Some of the most popular sunscreen lotions on the market might not adequately protect against cancer, the Federal Drug Administration acknowledged when it release new sunblock regulations last week.
To understand why requires some science. There are two types of ultraviolet rays from sunlight that cause skin aging and cancer: UVA rays and UVB rays. The typical sunscreen lotion protects against UVB, which causes sunburns, but not necessarily to UVA, which also causes skin cancer.
Next year, the FDA will require a stricter test for all sunscreens to ensure they protect against both ultraviolet wavelengths. They are also proposing to make SPF 50 the highest possible rating.
"Consumers should look for a product labeled 'broad spectrum' in the same font, color and size as the SPF label," the FDA's Dr. Reynold Tan said. "These are the only products that are proven to protect against both skin cancer and sunburns, both UVA and UVB rays."
If the bottle does not say "Broad Spectrum SPF 15" or higher, then the product only protects against sunburn, and the FDA will require a warning label. Bottles marked simply "SPF 30" or "Broad Spectrum SPF 10" are not known to adequately protect against cancer or aging.
HOW SPF IS MEASURED (AND WHAT THE NUMBERS REALLY MEAN)
Sunscreen is often tested on the backs of human subjects. One "site" is treated with sunscreen, and one site is left bare. Scientists zap the unprotected site with a solar lamp that simulates the sun's radiation. Then scientists zap the site treated with sunscreen until it causes the same sunburn. If the product-treated site takes 30-times longer to burn that the untreated site, the product gets the rating SPF 30.
Each minute wearing SPF 30 sunscreen lotion, you get a 1/30th, or 3.33%, of UV exposure that you would get without the lotion. Subtracting that 3.33 percentage points from 100, you're protected from 97% of UVB rays with SPF 30 sunscreen.
The takeaway: If you want to impress friends at your local CVS, take the inverse of the number after SPF and subtract it from 100. That's the percent protection you're getting with the sunscreen lotion. It also explains why you see diminishing returns. SPF 10 is 90% protection. SPF 30 is 97%. SPF 80 sounds more than twice as strong. But with 99% sunblock, it provides almost identical protection.
'MORE THAN A THIRD'
Whether UVA or UVB contributes more to skin cancer has not been determined. Some animal data shows that UVB contributes more to cancer and aging than UVA. But humans get more UVA exposure than UVB. "Nobody has determined UVB or UVA's percent contribution to skin cancer and early aging," Dr. Tan said.
Consumers expect sunscreen products to protect against sunburn and skin cancer. But according to one study mentioned by Dr. Tan, more than a third of the sunscreen products with SPF protection might not pass the FDA's new regulations. Men's skin cancer rates have doubled in the last 30 years.
"Unfortunately, we've allowed manufacturers to use the term 'broad spectrum' if the lotion provided any protection against UVA. But there are products on the shelves today that have the 'broad spectrum' label but do not meet our broad spectrum standards" he said.