Sunlight, vitamin D and safe sun time

By Trung Hieu Lam, developer of UV Tracker · Published 1 September 2026

How skin makes vitamin D

UVB exposure converts a cholesterol derivative in the skin into previtamin D3, which the body then processes into usable vitamin D. It's one of the few nutrients your body can largely produce on its own, given the right sunlight.

Less than you think

Meaningful vitamin D synthesis happens well within the safe sun time figures on this site — a fraction of the exposure needed to reach the sunburn threshold, for most skin types. And production isn't linear with exposure time: the skin's vitamin D synthesis plateaus, so once you've had enough sun, more exposure doesn't make more vitamin D — it only adds sunburn risk.

Skin type matters

More melanin means more natural UV filtering, so darker skin types generally need longer exposure to synthesize the same amount of vitamin D. See the Fitzpatrick skin types article and the safe sun time tables for how your type compares.

Don't tan for vitamin D

Deliberate tanning — indoors or out — is not a recommended way to get vitamin D. The World Health Organization and international cancer research bodies classify UV tanning as carcinogenic; the DNA damage behind a tan is permanent, while the vitamin D benefit can be obtained far more safely through brief incidental sun exposure, diet, or supplements.

Testing and supplements

If you have limited sun exposure — due to climate, indoor lifestyle, covering clothing, or darker skin at higher latitudes — a blood test can check your vitamin D level, and a doctor can advise on supplementation. This is a conversation for a doctor, not a phone app.

UV Tracker measures the UV where you stand — sky scan for local conditions, one photo for your skin type, and your safe sun time on one card. Free forever.