Sunscreen Science: SPF, UVA/UVB, and Daily Protection Explained

Most people think sunscreen is just for beach days. But if you’re not using it every day, you’re putting your skin at risk - even in Halifax in November. The sun doesn’t need to feel hot to damage your skin. UVA rays, the silent aging agents, cut through clouds, windows, and winter coats. They’re there every daylight hour, year-round. And they’re not the only threat.

What SPF Actually Means (And Why It’s Not Linear)

SPF stands for Sun Protection Factor. It measures how well a sunscreen blocks UVB rays - the ones that cause sunburn. But here’s the catch: SPF doesn’t work like a linear scale. SPF 15 blocks 93.3% of UVB rays. SPF 30 blocks 96.7%. SPF 50 blocks 98%. That’s it. The jump from 30 to 50 is just 1.3% more protection. So why do dermatologists push for SPF 30+? Because most people apply too little. If you use half the amount you should, SPF 30 drops to the level of SPF 15. SPF 50 drops to SPF 25. That’s why experts recommend SPF 30 as the minimum - it gives you a safety net for under-application.

The formula is simple: absorption = 100 - (100 / SPF). That means SPF 30 isn’t twice as strong as SPF 15. It’s barely stronger. But in real life, where people smear on a pea-sized amount instead of a shot glass full, higher SPF gives you breathing room.

UVA vs. UVB: The Hidden Enemy and the Obvious One

UVB rays are the burners. They hit the top layer of your skin, cause redness, peeling, and directly damage DNA. That’s how skin cancers like basal cell carcinoma start. But UVA rays? They’re the slow destroyers. They make up 95% of the UV radiation that reaches Earth. They go deeper - past the epidermis, into the dermis - where they break down collagen and elastin. That’s what gives you wrinkles, sagging, and sun spots. UVA doesn’t burn. It just ages you. And it doesn’t care if it’s cloudy, winter, or you’re sitting by a window.

That’s why “broad spectrum” isn’t just a marketing term. It’s a legal requirement. To be labeled broad spectrum in the U.S., a sunscreen must pass a critical wavelength test of at least 370 nanometers. That means it protects against a wide enough range of UVA rays to match its SPF level. The FDA requires UVA protection to be at least one-third of the labeled SPF. So SPF 30 should offer at least SPF 10 UVA protection. But the Skin Cancer Foundation now demands more. For their Seal of Recommendation, SPF 50+ products must have UVA protection equivalent to SPF 20 or higher. That’s a big upgrade from 2020 standards.

Mineral vs. Chemical: Which One Should You Use?

There are two main types of sunscreens: mineral and chemical. They work differently, and your skin type should decide which one you pick.

Mineral sunscreens (also called physical) use zinc oxide and titanium dioxide. These sit on top of your skin and reflect UV rays like tiny mirrors. They start working immediately. They’re less likely to irritate sensitive skin or cause breakouts. That’s why 78% of people with acne or rosacea report fewer reactions when switching to mineral formulas. But they can leave a white cast, especially on darker skin tones. Newer versions use micronized particles and tinted bases to fix this. Brands like Suntribe and CeraVe have made big strides here.

Chemical sunscreens use ingredients like avobenzone, octinoxate, and octocrylene. These absorb UV rays and turn them into harmless heat. They’re usually lighter, more cosmetically elegant, and blend in better. But they need 15-20 minutes to activate. And some ingredients - especially oxybenzone - can irritate skin or cause stinging around the eyes. About 28% of negative reviews on Amazon and Trustpilot mention eye stinging from chemical sunscreens. If you have sensitive skin, eczema, or acne, start with mineral. If you hate the white cast and don’t react to chemicals, go with a modern chemical formula like La Roche-Posay Anthelios.

Here’s what the data shows:

Daily Use Isn’t Optional - Even in Winter

Dr. Leslie Baumann, a board-certified dermatologist and author of Cosmetic Dermatology, says it plainly: “UVA rays are equally intense during all daylight hours throughout the year and can penetrate glass.” That means your car window, office window, and kitchen window don’t block them. If you sit near a window for 30 minutes a day, you’re accumulating damage. Over years, that adds up to photoaging - fine lines, dark spots, uneven tone.

