Background Personal annual ultraviolet (UV) radiation data based on daily records are needed to develop protective strategies.
Objectives To compare UV radiation exposure patterns in the winter half-year (October–March) and the summer half-year (April–September) for Danish indoor workers.
Methods Nineteen indoor workers (age range 17–56 years) wore personal UV dosimeters, measuring time-stamped UV doses continuously during a year. The corresponding sun exposure behaviour was recorded in diaries. Similar data were collected for 28 volunteers during sun holidays in the winter half-year. The relationship between UV dose and sun exposure behaviour was analysed.
Results The ambient UV dose during the winter in Denmark (at 56°N) was 394 standard erythema doses (SED) or 10·5% of the annual ambient UV dose. In winter compared with summer the subjects had: (i) a lower percentage of ambient UV radiation, 0·82% vs. 3·4%; (ii) a lower solar UV dose in Denmark, 3·1 SED (range 0·2–52) vs. 133 SED (range 69–363); (iii) less time outdoors per day with positive dosimeter measurements, 10 min vs. 2 h; and (iv) no exposure (0 SED) per day on 77% vs. 19% of the days. Sun holidays outside Denmark in winter gave a median 4·3 SED per day (range 0·6–7·6) and 26 SED (range 3–71) per trip.
Conclusions In the winter half-year indoor workers received a negligible UV dose from solar exposure in Denmark and needed no UV precautions. No UV precautions are needed from November to February during holidays to latitudes above 45 °N, while precautions are needed the whole year around at lower latitudes.