How do sunscreens work?
Sunscreen products contain ingredients that help in absorbing, reflecting and/or scattering UV rays. These products are formulated to provide various levels of protection from UVA, UVB, and IR rays. The Sun Protection Factor rating system has been established by the Food and Drug Administration to measure primarily the amount of UVB sunburn protection the product will provide. No rating system has yet been established for measuring UVA protection.
The Sun Protection Factor (SPF) indicates how much longer an individual can be in the sun before becoming burned when using a sun protection product, than if he or she did not. For example, a fair skinned person who would normally start to burn after 10 minutes in the sun would receive 15 times that with an SPF 15 (150 minutes or 2 1/2 hours). If a person with darker skin takes longer to burn without protection, say 20 minutes, an SPF 15 would give him 300 minutes (5 hours). Reapplication using the same SPF reinforces the initial protection, but does not add time to the protection period. Application of a significantly higher SPF may extend the protection period, but should not be relied on if you suspect sunburn has begun to occur. Then it's time to seek shade.
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