Solar ultraviolet radiation sensitivity of SARS-CoV-2

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), can be life-threatening; the outbreak of this disease was characterised as a pandemic by WHO on March 11, 2020.1WHORolling updates on coronavirus disease (COVID-19).https://www.who.int/emergencies/diseases/novel-coronavirus-2019/events-as-they-happenDate accessed: April 4, 2020Google Scholar COVID-19 is currently a global health issue and governments are taking a series of measures to reduce the spread of the virus among communities. Factors affecting the survival of SARS-CoV-2 need to be clarified and the effect of solar ultraviolet radiation is one subject under discussion.2van Doremalen N Bushmaker T Morris DH et al.Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1.N Engl J Med. 2020; (published online March 17.)DOI:10.1056/NEJMc2004973Crossref Scopus (6403) Google Scholar The output of both the popular and scientific press have included hopeful speeches by politicians or scientists suggesting that the transmission of SARS-CoV-2 will be contained within the coming months because of increasing temperatures and solar ultraviolet radiation during the summer season in countries in the northern hemisphere. However, solar ultraviolet radiation and ultraviolet germicidal irradiation (UVGI) are not the same. To evaluate effectiveness of ultraviolet radiation, we need to consider the virology of SARS-CoV-2, an enveloped, single stranded RNA virus. SARS-CoV-2 is a novel betacoronavirus and showed 88% genome similarity with two SARS-like coronaviruses (SL-CoV) found in bats: SL-CoV-ZC45 and SL-CoVZXC21.3Li X Geng M Peng Y Meng L Lu S Molecular immune pathogenesis and diagnosis of COVID-19.J Pharm Anal. 2020; (published online March 5.)DOI:10.1016/j.jpha.2020.03.001Crossref Scopus (1097) Google Scholar Human-to-human transmission occurs via droplets (by direct or indirect contact)4Ceccarelli M Berretta M Rullo EV Nunnari G Cacopardo B Differences and similarities between severe acute respiratory syndrome (SARS)-coronavirus (CoV) and SARS-CoV-2. Would a rose by another name smell as sweet?.Eur Rev Med Pharmacol Sci. 2020; 24: 2781-2783PubMed Google Scholar and viral droplets can survive on various surfaces for several hours, despite reductions in the viral load.2van Doremalen N Bushmaker T Morris DH et al.Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1.N Engl J Med. 2020; (published online March 17.)DOI:10.1056/NEJMc2004973Crossref Scopus (6403) Google Scholar This information shows that, despite their enveloped structure, SARS-CoV-2 is quite resistant to environmental conditions. During epidemics and pandemics, disinfection of environments is crucial, particularly for airborne diseases.5Walker CM Ko G Effect of ultraviolet germicidal irradiation on viral aerosols.Environ Sci Technol. 2007; 41: 5460-5465Crossref PubMed Scopus (212) Google Scholar The advantages offered by ultraviolet radiation make UVGI (ultraviolet disinfection) a very effective disinfection tool.6Wallace RL Ouellette M Jean J Effect of UV-C light or hydrogen peroxide wipes on the inactivation of methicillin-resistant Staphylococcus aureus, Clostridium difficile spores and norovirus surrogate.J Appl Microbiol. 2019; 127: 586-597Crossref PubMed Scopus (20) Google Scholar Ultraviolet light can be classified into three subtypes by radiation wavelengths: ultraviolet A (320-400 nm), ultraviolet B (280–320 nm) and ultraviolet C (200–280 nm).7Kowalski W Ultraviolet Germicidal Irradiation Handbook. Springer, Berlin2009Crossref Scopus (354) Google Scholar The commonly used wavelength for UVGI is ultraviolet C because its germicidal effectiveness peak wavelength is 260–265 nm, which is equivalent to the peak of ultraviolet radiation absorption of nucleic acids.7Kowalski W Ultraviolet Germicidal Irradiation Handbook. Springer, Berlin2009Crossref Scopus (354) Google Scholar It is known that as the ultraviolet wavelength decreases, the germicidal effect of ultraviolet radiation increases.8Pozo-Antonio JS Sanmartín P Exposure to artificial daylight or UV irradiation (A, B or C) prior to chemical cleaning: an effective combination for removing phototrophs from granite.Biofouling. 