(characterized by low blood pressure, fainting, and profound weakness). Several days after a sunburn, people with naturally fair skin may have peeling in the burned area, usually accompanied by itching. These peeled areas are even more sensitive to sunburn for several weeks. The best – and most obvious – way to prevent sun damage is to stay out of strong, direct sunlight. Clothing and ordinary window glass filter out most of the damaging rays. Water is not a good filter: ultraviolet A (UVA) and ultraviolet B (UVB) light can penetrate a foot of clear water. Clouds and fog are also not good filters of UV light; a person can get sunburned on a cloudy or foggy day.
Before exposure to strong direct sunlight, a person should apply a sunscreen, an ointment or cream containing chemicals that protect the skin by filtering out UV light. Sunscreens contain substances, such as para-aminobenzoic acid (PABA) and benzophenone, that absorb UV light. Sunscreens containing PABA must be applied 30 to 45 minutes before going out in the sun or into the water. Many sunscreens contain both PABA and benzophenone or other chemicals; these combinations provide protection from a broader range of UV light. Many sunscreens claim to be either waterproof or waterresistant, but most of these nonetheless require more frequent application in people who are swimming or sweating.
In the United States, sunscreens are rated by their sun protection factor (SPF) number – the higher the SPF number, the greater the protection. Sunscreens rated between 0 and 12 provide minimum protection; those rated between 13 and 29 provide moderate protection; those rated 30 and above provide maximum protection.