How Much UV Do You Need?

Or how long should you spend outdoors in the sun each day ...

Note added March 31 2021. Dowdy et al, 2010 discussed a correction to account for differences in the spectral distribution of sunlight compared with the artificial lamps used to calibrate the amount of vitamin D production for a given UV exposure. With this correction, all times below would be reduced by a factor of 0.7.

In my last post I talked about how your body’s vitamin-D might help in the battle against covid-19. Most of that vitamin-D is made in your skin through its exposure to UV from sunlight.

But how long do you need to expose yourself to sunlight to maintain adequate levels of vitamin-D? There’s no one-size-fits-all answer. It depends on the amount of UV present, which varies hugely with location, season, time of day, and cloud cover. For example, at mid-latitudes, the amount of vitamin-D-producing UV is about twenty times greater at noon on a clear day in summer than in winter. If we include times outside the midday period, it gets even more complex.

And it also depends on how much of your skin is exposed to sunlight. You’ll need to expose at least ten percent of your skin (e.g., more than just face and hands). The more skin area you can comfortably expose, the better.

Here I’ll show how you can easily work out the optimal exposure time to make enough vitamin D without causing skin damage. All that’s needed is knowledge of the how much UV is present (i.e., the UV Index, or UVI), and some simple arithmetic.

The UVI is hard to estimate as our bodies have no sensors that directly detect it. The best we have is sunburn, but that becomes apparent only hours after exposure - far too late to be useful. Luckily though, the UVI is always available to us from our own smartphone apps like GlobalUV (for anywhere in the world), or UVNZ (better for the New Zealand region).

Once you know the UVI, you can read off the required exposure times from the graph below (or you can use information already provided by those two apps, as I’ll explain later). I originally produced a graph like this for a paper we published in 2009. The one below is a simplified version that I adapted for my book.

The red curve (which I must say is very appropriately coloured) gives the approximate exposure times for visible damage to fair skin as a function of UVI. This damage is called “erythema”, which is just a fancy word for “skin reddening”. Also shown are the corresponding times for adequate vitamin-D production (the blue curves) for several amounts of skin exposed. Results are shown for fair skin (type II - using the left axis) and olive skin (type IV - using the right axis). The times for darkest skins are more uncertain. Based on propensity to sunburn, one might expect the times for vitamin-D synthesis would be much longer for skin type VI than for skin type II, but recent evidence suggests a much smaller difference in the capacity of darker skin types compared with lighter skin types.

The graph shows that it’s best to get short sharp bursts of UV with a large area of skin exposed. For example, if the UVI is 10 (summer noon in southern New Zealand), skin damage would result from about 15 minutes of exposure for fair skin (Type II). At the same time, if only the hands and face are exposed, it would take about 10 minutes - close to the sunburn limit - to maintain your daily supply of vitamin-D. But with full body exposure, only about 1 minute would be needed - far below the threshold for skin damage.

As the UVI reduces, exposure times required for vitamin-D sufficiency increase faster than those for erythema (the two effects aren’t quite linear). For example, if the UVI is 2 (winter noon in northern New Zealand), the time for erythema increases by a factor of 6 to 90 minutes. For those conditions, the time needed for vitamin D is 75 minutes if just the hands and face are exposed, but only 12 minutes if the face arms and legs are exposed. The two upper curves actually cross near UVI = 1 (not shown) and for UVI values less than that it wouldn’t be possible to get enough vitamin D without sunburn if just the hands and face are exposed.

Good news!

If you have one of the smartphone apps mentioned above, it’s much easier to work out the exposure times needed. You don’t need to refer to this graph, which is shown here just to help explain the rationale. The times for skin damage are already reported by the apps for the skin type you selected. They don’t currently report the exposure times for vitamin-D because there are larger uncertainties. For example, there’s controversy over whether the blocking effect of the melanin in darker skins is the same for vitamin-D and erythema. The effects could be different because of the different skin depths of the processes involved. However, very approximate exposure times for vitamin-D sufficiency can easily be estimated.

Here’s a good rule of thumb to do that, that I think is quite sensible (sorry, I can’t find the original reference to it).

Aim for daily exposures to sunlight corresponding to no more than 1/3 of the time for an erythemal dose, while exposing an area of at least 1/3 of your body area (roughly equivalent to a bit more than the area of your head, hands, and arms).

An earlier version of this rule (below) has recently been criticised, but the idea is very similar. The above version goes some way towards addressing those concerns. Obviously it makes a little less vitamin-D (but more than the factor 9/16 you might guess because the rate of production is non-linear).

Aim for daily exposures to sunlight corresponding to no more than 1/4 of the time for an erythemal dose, while exposing an area of at least 1/4 of your body area (roughly equivalent to a bit less than the area of your head, hands, and arms)

As you can see from the above plot, both rules appear to work reasonably well over a wide range of UVI. It’s not an exact science! It assumes that different parts of your skin have the same capacity for vitamin-D production, which is probably wrong.

Vitamin-D production without skin damage is most easily achieved over the noon period, but is rarely advised by health professionals because they’re nervous about the rapidly increasing risk of skin damage from exposures for longer exposures. Don’t ever exceed the time for skin damage given by the app without protection. Aim for no more than half that time - preferably no more than 1/4 or 1/3 - as suggested by those “rules” above.

It gets a little more complicated for low UV levels, as in winter. Applying the above rule for a UVI of 1 would suggest that at least 20 minutes of exposure to 1/3 of your body (face, arms and legs) would be required for vitamin-D sufficiency. But the graph suggests that a longer period is needed. The rule fails because for these low UVI values, non-linearity between the two effects, as mentioned above, become important.

Even better news?

The apps could help more. Currently they don’t tell you the maximum recommended exposure time when the UVI is less than 3. That’s because the times for skin damage are so long that changes over the period would become important enough to affect the message (more than an hour in the case of UVI = 3, or more than three hours for UVI = 1). But we could easily fix that …

If there’s a perceived need, the apps could be easily extended to report the time for vitamin-D sufficiency assuming say 1/4 of your skin area - e.g., your head, hands and lower arms - are exposed. If you have a strong opinion on that, let me know what you think. We might look into that further.

If the peak UVI is less than 2, the uncertainties in calculating the time needed become larger (as shown here in Figures 4 and 5 if you want to know the gory details). But in such cases, you probably won’t be able to get your full daily dose of vitamin-D from sunlight alone anyway, and may need to complement it orally.

Exposures to UV aren’t needed every single day because the vitamin-D has a lifetime of several weeks in your body. But, with regular daily exposure it’s easier to maintain your vitamin -D without inducing skin damage. For periods when the necessary sun exposures can’t be maintained, use a daily supplement of 1000 to 2000 IU per day instead.

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Thanks for listening. Previous posts on the intersection between Ozone, UV, Climate, and Health can be found at my UV & You area at Substack. Click below to subscribe for occasional free updates.