All of us have done it- spent too much time in the sun. The result is a painful sunburn, which is the body's reaction to overexposure. As the hole in the ozone layer gets bigger, we will get more and more UV radiation. Most of our UV exposure comes from the sun, and skin cell damage is happening faster and faster, resulting in millions of cases of skin cancer.
Many wavelengths of light are emitted by the sun, but the primary source of damage comes from UVA and UVB rays. UVA causes the skin cells to age rapidly and leads to age spots, reduced elasticity and wrinkles. UVB causes DNA damage in the form of a sunburn. A person's level of exposure is determined by a number of factors: time of day, time of year, cloud coverage and elevation. The UVI, or UV index, was developed by the EPA and the National Weather service to give people an idea of the amount of radiation that will make it to the surface of the earth. The index has a range of 1-11, with higher numbers indicating higher exposure.
When you are selecting a sunscreen, a product with a higher SPF (sun protection factor) will provide a higher level of protection. The SPF is the amount of protection from UVB only. Products that have at least three main ingredients are considered to be "broad spectrum" and they protect from both UVA and UVB rays. These ingredients can include ecamsule, titanium dioxide, zinc oxide, and avobenzone.
Though no sunscreen offers total protection, the ACS (American Cancer Society) recommends a sunscreen with a 15 or higher SPF. Other organizations like the AAD (American Academy of Dermatology) recommend a minimum SPF of 30. SPF 15 offers about 93% UVB protection, 30 offers 97% protection, and 50 offers almost 98% protection.
Most sunscreens need to be reapplied ever two hours or even less. Reapplication, sweating, toweling and swimming affect the level of protection; waterproof products can withstand 80 minutes or more of swimming, while water-resistant products offer enhanced protection for a minimum of 40 minutes. Back when I was working at The Arroyo Golf Club in Las Vegas, I saw golfers apply sunscreen before a round, and come back all burnt simply because they didn't reapply. According to the American Cancer Society, SPF 30 is the same as one minute of unprotected exposure every half hour.