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Sun Protection Factor (SPF) represents the length of time you can stay out in the sun without burning, multiplied by the corresponding number. So a person who would normally start to burn in 10 minutes, could theoretically have 150 minutes of sun protection with a sunscreen that has an SPF of 15.
Regardless of SPF number, testing of sunscreen does not really correspond to the amount of time you can stay out in the sun without burning because sweating, clothing rubbing against the skin, and water sports will all wear the sunscreen off. Tests also use much more sunscreen than do most people, so SPF number results can be inaccurate. Usually, an SPF 15 rated sunscreen will give your about an hour of protection before you should reapply sunscreen. This, of course, varies from person to person. It also helps to wear protective clothing, hats and sunglasses.
The SPF number only relates to ultraviolet B (UVB) protection. Ultraviolet A (UVA) protection is not measured through these numbers, and until recently, frequently was not available in sunscreens. UVB rays are more potent, quicker to produce sunburn, and have been linked to skin cancer. UVA is associated with aging of the skin, and along with UVB exposure, may increase risk or facilitate skin cancer. UVA though milder, is still not safe.
Some chemicals found in new sunscreens that also boast an SPF number can help block UVA rays. Chemicals like Parsol 1789 block both UVA and UVB rays. Unfortunately, these chemicals are a cause of concern to environmentalists because they have been found in water, groundwater and soil. Potential long-term effects of exposure are unknown.
What remains important to remember is that sunscreen strength is less important than frequent application, and avoidance of sun between 10 am and 4 pm, when UVB rays are strongest. Sunscreen protection should always be combined with protective clothing and sun avoidance during the hottest and brightest parts of the day.
Some people do pay more for higher SPF protection, and this may make sense for those with lighter skin. Doctors generally recommend purchasing an sunscreen of 15 or higher. There are lower ones, which will cause the skin to tan or burn in under an hour. It may be unnecessary to purchase an SPF higher than 15, except possibly for infants, if you can be vigilant about reapplying the sunscreen every hour, and after swimming or very vigorous physical activity.