Sunscreen is used to protect your skin from ultraviolet (UV) radiation, the invisible part of sunlight responsible for sunburns, premature aging, and skin cancer. It works by either absorbing or reflecting UV rays before they can damage skin cells and DNA. Beyond basic sun protection, sunscreen also plays a role in preventing hyperpigmentation, preserving collagen, and reducing skin reactions in people taking certain medications.
How UV Radiation Damages Your Skin
Sunlight contains two types of UV radiation that reach your skin: UVA and UVB. They cause different kinds of damage at different depths, which is why dermatologists recommend “broad-spectrum” sunscreen that blocks both.
UVB rays (the ones behind sunburns) are mostly absorbed by the outermost layer of skin. They directly damage DNA by fusing together building blocks in the genetic code, creating structural errors that cells must repair. When this repair goes wrong, it can trigger mutations that lead to skin cancer. UVB initiates melanoma through this direct DNA damage regardless of your skin tone or pigmentation.
UVA rays penetrate much deeper, reaching down into the dermis where collagen, elastin fibers, and tiny blood vessels live. UVA photon energy can be delivered at levels 100 times higher than UVB into the lower epidermis and upper dermis, the regions where melanocytes (pigment-producing cells) sit. UVA also generates reactive oxygen species, unstable molecules that cause additional DNA damage and oxidative stress. In melanocytes specifically, the pigment melanin can actually act as an internal photosensitizer, amplifying UVA damage rather than protecting against it.
Preventing Premature Aging
Photoaging, the visible skin aging caused by sun exposure, is distinct from the natural aging process. UVA light damages collagen and elastin fibers that give skin its firmness and bounce. Over years of unprotected exposure, this leads to wrinkles, sagging, rough texture, and uneven pigmentation. The changes happen at every level, from the surface epidermis down through the dermis to the capillaries beneath.
Regular sunscreen use slows this process considerably. Because UVA rays are present year-round and penetrate through clouds and windows, photoaging accumulates even on overcast days or during short outdoor errands. This is why consistent daily use matters more than occasional heavy application at the beach.
Reducing Skin Cancer Risk
By filtering UV radiation before it reaches your DNA, sunscreen reduces the accumulation of genetic mutations that can eventually become cancerous. This applies to all three major types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma. The mechanism is straightforward. Fewer UV photons reaching your cells means fewer DNA errors, which means fewer opportunities for a cell to lose control of its growth.
Protection for Photosensitive Medications
A wide range of common medications make your skin significantly more reactive to sunlight, a condition called photosensitivity. If you take any of these drugs, unprotected sun exposure can cause exaggerated sunburn, rashes, or other skin reactions that wouldn’t normally occur.
Medications that commonly cause photosensitivity include:
- Antibiotics like doxycycline and tetracycline
- Common pain relievers like ibuprofen and naproxen
- Cholesterol-lowering statins
- Oral contraceptives and estrogens
- Retinoids used for acne or skin conditions
- Certain diuretics and diabetes medications
- Antihistamines like cetirizine, diphenhydramine, and loratadine
Even alpha-hydroxy acids (AHAs) found in everyday skincare products can increase photosensitivity. The FDA recommends broad-spectrum sunscreen with SPF 30 or higher for anyone on these medications.
Mineral vs. Chemical Sunscreen
Sunscreens fall into two categories based on how they handle UV rays. Mineral sunscreens use zinc oxide or titanium dioxide, which sit on the skin’s surface and physically reflect and scatter UV radiation away. These are the only two active ingredients the FDA currently classifies as generally recognized as safe and effective (GRASE).
Chemical sunscreens use compounds like avobenzone, oxybenzone, and octinoxate. These absorb UV radiation and convert it into small amounts of heat that dissipate from your skin. They tend to feel lighter and spread more easily, but the FDA has noted it doesn’t yet have enough data to make a final safety determination on 12 chemical sunscreen ingredients still under review. Both types are effective at blocking UV when applied correctly.
What SPF Numbers Actually Mean
SPF measures how much UVB radiation a sunscreen filters. The numbers don’t scale the way you might expect. SPF 15 blocks 93% of UVB rays, SPF 30 blocks 97%, and SPF 50 blocks 98%. The jump from SPF 30 to 50 adds just one percentage point of protection, which is why most guidelines suggest SPF 30 as the practical sweet spot for daily use.
One thing SPF does not measure is UVA protection. That’s what the “broad-spectrum” label covers. A high SPF without broad-spectrum protection leaves your skin exposed to the deeper-penetrating UVA damage that drives aging and contributes to melanoma.
How Much to Apply
Most people apply far less sunscreen than what’s needed for the SPF rating on the bottle. The standard used in testing is two milligrams per square centimeter of skin. In practical terms, that means about a shot glass (two tablespoons) for all exposed areas of your face and body. For the face alone, a nickel-sized dollop is the minimum. Using half the recommended amount doesn’t give you half the protection; it drops the effective SPF dramatically.
Reapplication matters just as much as the initial coat. Sunscreen breaks down with UV exposure, sweating, and physical contact. Reapplying every two hours during continued sun exposure, or immediately after swimming or toweling off, maintains the level of protection you’re counting on.
Storage and Shelf Life
Sunscreen’s active ingredients degrade over time. Unless the label states otherwise, sunscreen lasts about three years from the date of manufacture. Heat accelerates this breakdown significantly. A bottle left in a hot car, sitting in direct sunlight at the pool, or stored in a humid bathroom loses effectiveness faster than one kept in a cool, dry place. If you’re outdoors, tuck the bottle under a towel or in a bag between applications. If the texture, color, or smell has changed, the product has likely degraded and should be replaced regardless of the expiration date.