When you've shopped for sunscreen , you have probably noticed products with very high sun protection factor (SPF) ratings.

But is a 100+ or a 90+ sunscreen really that much better than one with an SPF of 30?

SPF refers to the ability of a sunscreen to block ultraviolet B (UVB) rays, which cause sunburns, but not UVA rays, which are more closely linked to deeper skin damage. Both UVA and UVB contribute to the risk of skin cancer.

The SPF rating is a measure of the time it would take you to sunburn if you were not wearing sunscreen as opposed to the time it would take with sunscreen on.

"SPF is not a consumer-friendly number," says Florida dermatologist James M. Spencer, MD. "It is logical for someone to think that an SPF of 30 is twice as good as an SPF of 15 and so on. But that is not how it works."

According to Spencer, an SPF 15 product blocks about 94% of UVB rays; an SPF 30 product blocks 97% of UVB rays; and an SPF 45 product blocks about 98% of rays.

"After that, it just gets silly," he says.

Sunscreens with higher SPF ratings block slightly more UVB rays, but none offers 100% protection.

Spencer recommends SPF 30 products to his patients.

Farah Ahmed, who is general counsel for the cosmetics industry group Personal Care Products Council, concedes that the difference in sunburn protection between the medium- and high-SPF sunscreens is not great.

But, she says, the high SPF products may protect better against long-term skin damage and exposure-related skin cancers.

Whatever product you choose, experts recommend using a water-resistant sunscreen applied liberally one half hour before going outdoors. Sunscreen should be reapplied at least every two hours or after swimming, drying off, or sweating.

"The best way make sure you are protected is to reapply sunscreen often," Spencer says. "You just can't put it on in the morning and forget about it. I don't care if it's SPF 800 or the best UVA protection, after a few hours it's gone."