2018; 34: 851-869Crossref PubMed Scopus (27) Google Scholar Therefore, ultraviolet wavelengths below 320 nm are classed as actinic—ie, causing photochemical reactions. Since ultraviolet A radiation is insufficiently absorbed by viral nucleic acid, ultraviolet A radiation is not considered germicidal.7Kowalski W Ultraviolet Germicidal Irradiation Handbook. Springer, Berlin2009Crossref Scopus (354) Google Scholar Unfortunately, ultraviolet A is the major ultraviolet component of sunlight reaching the ground.8Pozo-Antonio JS Sanmartín P Exposure to artificial daylight or UV irradiation (A, B or C) prior to chemical cleaning: an effective combination for removing phototrophs from granite.Biofouling. 2018; 34: 851-869Crossref PubMed Scopus (27) Google Scholar Ultraviolet B radiation can also have a small germicidal effect,7Kowalski W Ultraviolet Germicidal Irradiation Handbook. Springer, Berlin2009Crossref Scopus (354) Google Scholar but only a small portion of it reaches the Earth's surface as most is absorbed by the atmosphere.8Pozo-Antonio JS Sanmartín P Exposure to artificial daylight or UV irradiation (A, B or C) prior to chemical cleaning: an effective combination for removing phototrophs from granite.Biofouling. 2018; 34: 851-869Crossref PubMed Scopus (27) Google Scholar Ultraviolet radiation that is totally absorbed by the ozone layer is accepted as having the optimum germicidal wavelength—ie, ultraviolet C radiation.8Pozo-Antonio JS Sanmartín P Exposure to artificial daylight or UV irradiation (A, B or C) prior to chemical cleaning: an effective combination for removing phototrophs from granite.Biofouling. 2018; 34: 851-869Crossref PubMed Scopus (27) Google Scholar Unifying all these principles, it is clear that sunlight reaching the ground lacks germicidal ultraviolet C radiation.7Kowalski W Ultraviolet Germicidal Irradiation Handbook. Springer, Berlin2009Crossref Scopus (354) Google Scholar, 8Pozo-Antonio JS Sanmartín P Exposure to artificial daylight or UV irradiation (A, B or C) prior to chemical cleaning: an effective combination for removing phototrophs from granite.Biofouling. 2018; 34: 851-869Crossref PubMed Scopus (27) Google Scholar Studies show that UVGI methods can be used effectively to eliminate viruses—eg, in health-care facilities, schools, indoors, etc—by using special ultraviolet radiation systems (ultraviolet C lamps, chambers). Also, it is known that the ultraviolet absorption peak of RNA viruses is around 250 nm wavelength.7Kowalski W Ultraviolet Germicidal Irradiation Handbook. Springer, Berlin2009Crossref Scopus (354) Google Scholar Therefore, the germicidal effectiveness of ultraviolet C radiation is limited to such applications and sunlight is not an alternative. However, some studies based on SARS-CoV, have shown that at least 60°C (which the earth does not reach to this temperature) and a minimum of 90 min are required to inactivate SARS-CoV-2.9Derraik JGB Anderson WA Connelly EA Anderson Y Rapid evidence summary on SARS-CoV-2 survivorship and disinfection, and a reusable PPE protocol using a double-hit process.medRxiv. 2020; (published online April 6.) (preprint).DOI: 10.1101/2020.04.02.20051409Google Scholar In the absence of scientific evidence showing ultraviolet B radiation's germicidal effectiveness on SARS-CoV-2, both politicians and scientists should avoid voicing assumptions on the effect of sunlight on viral transmission. Such uninformed statements can promoted misunderstanding and offer unrealistic hope to communities. This misunderstanding can also cause lethargy with regard to the government measurements in place in the community. Further studies should be done by simulating complex environmental conditions, in which a number of variables will test the effectiveness of ultraviolet B radiation against environmental SARS-CoV-2. We declare no competing interests.