The higher the SPF, the greater the protection against sun rays. Under perfect laboratory conditions, an SFP 15 blocks 93 percent of ultraviolet 'B' radiation and SPF 30 blocks 97 percent, says Pesqueira. However, variation in application technique may result in lower than advertised SPF effect. So a 30-50 SPF product gives an added margin of protection over just a 15 SPF when you factor in the application of a sunscreen. For maximum protection when using a sunscreen, apply it liberally and on a daily basis about 30 minutes before going outside if you'll be outdoors for more than 10 minutes.
Cloudy days are no exception.
Clouds may block the light but they do not block the ultraviolet rays. Even if the sunscreen is waterproof or water resistant, it should be reapplied for continuous protection after swimming, toweling, or any vigorous activity that causes heavy perspiration. The use of sunscreens is not limited to adults. Although sunscreens are not recommended for babies younger than six months, evidence suggests wearing sunscreen every day during the first 18 years of life can reduce the risk of worst skin cancer by 78 percent. And wearing it throughout your entire life can reduce the chances by 98 percent.