How Does Sun Cream Work? | UV Filters Explained

Sun cream—also called sunscreen—reduces your skin’s exposure to ultraviolet radiation. Most modern formulas combine filters that absorb UV energy and others that reflect or scatter some UV rays. The result is a protective layer that stops most of the sun’s damaging wavelengths from penetrating your skin in the first place.

Here’s what happens on a chemical level and what you actually need to know about SPF, broad-spectrum coverage, and getting the protection you paid for.

What Does SPF Actually Measure?

SPF stands for Sun Protection Factor, and it measures UVB protection—the wavelengths that cause sunburn. The FDA defines SPF as the ratio of UV exposure needed to cause sunburn on protected skin versus unprotected skin.

Higher numbers mean more protection, but the gains shrink as you climb:

• SPF 15 blocks roughly 94% of UVB rays
• SPF 30 blocks about 97% of UVB rays
• SPF 50 blocks approximately 98% of UVB rays

No sunscreen blocks 100% of UV radiation. The FDA notes that all sunscreens are tested by comparing how much UV is needed to cause sunburn with and without the product applied.

Why Broad Spectrum Matters

SPF alone only tells you about UVB protection. UVA rays don’t cause visible sunburn as quickly, but they penetrate deeper and contribute to premature skin aging and skin cancer risk.

That’s why the FDA requires sunscreens labeled “broad spectrum” to pass a separate test measuring UVA protection too. To earn that label, a product must have a critical wavelength of at least 370 nanometers—meaning it blocks a meaningful portion of UVA rays as well as UVB. In the U.S., a sunscreen must be broad spectrum AND have an SPF of at least 15 before it can claim to help prevent skin cancer and early skin aging.

For digging in the garden or spending a day outdoors, our tested picks for the best sun protection creams cover the formulas that hold up best under real conditions.

How Much Sunscreen Should You Use?

The most common mistake is applying far too little. Most adults need about 1 ounce—a full shot glass—to cover all exposed skin on their body. For the face alone, the AAD recommends at least 1 teaspoon.

Apply sunscreen about 15 minutes before going outside so the product can bond with your skin and start working. Cover every exposed area: neck, ears, face, tops of feet, legs, and arms. For your back and other hard-to-reach spots, ask someone for help rather than skipping it.

Don’t assume one application lasts all day. Reapply every 2 hours and immediately after swimming, heavy sweating, or towel-drying. Even water-resistant formulas eventually break down.

Sprays, Sticks, and Common Mistakes

Spray and stick sunscreens work fine as long as they’re broad spectrum, water-resistant, and SPF 30 or higher. For sprays, apply generously until the skin visibly glistens, then rub it in—a quick misting doesn’t provide adequate coverage.

Tinted sunscreens containing iron oxide may offer better protection against dark spots and hyperpigmentation because iron oxide blocks visible light in addition to UV radiation.

The three most common reasons people still get burned: using too little product, forgetting to reapply, and using expired sunscreen. Sunscreen does lose effectiveness over time, so check the expiration date and replace it when needed.

Chemical, physical, and hybrid formulas all form a protective layer on your skin. Physical sunscreens (zinc oxide, titanium dioxide) reflect some rays in addition to absorbing them, but both types are effective when used correctly.