Ultraviolet rays produced by the sun could help kill the coronavirus, according to a team of Italian astrophysicists, who said that the impact of Covid-19 outbreaks around the world might have been affected by the intensity of such light.
As well as the better-known UVA and UVB forms of ultraviolet light, solar radiation contains UVC, which has a shorter, more energetic wavelength that is powerful enough to break up genetic material. Thankfully for humans, most UVC is filtered out by the ozone layer.
However, a team of researchers from the National Institute for Astrophysics in Rome, led by Dr Fabrizio Nicastro, calculated a dose of UVA and UVB radiation capable of causing the same damage to the coronavirus as an equivalent blast of UVC.
They then built a model to estimate how long it would take to kill the virus in more than 100 countries.
The results varied, but generally, from January to April in countries between 40 to 60 degrees north of the equator, exposure to the UV light for between 30 minutes and 14 hours a day was required to kill 63 per cent of the pathogen.
That region encompassed many of the areas – including China, Italy, Spain, Britain and the United States – that were hardest hit by the pandemic, the team said in a paper published on the preprint website arXiv.org last week, which means it has not been peer-reviewed.
“In northern countries the outbreaks proceeded at high rates for tens of days … despite the severe measures of social distancing adopted by the majority of these countries,” the scientists said, adding that there was “concurring circumstantial evidence suggesting the evolution and strength of the pandemic might have been modulated by the intensity of UVB and UVA solar radiation”.
The researchers then turned their attention to southern parts of the world and found that over the same period, in regions between 40 and 60 degrees south of the equator, it would have taken between four and 35 minutes of sunlight to kill the virus.
The regions included Australia, New Zealand, South Africa, Chile, Argentina and Zimbabwe, many of whose governments imposed less strict lockdown measures, and where infection levels were relatively low.
However, not all southern locations fared so well. Brazil has experienced high rates of infection since March.
But “epidemics efficiently develop in areas where typical UVB/A virus-inactivation exposures are longer than about 20 minutes”, the study said, adding that that was something policymakers around the world should take into consideration.
Also, there were signs emerging that infection rates in southern regions had increased as UV exposure levels were falling due to the change of season.
“The situation is almost reversed,” Niscastro said.
Professor Li Ying, an astronomer at the Purple Mountain Observatory in the east China city of Nanjing, said scientists faced many challenges in establishing a firm link between solar radiation and the spread of Covid-19.
Many weather elements, like water droplets in clouds that absorb or deflect sunlight, could affect the modelling, she said, which meant that the level of UV radiation could remain low even if the sun was directly above a region.
“The spread of the virus is affected by so many forces,” she said. “I’m afraid the UV signal, if it really exists, will be drowned out in the noise.”
Some researchers have suggested that changes in solar activity in the past few months have reduced the amount of radiation hitting the Earth, which might have contributed to the emergence and spread of the virus.
But Li said there was no proof to support such speculation and that there were signs a new sun circle would begin soon, if it had not already.
“We expect solar activity will have reached a high point in two or three years,” she said.