Sunscreen Ratings: Everything You Need To Know About SPF, UVA, UVB & More

SPF is an acronym we all use when referring to sunscreen but don't always completely understand. If you're not careful, a high SPF can lure you into a false sense of security against the ultraviolet rays of the sun.

More than 5 million cases of skin cancer are diagnosed each year in the United States—more than all other cancers combined. Currently, one in five Americans will develop some form of it by the time they hit 70. The most common (but least lethal) form of skin cancer is basal cell carcinoma. Squamous cell carcinoma is four times less common than basal cell but causes five times as many deaths per year. And melanoma, the big bad wolf of skin cancer, is the least common (1 percent of all skin cancers) but causes the majority of skin cancer deaths—approximately one American per hour dies of melanoma.

UVB rays cause sunburn; think B for burn. It’s those severe, childhood sunburns that can cause melanoma many years down the line. UVB light also contributes to basal and squamous cell carcinomas. UVB rays are most prevalent between 10 a.m. and 4 p.m. and do not penetrate glass (car windows, for example) to a significant degree. As explained above, the SPF rating system is based solely on UVB exposure.

UVA is far more prevalent and penetrates more deeply into the dermal layer. It is present any time there is daylight and can penetrate glass. It causes squamous cell carcinoma and contributes to basal cell carcinoma. UVA light causes photo-aging by damaging collagen and causes immune suppression, possibly making it easier for skin cancer and other cancers to gain ground.

This is one of the most confusing aspects of the SPF system. Most people believe that an SPF 30 product offers twice the sun protection of an SPF 15 product, and that would make a lot of sense. In reality, though, an SPF of 15 blocks 93 percent of the UVB rays, whereas an SPF 30 blocks 97 percent, making the SPF 30 product only 4 percent more effective at blocking UVB rays. Bump your SPF to 50, and you’re only getting 1 percent more protection by blocking 98 percent of UVB radiation. In short, choose an SPF of at least 15, and don’t fall for the SPF 100 trick—you never need more than a 50.

Zinc oxide is the superstar of the sun protection world. It protects against UVA and UVB rays and does not get absorbed by the skin. Instead, it sits on top and deflects the sun's rays like a mirror would.

Titanium dioxide is another barrier sunscreen but is mainly active against UVB rays. Avobenzone is one of few ingredients approved in the United States for UVA protection and has a low toxicity profile. Mexoryl is another UVA screen with a similar profile, currently awaiting FDA approval.

The two simplest and most effective ways to limit sun damage are to wear more clothes and stay out of the sun between 10 a.m. and 4 p.m., when its rays are most perpendicular to the earth.

UPF clothing is an incredible way to protect your skin from both UVA and UVB radiation. The UPF rating system is more reliable than the SPF system, so if it says UPF 50, you’re blocking out at least 98 percent of the sun’s damaging rays. If UPF clothing is not in the budget, look for tightly woven cotton, which also has a UPF close to 50.

And remember that the amount of solar exposure is more important than the number of minutes spent in the sun: One hour on a mountain in sunny Colorado is far more impactful than an hour at sea level on an overcast day.

Skin cancer is an almost entirely preventable disease. So it’s time to get educated, share your knowledge, and take action to keep yourself and your loved ones safe from the sun.