The American Academy of Dermatology recommends daily sunscreen use for everyone over age 6 months. That’s not a suggestion. It’s a medical standard. And it’s backed by data: 90% of visible aging is caused by sun exposure. Not genetics. Not stress. Sun.

Even in Halifax, where winters are long and gray, the UV index can hit 3 or 4 in January. That’s enough to cause damage. Snow reflects up to 80% of UV rays - more than sand at the beach. So yes, you need sunscreen on your face, neck, and hands every single day, no matter the season.

How to Apply Sunscreen Right (Most People Get It Wrong)

Applying sunscreen is like taking medicine. If you take half the dose, it doesn’t work. The standard amount for your face and neck is 1/4 teaspoon. That’s about the size of a nickel. Most people use half that - maybe a pea-sized blob. That means your SPF 30 becomes SPF 15. Or worse, SPF 7.5.

Here’s how to get it right:

1. Use 1/4 teaspoon for your face and neck.
2. Apply after moisturizer, before makeup.
3. Wait 15 minutes before putting on foundation - especially with chemical sunscreens.
4. Reapply every two hours if you’re outside. If you’re sweating or wiping your face, reapply every 40-80 minutes, depending on the label.
5. Don’t forget ears, neck, lips (use a lip balm with SPF), and the back of your hands.

Common problems? Pilling. That’s when sunscreen rolls up under makeup. Fix it by letting your sunscreen fully absorb before applying anything else. Use a lightweight formula. Avoid mixing too many thick products. Some brands now make “non-pilling” sunscreens specifically for makeup wearers.

What’s Changing in 2025 - And What to Watch For

The FDA is cracking down. By December 2025, any sunscreen sold in the U.S. that doesn’t clearly label “broad spectrum” will be pulled from shelves. That’s a big deal. It means brands can’t hide weak UVA protection behind a high SPF number anymore.

Also, the Skin Cancer Foundation updated its Seal of Recommendation in 2023. Now, SPF 50+ products must prove they offer UVA protection equivalent to SPF 20 or higher. That’s stricter than the FDA’s minimum. Look for the seal - it’s a real indicator of quality.

And here’s what’s coming: blue light and infrared protection. Dermatologists are starting to test sunscreens for protection against light from screens and heat radiation. While it’s not standard yet, 68% of dermatologists expect multi-spectrum protection to be normal within five years. For now, stick with broad-spectrum SPF 30+ and know that zinc oxide also offers some blue light defense.

What to Buy and What to Skip

Top brands in the U.S. market - La Roche-Posay, CeraVe, Neutrogena - make up 45% of sales. Why? They deliver on performance, texture, and broad-spectrum coverage.

For sensitive skin: CeraVe Hydrating Mineral Sunscreen SPF 30 - no fragrance, no parabens, contains ceramides to repair the skin barrier.

For oily or acne-prone skin: La Roche-Posay Anthelios Clear Skin SPF 60 - oil-free, non-comedogenic, no white cast.

For daily wear under makeup: Supergoop! Unseen Sunscreen SPF 40 - invisible, silicone-based, blends like a primer.

Avoid: sunscreens with oxybenzone if you’re concerned about coral reefs or skin irritation. Skip sprays unless you’re reapplying on a beach - they’re hard to apply evenly and often under-dosed.

Price doesn’t always matter. Some drugstore sunscreens outperform luxury ones. Look at the ingredient list, not the label. Zinc oxide and avobenzone are the gold standards for UVA protection. If you see “SPF 100” with no mention of broad spectrum - walk away.

Final Rule: Sunscreen Is Non-Negotiable

You don’t need to spend hours on skincare. You don’t need 10 steps. But you do need sunscreen - every single day. It’s the single most effective anti-aging product you’ll ever use. It’s the best defense against skin cancer. It doesn’t matter if you’re 18 or 80. It doesn’t matter if you’re white, Black, or Brown. UV damage doesn’t discriminate.

Use SPF 30+. Choose broad spectrum. Apply enough. Reapply when needed. Skip the myths. Don’t wait for a sunny day. Your skin won’t thank you tomorrow - but it will thank you in 10 years.