How Does Sunscreen Work?
Sunscreen works in two ways depending on type: mineral sunscreens (zinc oxide, titanium dioxide) sit on skin and physically reflect/scatter UV rays; chemical sunscreens absorb UV rays and convert them to heat. Both prevent UV radiation from damaging skin cells. SPF indicates protection level against UVB (burning) rays.
Key Takeaways
- UV radiation from the sun comes in two main types that affect skin: UVA (aging rays, penetrate deeper) and UVB (burning rays, affect surface).
- SPF 30 blocks about 97% of UVB rays; SPF 50 blocks about 98%—higher numbers offer diminishing returns.
- No sunscreen is "waterproof"—water-resistant formulas need reapplication after swimming.
Explanation
UV radiation from the sun comes in two main types that affect skin: UVA (aging rays, penetrate deeper) and UVB (burning rays, affect surface). This is the same sunlight that makes the sky appear blue. Both contribute to skin damage and cancer risk. Effective sunscreen protects against both, which is why "broad spectrum" labeling matters.
Mineral (physical) sunscreens use zinc oxide or titanium dioxide particles that sit on the skin surface. These minerals reflect and scatter UV rays before they can penetrate. They start protecting immediately upon application and are less likely to irritate sensitive skin.
Chemical sunscreens contain compounds like avobenzone or oxybenzone that absorb UV radiation. The energy transforms into harmless heat that dissipates from the skin. These tend to rub in more easily without leaving white residue but need 15-20 minutes to become fully effective.
Newer mineral sunscreen formulations use micronized or nano-sized zinc oxide particles (20-200 nanometers) that reduce the white cast traditionally associated with physical sunscreens. Tinted mineral sunscreens add iron oxides that also block visible light and blue light, providing additional protection that untinted formulas miss. These advances have made mineral sunscreens much more cosmetically acceptable for daily wear across all skin tones. Check whether makeup expires on a similar timeline to sunscreen.
Recent research has raised questions about certain chemical UV filters. Studies found that ingredients like oxybenzone, octinoxate, and homosalate can be absorbed into the bloodstream at levels exceeding FDA safety thresholds after a single application. Hawaii banned oxybenzone and octinoxate in reef areas because they contribute to coral bleaching at concentrations as low as 62 parts per trillion. Zinc oxide and titanium dioxide remain classified as GRASE (Generally Recognized as Safe and Effective) by the FDA.
Things to Know
- SPF 30 blocks about 97% of UVB rays; SPF 50 blocks about 98%—higher numbers offer diminishing returns.
- No sunscreen is "waterproof"—water-resistant formulas need reapplication after swimming. Staying hydrated is equally important during sun exposure.
- The "teaspoon rule": you need about a teaspoon for your face and a shot glass worth for your body.
- Expired sunscreen loses effectiveness—check dates and replace yearly.
- Clothing with UPF (Ultraviolet Protection Factor) ratings provides consistent sun protection without reapplication—a UPF 50 shirt blocks 98% of UV, equivalent to SPF 50 sunscreen.