Influence on SPF of the quantity of sunscreen product applied

It is now a clearly established fact that ultraviolet radiation is involved in the induction of skin cancer such as carcinoma and melanoma. The most efficient means of protection is by using clothes. For uncovered areas, it is essential to apply a sun product. For some time, various authors have been wondering about the inefficiency or indeed the dangerousness of sun products. Cases of sunburn are still just as frequent in children. We have chosen to study the influence of the quantity of sun product applied on the skin on the level of efficiency obtained. Twenty commercially-available products with SPFs varying between 10 and 50+ and belonging to all levels of protection, that is to say "low", "medium" and "high", were tested in vitro at 5 different doses (5.0, 7.5, 10.0, 12.5 and 15.0 mg for a surface area of 25 cm(2)). We were able to demonstrate that there was a polynomial relationship between the SPF and the quantity of product on the skin's surface. It can be seen that when the dose is halved, the SPF is divided by a variable factor according to the product, from 1.5 to 3.8.