularly on single-stranded RNA viruses. Here we combine our measurements of the action spectrum of Covid-19 in response to UV light, Solar irradiation measurements on Earth during the SARS-CoV-2 pandemics, worldwide recorded Covid-19 mortality data and our “Solar-Pump” diffusive model of epidemics to show that (a) UV-B/A photons have a powerful virucidal effect on the single-stranded RNA virus Covid-19 and that (b) the Solar radiation that reaches temperate regions of the Earth at noon during summers, is sufficient to inactivate 63% of virions in open-space concentrations (1.5 × 103 TCID50/mL, higher than typical aerosol) in less than 2 min. We conclude that the characteristic seasonality imprint displayed world-wide by the SARS-Cov-2 mortality time-series throughout the diffusion of the outbreak (with temperate regions showing clear seasonal trends and equatorial regions suffering, on average, a systematically lower mortality), might have been efficiently set by the different intensity of UV-B/A Solar radiation hitting different Earth’s locations at different times of the year. Our results suggest that Solar UV-B/A play an important role in planning strategies of confinement of the epidemics, which should be worked out and set up during spring/summer months and fully implemented during low-solar-irradiation periods.
Highlights
It is well known that 200–290 nm ultraviolet photons are absorbed by both the nucleic acid (RNA and DNA) and proteins and are endowed with germicidal properties that are effective on viruses because they induce irreversible modifications that inhibit the virus r eplication1–9
We present a number of concurring circumstantial evidence suggesting that the evolution and strength of the recent SARS-Cov-2 pandemics41,42, has been modulated by the intensity of UV-B and UV-A Solar radiation hitting different regions of Earth during the diffusion of the outbreak between January 2020 and April 2021
The daily fluence on Earth of this UV radiation depends critically on Earth location and time of the year and results in the potential of Solar UV-B/A photons to inactivate a virus in air or surfaces
Summary
It is well known that 200–290 nm ultraviolet photons (hereinafter UV-C radiation) are absorbed by both the nucleic acid (RNA and DNA) and proteins and are endowed with germicidal properties that are effective on viruses because they induce irreversible modifications that inhibit the virus r eplication (see “Methods”). About 10% of the photons emitted by the Sun in the range 290–315 nm (UV-B) and as much as 95% of those in the range 315–400 nm (UV-A), reach the Earth. The effect of these photons on Single- and Double-Stranded RNA/ DNA viruses and the possible role they play on the seasonality of e pidemics, are little studied and highly debated in alternative or complementarity to other environmental causes. We present a number of concurring circumstantial evidence suggesting that the evolution and strength of the recent SARS-Cov-2 pandemics, has been (and is being) modulated by the intensity of UV-B and UV-A Solar radiation hitting different regions of Earth during the diffusion of the outbreak between January 2020 and April 2021. Our findings could help in designing the social behaviors to be adopted depending on season and environmental conditions
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