How Sunblock Protects Your Skin: The Science Behind Its Effectiveness

Sunscreen protects skin by absorbing or reflecting ultraviolet rays. Mineral sunscreens such as zinc oxide reflect UV, and chemical sunscreens absorb it.

How it works

Sunscreens protect against UV radiation (UVA and UVB) in two ways:

• Chemical filters (avobenzone, octocrylene, oxybenzone) absorb UV and convert it to heat.
• Mineral filters (zinc oxide, titanium dioxide) mostly reflect and scatter UV, and also absorb some.

SPF

SPF 15 blocks about 93 percent of UVB, SPF 30 about 97 percent, SPF 50 about 98 percent.

Broad spectrum

Protects against both UVA (aging) and UVB (burning).

How to use

Apply 1 ounce (a shot glass) for the body, reapply every 2 hours and after swimming. Apply 15 minutes before sun.

FDA

The FDA regulates sunscreens as drugs.

Other protection

Hats, shade and clothing.

How it works

Sunscreens contain active ingredients that protect against ultraviolet (UV) radiation. Chemical sunscreens (avobenzone, octocrylene, oxybenzone, homosalate) absorb UV and convert it into small amounts of heat. Mineral sunscreens (zinc oxide and titanium dioxide) sit on the skin and mainly reflect and scatter UV, along with some absorption. Many products combine both types.

SPF

SPF (sun protection factor) measures protection against UVB rays, which cause sunburn. SPF 15 blocks about 93% of UVB, SPF 30 about 97%, and SPF 50 about 98%. The increase is small at higher numbers: SPF 100 blocks about 99%. SPF is tested at 2 milligrams of sunscreen per square centimeter of skin, and most people apply only 25% to 50% of that, so real protection is lower.

Broad spectrum

UVA rays penetrate deeper and cause aging and contribute to skin cancer. “Broad spectrum” on a label means the product protects against both UVA and UVB, and passes an FDA test. The American Academy of Dermatology recommends broad spectrum, water-resistant sunscreen with SPF 30 or higher.

How to use

• Apply about 1 ounce (a shot glass) to the whole body and about a nickel-size amount to the face.
• Apply 15 to 30 minutes before going out, though mineral sunscreens work right away.
• Reapply every 2 hours, and after swimming or sweating, even if the label says water-resistant (40 or 80 minutes).
• Cover the ears, neck, tops of feet and scalp part.
• Use sunscreen on cloudy days, since up to 80% of UV passes through clouds.
• Check the expiration date. Most expire in about 3 years.

FDA

The FDA regulates sunscreens as over-the-counter drugs. It considers zinc oxide and titanium dioxide generally recognized as safe and effective. Studies in 2019 and 2020 showed that some chemical ingredients are absorbed into the blood, but the significance is unclear, and the FDA asked for more data. Hawaii and some other places have banned oxybenzone and octinoxate to protect coral reefs.

Other protection

• Seek shade, especially between 10 a.m. and 4 p.m.
• Wear a wide-brimmed hat, UV-blocking sunglasses and UPF 50 clothing.
• Avoid tanning beds.

Regular sunscreen use cuts the risk of squamous cell carcinoma by about 40% and melanoma by 50%, according to studies.

Ingredients and labeling rules

Sunscreens work by absorbing UV (chemical filters like avobenzone and octocrylene) or by reflecting and scattering it (mineral filters zinc oxide and titanium dioxide). SPF 15 blocks about 93 percent of UVB rays, SPF 30 blocks about 97 percent and SPF 50 about 98 percent, according to the American Academy of Dermatology. The AAD recommends a broad spectrum, water resistant sunscreen of SPF 30 or higher, applying about 1 ounce, a shot glass amount, to the body and reapplying every 2 hours or after swimming or sweating. Under FDA rules, “water resistant” must state 40 or 80 minutes, and labels cannot claim “waterproof” or “sunblock.” Zinc oxide and titanium dioxide are the only ingredients the FDA currently considers generally recognized as safe and effective. Sunscreen should be used with shade, hats and protective clothing. Babies under 6 months should be kept out of direct sun.

Based on data from the American Academy of Dermatology and the FDA