The surface of the skin is constantly replenished with cells. The cells produced in the lower part of the epidermis take about one month to reach the horny layer. On their journey to the surface of the skin they become increasingly dehydrated and are finally rejected as dead skin cells. That is how the skin constantly regenerates itself. Eighty percent of the aging process is due to external factors that damage the skin. UV radiation causes the greatest stress to skin. UV light promotes the formation of free radicals, which in turn activate enzymes that specialize in breaking down collagen.