Sun: friend or foe?

Solar radiation contains UV rays, which are not visible to the naked eye: UVCs are stopped by the atmosphere, UVBs are stopped by glass, UVAs (short and long) are able to cross glass, whilst infrared diffuse heat. The quality of solar radiation varies according to latitude, season, cloud cover, altitude, pollution and time of day. Sunshine results from direct radiation, solar radiation diffused in the atmosphere, and radiation reflected by the ground (snow reflects from 75 to 95%, sand from 15 to 25%, water from 10 to 20%, grass from 0.4 to 4%).

Due to protection from the ozone layer and the atmosphere, only a part of solar radiation actually arrives at the surface of the Earth. Skin is mostly penetrated by UVBs (70%) which are absorbed by the cornea layer; 20% reach the epidermis, 10% go as far as the superficial dermis, and only 20 to 30% reach the deep dermis. The infrared rays cross the epidermis and dermis to reach the hypodermis.
 
Precautions to take
Avoid going out into the sun between 11am and 3pm. Exposure to the sun should be progressive. Use products which are adapted to each skin-type. Reapply protection regularly, especially after being in water or playing sport. Protect your eyes from the sun. Mind out for glares: even under a hat or parasol, you are still at risk from sunburn.