Sunscreens work either by absorbing ultraviolet light before it reaches the skin or by reflecting light away from it. SPF, the familiar measurement of most sunscreens, refers to the level of protection against UVB rays. Sunscreens with SPF 15 or 30 block 93% and 96.7% of UVB light, respectively. Dermatologists recommend blocks with a high SPF and ingredients such as Parsol 1789, titanium dioxide or zinc oxide, which offer protection against a broader spectrum of light, including dangerous UVA and UVB rays.
Sunscreen should be applied 30 minutes before sun exposure and reapplied every two hours. Factor in perspiration, temperature, humidity and time spent in the water and even more frequent reapplication is necessary even if the product is "waterproof". The Journal of American Academy of Dermatology reported recently that most people don't apply enough sunscreen to reach the level of SPF listed on the label: the best bet is to opt for SPF 30 to ensure that you are getting effective coverage. To further maximize your protection, avoid direct sunlight during 10am and 4pm and wear a hat with a wide brim or clothes of woven fabrics scientifically designed to minimize UV absorption.