Vitamin D is made by the body when the skin is exposed to the sun’s UVB rays.
If you can swim outside for about 10 to 15 minutes, two or three times a week, you're all set, because that's how much sun it takes to produce the vitamin D you need – about 10,000 to 15,000 IU in each session.
This is based on a UV index of 7, around the midday UV index in spring in hotter places such as Australia or summer in cooler places such as the UK.
The right dose of sun is just short of getting some color in the skin on each occasion. This amount of whole body sun exposure generates the maximum amount of vitamin D possible.
Longer exposure won't produce more vitamin D, but it may raise the risk of other diseases, particularly skin cancer.
Even SPF 8 almost completely eliminates UVB rays from reaching the skin, thereby preventing vitamin D manufacture. OMS advises that you expose the skin to sunlight before applying sunscreen.
If the skin starts to become red or painful, then regardless of length of time, sunscreen should be applied immediately to prevent burning. It has been shown that small, regular doses of sun exposure may actually be protective against skin cancers, but the skin should not be allowed to burn. The same principle applies to children.
The UV index shows the intensity of UV rays from the sun. Check online to find the UV index forecast for where you are; the higher the index reading, the shorter the time you will need to spend in the sun. Find out more.
15 - 20 mins
UV index 5 - midday, uncovered
10 - 15 mins
UV index 7 - midday, uncovered
5- 7 mins
UV index 14 - midday, uncovered
Time of year
If you live in a cooler climate, you are unlikely to be able to produce enough vitamin D in the winter months and will need to take supplements (5,000 – 10,000 IU vitamin D3 daily)
Duration of time spent outside
The time needed to achieve the desired amount of vitamin D is dependent on the UV index locally. This may be achieved in just a few minutes at midday (e.g. 10 -15 mins when the UV index is 7 and your skin is mostly uncovered, if the UV index is 14 it would be half that time).
If the weather is cloudy, you probably aren’t getting your required amount of vitamin D.
Note: you also cannot get vitamin D through windows, as glass will absorb UVB rays.
OMS suggests as close to full-body skin exposure as possible. Clothing will prevent most UVB rays from reaching the skin. Exposing a smaller area of skin for longer doesn't work, because once a given patch of skin produces its daily dose of vitamin D, no more can be made that day.
People with dark skin, such as those of African, Afro-Caribbean or South-Asian origin, will need to spend longer in the sun to produce the same amount of vitamin D as someone with light skin.