Vitamin D and Humidity

Humidity's effect on UVB is small. Atmospheric water vapour absorbs some UV but the effect is dwarfed by cloud cover, latitude, and time of day. The practical relevance for vitamin D is minimal — humid tropical climates generally still deliver plentiful UVB.

Water vapour and UVB

Water vapour absorbs UV weakly across the 290–315 nm band relevant to vitamin D synthesis. Even in very humid conditions (dew point > 24 °C), the reduction in ground-level UVB is at most 3–8% versus a dry-air reference — negligible compared with a 30–50% reduction from moderate cloud cover.

Sweat and skin synthesis

A common myth: showering right after sun exposure "washes off" freshly made vitamin D. Not true. Cholecalciferol is produced inside the epidermis (not on the skin surface) from 7-dehydrocholesterol, and it gradually diffuses into the bloodstream over hours to a day. Sweat, water, and soap do not remove it.

The behavioural angle

What humidity does affect is behaviour — hot humid climates drive people indoors during midday hours. Bangkok, Miami, Singapore all have high UVB years-round but urban deficiency rates that rival northern climates because residents avoid outdoor midday time. See our hot climates page.