Is 5-minutes of summer sun a safe amount of time for Vitamin D?

There is no straightforward answer to this question since it depends on a range of variables, beginning with the definitions of ‘safe amount of sun exposure’ and ‘healthy level of vitamin D’, followed by consideration of latitude, season, time of day, skin type and unprotected skin area exposed.

In recent work from the UK (latitudes 50-60N) we have shown that 9 minutes exposure every day (whatever the weather) in the lunchtime period during the months March to September would provide for vitamin D needs. The caveats are: 1 standard erythema dose (SED) is the maximum UV dose that could be received in the UK for this duration of exposure, so there is no risk of sunburn for the general population (our definition of ‘safe sun exposure’); ‘healthy level of vitamin D’ means the avoidance of deficiency (25(OH)D < 25 nmol/L) throughout the year; 35% skin area is exposed in June, July, August (equivalent to wearing modest shorts/skirt and T-shirt), with 10% (hands and face) exposed in the cooler months; this is for white-skinned individuals; exposure occurs in an open, unshaded place e.g. field, park, large piazza, rather than a city street surrounded by tall buildings.

In the UK there is insufficient UV radiation in sunlight from about October – February to initiate any appreciable vitamin D synthesis. Vitamin D status declines through the winter so summer exposure has to provide for winter-time vitamin D needs.  At lower latitudes than the UK the so-called vitamin D winter is shorter and the noon-time sunlight contains more UV radiation, so exposure times can be shorter. At latitudes higher than the UK the opposite is true. North America/Canada and Europe cover latitudes both higher and lower than the UK.

For people with skin type V (brown, South Asian) the equivalent exposure time in the UK is 25 minutes per day. This means the exposure maximum is a little less than 3SED, but this is also well below the sunburn limit for skin type V.

Exposing a greater area of unprotected skin (if practically possible) would enable exposure times to be reduced further. Maintaining our ‘safe sun exposure’ of 9 (or 25) minutes daily and exposing only hands and face year round does not promote sufficient vitamin D synthesis for year-round avoidance of deficiency.

Finally, redefining a ‘healthy level of vitamin D’ would change the exposure times, and the possibilities of meeting vitamin D requirements through safe sun exposure.