Sun drying requires little more than racks and sufficient sunlight. Populations with light skin pigmentation evolved in climates of little sunlight. The function is to protect against damage from the ultraviolet radiation of sunlight. Structure provides protection from predators, bright sunlight, and higher water temperatures. Natural skin color can darken as a result of tanning due to exposure to sunlight. It is designed to protect a person against rain or sunlight. Solar water disinfection has been researched for cheaply treating contaminated water using natural sunlight. Sunlight and heat may weaken the adhesive. The effects of attack increase with exposure time and sunlight intensity. It competes with other species for sunlight, moisture and nutrients. Adaptations include resistance to drought and tolerance of intense heat and sunlight. These properties include resistance to heat, acids, alkalies, mildew, bleaches, aging, sunlight, and abrasion. UV degradation is one form of polymer degradation that affects plastics exposed to sunlight. Vitamin D production depends on exposure to sunlight. Risk factors include diabetes, smoking tobacco, prolonged exposure to sunlight, and alcohol.