The sun-protection-factor rating system, which was introduced in 1978, is mainly a measure of the time it takes your skin to burn – and that depends on how fair or dark your skin is. If, say, your unprotected skin turns pink after 10 minutes of sun exposure, then it would take 15 times as long, or 150 minutes, when you’re wearing an SPF15. The SPF number also indicates what percentage of UV rays the product blocks. An SPF2 blocks 50 percent of rays, and SPF15 blocks 93 percent and and SPF30 97 percent. Any SPF above 30 give you only marginally more protection, which is why doctors often say that SPF30 is as high as anyone needs to go.

How to protect yourself

Since applying the right sunscreen is your first priority, use the following checklist to see if a product measures up
Broad-spectrum protection: Products with these words on their labels will filter out most UVA and UVB rays.
Physical blocks: Sunscreens that contain titanium dioxide and zinc oxide reflect the widest range of UVA and UVB rays. Physical blocks work like a mirror, reflecting UV rays back into the environment. And these days you don’t need to sport a white nose to wear a physical block – the zinc and titanium particles are now micronised, which means they’re so fine they’re invisible, but still strong enough to form a curtain-like barrier. Apply physical blocks after moisturizing.
Chemical blocks: Synthetic sunscreens contain benzophones, cinnamates, Parsol 1789 and salicylates that absorb UV light instead of reflecting it. Dermatologists agree that suncreens should ideally contain physical and chemical blocks. If your skin is sensitive, however, look for sunscreens with only physical blocks to avoid skin reactions. Apply chemical blocks before moisturizing.
Water- and sweat-resistant: These products will maintain their SPF for up to 40 minutes in water, and will stand up to humidity and perspiration.

Apply it right

You are the biggest block to a sunscreen’s effectiveness. Here are tips to avoid damage.
Coat your entire face and body before going outside. You don’t want to get to the beach, spend half an hour setting up your towel and umbrella, and then sit down to put on sunscreen. Chemical sunscreens take 20 to 30 minutes to seep into the skin to protect you properly. And remember to apply sunscreen to your feet if you intend to tan your bum.
Use it liberally. Studies have shown that using only half the necessary amount of sunscreen can turn what you think is a coating of SPF30 into no more than and SPF2. The proper amounts for complete coverage: a full shot glass on you body, one teaspoon for your face, and another teaspoon on your neck.
Reapply it often. Depending on what you’re doing, apply between every two to four hours.