Sunburn is caused by ultraviolet (UV) radiation from the sun damaging the DNA inside your skin cells. When that damage is severe enough, your body launches an inflammatory response: blood vessels dilate, immune cells flood the area, and the result is the redness, heat, swelling, and pain you recognize as a sunburn. The process starts within minutes of exposure, but you typically won’t feel it until hours later.
How UV Radiation Damages Skin Cells
Sunlight contains two types of UV radiation that reach your skin: UVA (320 to 400 nanometers) and UVB (280 to 320 nanometers). A third type, UVC, is filtered out by the atmosphere before it reaches the ground. UVB is the primary cause of sunburn. It penetrates the outermost layer of skin and directly interacts with the DNA in your skin cells, creating structural defects called pyrimidine dimers. These are spots where two neighboring building blocks of DNA fuse together abnormally, distorting the DNA strand and preventing it from being read correctly.
UVA penetrates deeper into the skin because of its longer wavelength, reaching layers below the surface. It contributes to sunburn less directly, mainly by generating unstable molecules (free radicals) that damage cells from the inside out. Both types of radiation matter, but UVB is the one most responsible for the acute burn you see and feel.
Why Your Skin Turns Red and Hurts
The redness, swelling, and pain of a sunburn are not the UV damage itself. They’re your immune system’s response to it. When skin cells sustain enough DNA damage, they release signaling molecules, particularly one called TNF-alpha, that essentially sound an alarm. This triggers a cascade: blood vessels in the skin widen (which is why the skin looks red and feels warm), and immune cells rush to the damaged area. Macrophages, a type of immune cell, arrive and release compounds that generate an “oxidative burst,” which helps clear damaged tissue but also intensifies inflammation and prolongs the injury.
This inflammatory process explains the delayed timeline of sunburn. Pain typically starts within a few hours of sun exposure and peaks around 24 hours after the burn. You can be badly burned on a beach at noon and not realize it until that evening, because the immune response takes time to fully ramp up. Redness and tenderness can last several days as the body repairs or removes the damaged cells.
Why Some People Burn Faster Than Others
The pigment melanin is your skin’s built-in UV shield. It sits in the upper layers of skin and absorbs UV radiation before it can reach the DNA in deeper cells. But not all melanin works the same way. There are two types: eumelanin, which is dark brown to black, and pheomelanin, which is red to yellow. Eumelanin is far more protective. It absorbs UV strongly and also acts as an antioxidant, neutralizing free radicals before they cause further damage.
Pheomelanin, on the other hand, can actually make things worse. After UV exposure, pheomelanin can become a photosensitizing agent, meaning it generates free radicals instead of neutralizing them. This is why people with red hair and very fair skin, who produce predominantly pheomelanin, burn easily and are at higher risk of skin cancer. People with darker skin produce more eumelanin and can tolerate significantly more UV exposure before burning, though they are not immune to UV damage.
Environmental Factors That Increase UV Exposure
The intensity of UV radiation reaching your skin depends on more than just the time of day and cloud cover. Surfaces around you reflect UV rays back at you, effectively increasing your dose. Fresh snow reflects about 85% of UV radiation, which is why skiers burn so easily at high altitudes. Dry sand reflects around 17%. Water reflects only about 5% at most angles, but reflection climbs steeply as the sun sits lower in the sky, approaching nearly 100% at very shallow angles.
Altitude also matters. The atmosphere absorbs some UV, so at higher elevations there’s less air between you and the sun. Every 1,000 feet of elevation gain increases UV exposure by roughly 4 to 5 percent. And while clouds block some UV, thin or scattered clouds let most of it through, which is why you can burn on overcast days without realizing it.
Medications That Make You More Vulnerable
Certain medications cause photosensitivity, a chemical change in the skin that makes it react more strongly to UV radiation. If you’re taking one of these drugs, you can burn faster, burn more severely, or develop a rash from sun exposure that wouldn’t normally bother you. The U.S. Food and Drug Administration lists several common categories:
- Antibiotics: doxycycline, tetracycline, and several fluoroquinolones
- Anti-inflammatory painkillers: ibuprofen, naproxen, and celecoxib
- Diuretics: hydrochlorothiazide and related water pills
- Cholesterol drugs: statins like simvastatin and atorvastatin
- Retinoids: isotretinoin (used for acne) and acitretin
- Oral contraceptives and estrogens
- Diabetes medications: sulfonylureas like glipizide and glyburide
- Topical products: alpha-hydroxy acids (AHAs) found in many cosmetics and skincare products
Photosensitivity can be triggered by pills you swallow or products you apply to your skin. If you’re on any of these medications, your threshold for sunburn drops considerably, sometimes to as little as a few minutes of direct exposure.
What Happens After the Burn
Once the inflammatory response peaks at around 24 hours, your body shifts into repair mode. Enzymes scan the damaged DNA and snip out the fused pyrimidine dimers, filling in the gaps with correct DNA building blocks. Cells that are too damaged to repair undergo programmed cell death, which is what causes peeling a few days after a burn. The dead cells on the surface slough off to make way for healthy replacement cells beneath.
Your skin also responds by ramping up melanin production, which is why you may notice a tan developing in the days after a burn. This extra melanin provides some additional protection against future exposure, but it represents damage that has already occurred. There is no such thing as a “safe” tan from UV radiation. Each episode of sunburn, and each round of DNA damage, accumulates over a lifetime. The pyrimidine dimers that aren’t perfectly repaired can introduce permanent mutations, and these mutations are strongly linked to skin cancer development over time.