How Many Minutes in the Midday Sun?

Short sharp boosts best for vitamin D sufficiency ...

Here are the answers for those who don’t want to bother with any of the calculations I talked about in my last post …

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 in the tables below are reduced by a factor of 0.7. So for summer sun with 35 percent of skin type II exposed, the time required reduces from about 3 minutes to 2 minutes. Updated tables will be provided in a followup post.

With Vitamin-D production, you get more bang for the buck when the sun’s high in the sky. That’s when the ratio of beneficial UV (causing Vitamin D production) to harmful UV (causing skin damage) is greatest. But you have to be extra careful, because even short over-exposures at this time are potentially harmful.

So how long should you expose yourself to UV at noon to get your daily UV dose needed for optimal vitamin-D without damaging your skin?

The first table below - which you may recall seeing in my earlier ‘Madonna’ post - gives the mean UVI at noon for each month of the year at several cities, covering a wide range of latitudes in Australia, New Zealand and the UK. Sun protection is currently advised whenever the UVI is 3 or more - that’s everywhere except for the green cells, where the UVI is designated as ‘low’ (colour coding follows the WMO/WHO guidelines). For the highest UVI shown, visible damage to sensitive skin (skin reddening) will become apparent following exposures of 10-15 minutes.

The next table below - with colour coding to match the UVI ranges above - gives the corresponding calculated daily exposure times in minutes needed to maintain vitamin D levels for a fair skinned individual with 35 percent of their skin exposed (e.g., wearing ‘modest’ shorts and tee shirt). With that amount of skin exposed, the times for vitamin-D sufficiency are always much shorter than the times for skin damage. A couple of minutes is all you need on days with high UVI.

These results are for average weather conditions, including cloud effects. On clear days, the UVI will be typically about 25 percent greater than these mean values, so the exposure times should be shorter by a similar factor. And under heavy overcast conditions, the UVI could be less than half these mean values, in which case the exposure times should be doubled.

If less skin area is exposed, the exposure times should be increased proportionately. Longer periods will also be needed for darker skins. But not that much longer. Recent evidence suggests that the time needed may be less than doubled even for the darkest skin types, a much shorter time than was thought previously. But the sample size of the recent study was rather small.

Notice that for latitudes greater than 45 degrees (in both hemispheres), exposure times of more than 1 hour over the noon period would be needed in winter even if (an unlikely) 35 percent of the skin is exposed. Over that winter period, some supplementation (or perhaps a beach holiday at Acapulco? 😊 ) will be required to prevent a decline in vitamin-D. Note though that most health professionals now advise that a mean blood serum concentration of 50 nmol/litre is sufficient, which allows for a short winter period with slightly lower levels. Don’t panic.

Please don’t take these times as Gospel. There are large uncertainties. They could exceed 50 percent, so the times are indicative only. So those last two entries of 14 and 9 (for Glasgow in May and June) should be roundly interpreted as “10 to 20 minutes” and “5 to 10 minutes” respectively.

If your city isn’t listed in the above tables, your UVI values will be comparable with those at the closest latitude listed (but greater at clean sunny sites, and less at polluted cloudy sites).

Otherwise, if you know the UVI you can estimate the times needed. The table below compares the daily exposure times for skin damage (erythema) with the much shorter times needed for vitamin D sufficiency as a function of UVI for the full range that’s been (correctly) measured on the surface of the planet. The times - given in minutes - are approximate only.

Exposure times as a function of UVI for fair skin (assuming 2.5 SED per MED). The times for Vitamin D sufficiency assume that 35 percent of skin is exposed.

If that’s all still too hard, you’ll just have to wait for the update of our GlobalUV app (for anywhere in the world), or our UVNZ app (better for the New Zealand region). My friend Jerry Burke is currently working on improvements to them so they provide the times for any specified skin type and for a variety of clothing. I’ll keep you posted.

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Thanks for getting this far. 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.