To document the effect of season and environmental pollution on UVB irradiance; and to estimate cutaneous vitamin D synthesis in village women in different seasons.

Radiant UVB energy was measured by a spectroradiometer in different seasons and, in April and May, on successive days in open areas at the city outskirts, at a crowded inner-city area and the villages of our participants. Clothing, outdoor activity pattern and serum 25-hydroxycholecalciferol (25(OH)D) levels were documented.

Rural north India, latitude 26·8°N.

Pregnant women (n 139, aged 20–40 years).

UVB irradiance ranged from 56 µW/cm2 in January to 470 µW/cm2 in June. Proportion of skin exposed was 18·5 % in summer and 9·5 % in winter. Mean (sd) daily duration of sun exposure was 3·2 (0·2) h during winter and 2·1 (0·4) h during summer. Cutaneous vitamin D synthesis was estimated to be 19·25 µg (770 IU) during winter and 37·25 µg (1490 IU) during summer. Mean (sd) serum 25(OH)D was 28 (15) nmol/l during winter (92 % of participants with <50 nmol/l) and reached 56 (20) nmol/l during late summer (60 % with >50 nmol/l). Mean (sd) UVB irradiance at peak summer was significantly higher at the open areas and in the villages than at the inner-city location (340 (45) and 310 (60) v. 250 (50) µW/cm2, P=0·03).

In our population, at latitude 26·8°N, poor skin exposure is a limiting factor in all seasons. During winter, low UVB radiation energy also contributes. Particulate pollution limits UVB irradiance. Vitamin D supplementation during winter may be necessary.