Are You Using the Right Sunscreen? A Guide to SPF, Broad-Spectrum, and More

Standing in the sunscreen aisle can feel overwhelming when you’re trying to make an informed decision about your skin. Dozens of bottles promise protection, but they all list different sun protection factor (SPF) numbers, claim to be “broad-spectrum,” and use terms like “water-resistant” without explaining what any of it actually means.

Our board-certified dermatologists at Vivida Dermatology, with locations in Las Vegas and Henderson, Nevada, and St. George, Utah, help patients understand what to look for in sunscreen and how to use it correctly to prevent sun damage, premature aging, and skin cancer.

SPF only measures UVB protection

Sun protection factor (SPF) tells you how well a sunscreen protects against ultraviolet B (UVB) rays, which cause sunburn and contribute to skin cancer. The number indicates how much longer you can stay in the sun before burning compared to wearing no sunscreen.

SPF 30 blocks about 97% of UVB rays, while SPF 50 blocks roughly 98%. The difference between SPF 30 and SPF 100 is minimal in terms of actual protection. No sunscreen blocks 100% of ultraviolet (UV) radiation.

Most dermatologists recommend SPF 50 for extended sun exposure or if you have fair skin, a history of skin cancer, or conditions that increase sun sensitivity.

Broad-spectrum protection covers UVA rays, too

UVB rays cause sunburn, but ultraviolet A (UVA) rays penetrate deeper into your skin and cause aging, wrinkles, and also contribute to skin cancer. A sunscreen labeled “broad-spectrum” protects against both UVA and UVB radiation.

Many people don’t realize they can get significant sun damage without burning. UVA rays pass through clouds and windows, meaning you’re exposed even on overcast days or while driving. Broad-spectrum protection is the only way to guard against this invisible damage.

Physical vs. chemical sunscreens

Sunscreens use either physical (mineral) or chemical filters to block UV radiation. Physical sunscreens contain zinc oxide or titanium dioxide, which sit on your skin’s surface and reflect UV rays away. Chemical sunscreens absorb UV radiation and convert it to heat, which your skin then releases.

Physical sunscreens work immediately after application and tend to cause less irritation, making them better for sensitive skin, rosacea, or children. The downside is they can leave a white cast on darker skin tones, though newer formulations have improved.

Chemical sunscreens blend into skin more easily and feel lighter, but they take about 15-20 minutes to become effective after application. Some people experience irritation or allergic reactions to chemical filters, especially avobenzone and oxybenzone.

Neither type is inherently better — choose based on your skin type, preferences, and how well you’ll actually use it consistently.

Water-resistant doesn’t mean waterproof

No sunscreen is truly waterproof. “Water-resistant” sunscreens typically maintain their SPF for 40 or 80 minutes when swimming or sweating. After that time, you need to reapply.

If you’re swimming, sweating heavily, or toweling off, reapply sunscreen at least every two hours regardless of the water-resistant claim. Most people don’t apply enough sunscreen in the first place, and water washes away more than you think.

You’re probably not using enough sunscreen

Most people apply only 25-50% of the product listed on the bottle. You need about one ounce (a shot glass full) to cover your entire body, and a nickel-sized amount just for your face.

Reapplication matters as much as the initial application. Sunscreen breaks down with sun exposure, gets rubbed off by clothing or towels, and wears away with sweat. Reapply every two hours when you’re outdoors, or more frequently if swimming or sweating.

Don’t forget commonly missed spots — ears, the back of your neck, the tops of your feet, and your scalp if you have thin hair or a part line.

Pay attention to expiration dates

Sunscreen loses effectiveness over time. The active ingredients break down, especially when exposed to heat and light. Check the expiration date before using sunscreen, and throw away any bottles that have been open for more than a year or have changed color or consistency.

Storing sunscreen in hot cars or direct sunlight accelerates degradation. Keep it in a cool, dark place when possible.

Sunscreen and skin protection in Las Vegas, Henderson, and St. George

The right sunscreen is one you’ll actually use every day. Our dermatologists can recommend specific products for your skin type and answer questions about proper application.

Call Vivida Dermatology or book an appointment online at one of our Nevada or Utah locations.