It's just like wearing a seatbelt. You are not usually in an accident but you put it on anyway for protection. Ultraviolet rays are a form of radiation. Therefore you need to protect yourself from long term damage. Its effect is cumulative.
Sunscreens vary in the amount of protection they provide.
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.