Coronavirus and Sunlight

kw: medicine, viruses, coronavirus, ultraviolet, inactivation

I contend that the safest place to see others face-to-face is outside on a sunny, breezy day. Masks are not needed outdoors, only indoors. Here is why.

If someone is ill with the novel coronavirus SARS-Cov-2, the cause of Covid-19 disease, any droplets that leave their body with their breath will contain virus particles, called virions on medical literature. The virions are rather large, some 125 nm in diameter. That's about four times the size of rhinoviruses, the most common cause of the common cold, and a it's a little larger than influenza viruses. However, 125 nm is 1/8,000th of a millimeter. No mask you can afford will capture them, but that is not the point.

Indoors among "the public", the threat is not isolated virions but the droplets released in small amounts by speaking or heavy breathing, in larger amounts by a cough, and in very great amounts by a sneeze. The droplets are mostly between 1,000 and 10,000 times the size of the virions they might contain. Nearly any such droplets that encounter your face covering or mask will be caught in it. So when you wear a mask to the store, when you get home wash it with hand soap and let it dry, or replace it if you have a lot of the disposable kind.

What happens outside? Unless the humidity is very high, the droplets evaporate in a few minutes, or even a few seconds. The virions are now "free". Even before the droplets evaporate, however, if the sun is up, something wonderful happens! Ultraviolet light from the sun destroys the virus genome. The question is, how soon?

I found and downloaded an article, "Predicted Inactivation of Viruses of Relevance to Biodefense by Solar Radiation", by C. David Lytle and Jose-Luis Sagripanti, in Journal of Virology, v79, No. 22, Nov. 2005, p. 14244–14252. The work was supported by DoD and relates to defense against biowarfare agents.

The authors gathered data on solar UV and Hg-vapor UV (such as that used in hospital disinfection lamps), and how quickly different kinds of viruses are destroyed. There are a lot of details, but the relevant conclusions related to coronaviruses are these:

• Sensitivity units are reported two ways, as D37 and as 1-log.
• D37 is the dose of UV needed to destroy all but 37% of the particles. 0.37 is 1/e, and is related to natural logarithms.
• 1-log is the dose of UV needed to destroy 90% (all but 10%) of the particles. It is related to common logarithms, of base 10. This is more understandable and relevant to us.
• The viruses of greatest interest in the article are filoviruses such as Ebola or Marburg. D37 for these is about 7.4 and 1-log is 17. (These data are from Tables 2 and 4)
• The virus of interest today is a coronavirus; the family Coronaviridae has D37 in the range 2.5-3.9 (Table 2), for which I calculate 1-log of 5.7-9.0. Thus SARS-Cov-2 is about twice as sensitive to UV light as the Ebola virus. That's good news.
• The effectiveness of solar radiation depends on the angle of the sun. A specific datum relevant to the DE-PA border area where I live is for Davis, CA, on a typical, sunny July 15. The 1-log level of deactivation for a filovirus occurs in 55 minutes. (If you're lucky enough to live in Hawaii, the sun's UV is twice as strong there, and the relevant time is 21 minutes.)

From these data I calculate that 1-log deactivation (10% remaining viable) takes between 19 and 29 minutes. Let's round these to 20 and 30 minutes. Deactivation is a stochastic matter. If a UV photon passes through the right part of a coronavirus virion, it will damage the RNA. Most of them miss. So let's look at the 20-minute level for a theoretical virus. In 20 minutes, 10% are still viable. In 40 minutes, only 1% are still viable. In an hour, 0.1% are still viable, and so it goes. That is one in a thousand after an hour, and one in a million after two hours, and one in a billion after three hours. For a "tougher" coronavirus, at the 30-minute end of that range, viability after an hour is about 1%, and after three hours it is about one in a million.

So if you're worried that someone emitting viruses is outside, and they blow downwind, two things work in your favor. Firstly, the swirling wind will spread them out so only a few have the chance to reach you, and secondly, if the person is some distance "upwind" from you, sunlight will zap the virions at a steady rate.

There are two further matters I am still researching. 

• How many virions need to enter the body to cause infection? A very infectious virus such as influenza can stably cause infection if around 100 virions are breathed in. This is because many of them are "beaten" by the innate immune system. For more susceptible people, not as many are needed, but I don't know what the normal range of natural resistance is, for people who are not vaccinated. It may not yet be known what the "average infectious load" (my term) is for coronavirus.
• Whether it is sunny or not, oxygen also deactivates virions. How quickly? Whatever I can find out, I'll report.