The active UVB wavelengths are present in sunlight, and sufficient amounts of cholecalciferol can be produced with moderate exposure of the skin, depending on the strength of the sun. Time of day, season, and altitude affect the strength of the sun, and pollution, cloud cover or glass all reduce the amount of UVB exposure. Exposure of face, arms and legs, averaging 5–30 minutes twice per week, may be sufficient, but the darker the skin, and the weaker the sunlight, the more minutes of exposure are needed. Vitamin D overdose is impossible from UV exposure; the skin reaches an equilibrium where the vitamin degrades as fast as it is created.