Ultraviolet rays damage DNA and cause "dilation of the cutaneous blood vessels and recruitment of inflammatory cells," leading to swelling, pain and redness, says dermatologist Jeffrey M. Sobell, an assistant professor and the director of photomedicine at Tufts University School of Medicine. The ingredients in sunscreens absorb, scatter or reflect ultraviolet rays. Many people fail to apply enough sunscreen or to reapply it at the correct intervals. Darker skin or an existing tan do not prevent sunburn or skin cancer, and sunburn can occur on cloudy days.

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