Sun tan lotion, more accurately called sunscreen, works by preventing ultraviolet radiation from damaging your skin. It does this in one of two ways depending on the formula: either by absorbing UV rays before they reach your skin cells, or by physically reflecting them away. The result is less sunburn, a lower risk of skin cancer, and slower skin aging.
How Sunscreen Blocks UV Radiation
There are two categories of sunscreen, and they protect your skin through completely different mechanisms.
Mineral sunscreens (sometimes called physical sunscreens) contain tiny particles of zinc oxide or titanium dioxide. These sit on top of your skin like a shield, reflecting and scattering UV rays away before they can penetrate. Think of them as a mirror for sunlight.
Chemical sunscreens work more like a sponge. Their active ingredients absorb UV rays, convert that energy into a small amount of heat, and release it from your skin. You don’t feel this heat, but it’s the reason chemical sunscreens need about 15 to 20 minutes after application to start working. The molecules need time to bind to your skin and begin absorbing radiation effectively.
What UV Rays Actually Do to Your Skin
Sunlight contains two types of ultraviolet radiation that matter for your skin: UVA and UVB. They cause different kinds of damage at different depths, which is why you want protection from both.
UVB rays are absorbed mostly by the outermost layer of skin, the epidermis. These are the rays responsible for sunburn. They directly damage your DNA by creating structural defects in the genetic code of skin cells. Over time, this kind of repeated DNA damage is what leads to skin cancer.
UVA rays penetrate much deeper, reaching into the layer beneath the surface where collagen lives. UVA energy can be delivered at levels 100 times higher than UVB into the lower epidermis and upper dermis, the regions where pigment-producing cells sit. Rather than causing direct DNA damage the way UVB does, UVA triggers the production of unstable molecules called free radicals. These free radicals break down collagen, damage cell membranes, and cause the uneven pigmentation, fine lines, and leathery texture associated with sun-aged skin. UVA is also the primary driver behind tanning, which is itself a sign of DNA injury.
What SPF Numbers Actually Mean
SPF stands for sun protection factor, and it measures only UVB protection. The numbers follow a curve of diminishing returns:
- SPF 15 blocks 93% of UVB rays
- SPF 30 blocks 97% of UVB rays
- SPF 50 blocks 98% of UVB rays
The jump from SPF 15 to 30 is significant, but going from 30 to 50 gains you only one extra percentage point. SPF 30 is the standard recommendation for daily use. Higher numbers offer a small additional margin that can matter if you burn easily or spend long hours outdoors, but no sunscreen blocks 100% of UV.
To get UVA protection as well, look for the words “broad spectrum” on the label. In the U.S., the FDA requires a product to pass a specific test proving it filters across the full UV spectrum before it can carry that label. A sunscreen that isn’t labeled broad spectrum is only protecting you from sunburn, not from the deeper damage that ages skin and contributes to melanoma.
Skin Cancer and Aging Prevention
The most important thing sunscreen does is reduce your risk of skin cancer. A large Australian study found that daily sunscreen use would result in 1,055 fewer melanomas and nearly 17,000 fewer non-melanoma skin cancers per 100,000 people over a lifetime. That’s a massive reduction from a single daily habit.
The anti-aging effects are equally well documented. The landmark Nambour trial tracked participants over 4.5 years and found a 24% reduction in visible skin aging among people who used sunscreen daily compared to those who used it only when they felt like it. Shorter studies have consistently shown measurable improvements in wrinkles, dark spots, and skin texture within as little as 8 weeks of regular use. The evidence shows that consistent sunscreen use not only prevents new signs of aging but can partially reverse existing damage, as the skin gets a chance to repair itself without ongoing UV assault.
How Much to Apply and How Often
Most people apply far too little sunscreen to get the protection promised on the bottle. For your whole body, you need about one ounce, roughly the amount that would fill a shot glass. For your face alone, use about a teaspoon, which is approximately the amount needed to cover the length of your index and middle fingers laid side by side.
Reapply at least every two hours regardless of the SPF number. Sunscreen breaks down with UV exposure and loses effectiveness over time. If you’re swimming or sweating, reapply more frequently. Products labeled “water resistant” are required to specify whether they hold up for 40 or 80 minutes in water, so check the label and set a timer accordingly. No sunscreen is waterproof or sweatproof despite what older labels may have claimed.
Shelf Life and Storage
Sunscreen is required by the FDA to remain at its original strength for at least three years. If your bottle has an expiration date, follow it. If it doesn’t, write the purchase date on the bottle and replace it after three years. Sunscreen that has changed color, consistency, or smell should be discarded regardless of the date.
Heat and direct sunlight degrade sunscreen’s active ingredients faster than normal. Don’t leave bottles in your car, on the dashboard, or sitting in the sun at the beach. Keep them in the shade, wrapped in a towel, or inside a cooler.
Environmental Considerations
Some chemical sunscreen ingredients wash off in the ocean and damage marine ecosystems, particularly coral reefs. The two most studied offenders are oxybenzone and octinoxate, which several coastal regions have banned from sunscreen products. Other ingredients flagged for environmental harm include octocrylene, homosalate, parabens, and any form of microplastic beads.
If you swim in the ocean or near reefs, mineral sunscreens with non-nano zinc oxide or titanium dioxide are the safest option for marine life. “Non-nano” means the mineral particles are large enough that they don’t penetrate coral tissue. If the product rubs in completely clear without any white cast, it likely uses nano-sized particles, which can be toxic to coral in high concentrations.