This section is from the book "The Hygienic System: Fasting And Sun Bathing", by Herbert M. Shelton. Also available from Amazon: The Hygienic System Vol III Fasting and Sun Bathing.
Prolonged exposure of the unprotected skin to the sun's rays results in severe and painful burning, prostration and even death. Necessary and useful as is sunshine, it is a powerful chemical agent against which the body must protect itself to avoid serious damage. It .is estimated that the "normal," unprotected skin can endure fifty minutes of sunbathing without burn, but this estimate must not be taken too literally. First, we must ask: What is a normal skin? Next we must take into account the time and place for sunbathing. Is it to be taken at the shore or in the lowlands or on the mountains? Is it to be taken in the early morning or at noonday?
The body is well-equipped with defenses against the sun and burning can occur only when more sun is secured than the defenses can protect us against. There are three chief means of protection and these will be discussed in their logical order at this place.