Erythemal and vitamin D weighted solar UV dose-rates and doses estimated from measurements in mainland France and on Réunion Island.

Solar UV radiation causes beneficial and detrimental changes in human health. International and national Health agencies recommend avoiding sun exposure when the solar rays are strongest (typically 2 h before and after solar noon). In this study we detail and refine such recommendations. We estimated biologically-effective radiation (inductive of erythema and pre-vitamin D) using spectral solar UV radiation measurements on a horizontal plane at three French sites equipped with spectroradiometers: Villeneuve d'Ascq (VDA) (North of France); Observatoire de Haute-Provence (OHP) (French Southern Alps); and Saint-Denis de La Réunion (SDR) on Réunion Island, in the Indian Ocean. These sites are very different: VDA is a semi-urban site in a flat region, OHP a rural mountainous site and SDR a coastal urban site on a small mountainous island. Biologically active radiation was analyzed by studying erythema induction and measuring pre-vitamin D synthesis. Dose-rates, doses and times for sunburn induction and vitamin D production were derived. Regarding the level of vitamin D dose considered here (1000 IU), we found that at mainland sites time required for vitamin D synthesis was relatively long, even around solar noon, in winter months this could be 2-3 h for phototype II individuals exposing their face and hands. In the tropics vitamin D could always be synthesized in a reasonable time (e.g. 20 min in winter). By contrast, in summer, the required duration times (exposing face, hands, arms and legs) are very short, approximately 2-4 min on the mainland and 1 min in the tropics for phototype II individuals. In all skin phototypes the duration of sun exposure required to induce erythema was generally longer than that to produce vitamin D. These quantitative results, obtained using an instrument measuring on a horizontal plane and with an unobstructed view, do not represent realistic values for human exposure. To account for realistic human body exposure, received doses and times of exposure were adjusted. Our study shows that, mostly in summer, the time periods where limited solar exposure is recommended should be extended, especially at low latitude locations.