As I look out of my office window this morning, I see bright sunshine. And it’s very welcome. Sunlight exposure not only helps you sleep better and makes you feel more alert and more energised, recent evidence suggests that it may also help reduce COVID-19 infection risk and severity.

As I discussed in a previous article, outdoor light exposure (especially in the morning) helps to regulate your circadian rhythm and improve your sleep. And, as with exercise, there is a dose response – more is better.

As it turns out, sunlight may also help protect us from COVID-19, by:

• Increasing vitamin D levels
• Rapidly inactivating SARS-CoV-2 in the environment
• Protecting against cardiovascular disease (a major COVID risk factor)
• Inhibiting replication of SARS-CoV-2 in the body

UVB radiation – the spectrum of ultra-violet light with wavelengths from 280 to 315 nm – stimulates the production of vitamin D in the skin. Having sufficient vitamin D levels is associated with better COVID-19 outcomes. However, as I’ve written about before, these associations can’t be untangled from other (confounding) variables, never mind tell us anything about causation. That being said, vitamin D is essential for bone health – and probably many other aspects of health – so you should ensure your levels are topped up anyway, which may improve your COVID-19 defences as an added bonus. (A recent article in JAMA came to a similar conclusion). Sunlight is the best source of vitamin D – but UVB radiation in the UK is only high enough during spring and summer.

According to a study from 2020, UVB radiation also rapidly kills SARS-CoV-2 in saliva and cells on surfaces. This means that transmission of the virus via surfaces is likely to be significantly lower outside than inside. As you would expect, the virus inactivation is quicker in the height of summer (90% inactivated every 6.8 minutes) than in the depths of winter (90% inactivated every 14.3 minutes). It’s worth pointing out, however, that the levels of UVB radiation were set to mimic midday sunlight on a clear day at 40 degrees N latitude. This latitude crosses through the middle of the USA and the Mediterranean countries of Europe, which, sadly, means far more sunlight than we get here in the UK. So, we can assume the inactivation rates here will be slower, but certainly still fairly rapid in spring and summer.

But it’s not just UVB that happens to be an ally in our fight against COVID-19. UVA – the spectrum of ultra-violet light with wavelengths from 315 to 400 nm – may also have a role to play.

Higher UVA exposure is associated with lower incidence of heart attacks. And, as you know, cardiovascular disease is one of the major risk factors for severe illness and death from COVID-19. Higher UVA exposure is also associated with lower COVID-19 deaths, independent of cardiovascular disease and other major risk factors.

This data suggests that getting more UVA (from sunlight) may protect against cardiovascular disease (which has a knock-on effect of protecting against COVID-19) and may protect against COVID-19 directly. Now, these are only associations, there may or may not be a causal relationship. But the two studies do a decent job of controlling for confounding variables, and there appears to be a plausible biological mechanism in each case – the common denominator being nitric oxide.

UVA increases the production of nitric oxide in the skin. Nitric oxide dilates blood vessels, which reduces blood pressure (high blood pressure is a major risk factor for cardiovascular disease). But nitric oxide also appears to inhibit SARS-CoV-2 replication.

What levels of UVA are required? Don’t worry, you don’t need to move to the Equator to reap the potential benefits. The UVA-heart attack association was observed in Scotland (not exactly the Sahara) and the UVA-COVID-19 mortality association was observed in the USA, Italy, and England…in winter. So, there is plenty of UVA radiation in the UK across the seasons, you just need to spend more time in it.

So, there you have it. There is some evidence to suggest that outdoor light can help in our fight against COVID-19. It’s far from definitive, but there is enough here to justify including it in your toolkit. Every little helps. And now is the best time – over the next few months both UVA and UVB radiation will be at their highest.