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 triggers an inflammatory response to kill off the most heavily damaged cells before they can become cancerous. The redness, heat, swelling, and pain you feel are all part of that cleanup process, not the burn itself.
How UV Light Damages Your Skin
Sunlight contains two types of UV radiation that reach your skin: UVB and UVA. UVB is the primary cause of sunburn. It’s absorbed mostly by the outermost layer of skin (the epidermis), where it directly strikes the DNA inside your skin cells. This creates structural defects in the DNA strand, forcing two neighboring building blocks to fuse together abnormally. These defects are difficult for the cell to repair and, if left uncorrected, can lead to mutations that eventually cause skin cancer.
UVA rays penetrate deeper, reaching the layer beneath the surface (the dermis). Rather than hitting DNA directly, UVA generates unstable molecules called free radicals that damage DNA, break down collagen fibers, and trigger inflammation. UVA contributes to the burn, but its bigger role is long-term aging and tissue breakdown. Both types work together during a day in the sun, but UVB is the one doing most of the acute damage that turns your skin red.
Why Your Skin Turns Red and Hurts
The redness of a sunburn isn’t the UV damage itself. It’s your immune system responding to it. When skin cells accumulate too much DNA damage to safely repair, your body activates a self-destruct program. A protein called p53, often described as the “guardian of the genome,” detects the damage and signals heavily affected cells to die. This controlled cell death prevents those cells from replicating with dangerous mutations.
At the same time, your body floods the area with blood and immune signals, producing the classic signs of inflammation: redness, warmth, swelling, and pain. This response doesn’t happen instantly. The inflammatory reaction typically peaks 12 to 24 hours after sun exposure, which is why you can feel fine at the beach and wake up the next morning in serious pain. The delay is the time your body needs to assess the damage and mount its full response.
What Melanin Does to Protect You
Your skin’s built-in defense against UV is melanin, the pigment that determines skin color. Melanin works in two ways: it acts as a physical shield that scatters UV rays before they can reach cell DNA, and it functions as an absorbent filter that soaks up UV energy and converts it to harmless heat. On top of that, melanin neutralizes the free radicals that UVA generates, reducing collateral damage.
Not all melanin is equal. The darker brown-black form (eumelanin) is a far more effective UV shield than the reddish-yellow form (pheomelanin) found in higher concentrations in people with red hair and very fair skin. This is why people with lighter skin burn faster: they simply have less of the more protective type of melanin filtering out UV before it hits their DNA. A tan is your body producing more melanin in response to UV exposure, but it only provides modest protection, roughly equivalent to a very low SPF sunscreen.
First-Degree vs. Second-Degree Sunburn
Most sunburns are first-degree burns, meaning the damage stays within the outer layer of skin. You’ll see redness, feel tenderness, and the skin will peel as dead cells shed over the following days to a week. This is uncomfortable but heals on its own.
A second-degree sunburn reaches the middle layer of skin, the dermis. The hallmark is blistering. Blisters form because the damage is deep enough to separate skin layers, and fluid fills the gap. These burns can take weeks to heal and sometimes need medical treatment. The pain is more intense, and you may also experience chills, nausea, or fever as your body deals with the widespread cell damage.
Factors That Make Sunburn Worse
Your environment changes how much UV actually reaches your skin, sometimes dramatically. Snow reflects between 50 and 88% of UV radiation back at you, essentially doubling your exposure. White sea foam reflects 25 to 30%, and dry beach sand reflects 15 to 18%. This is why you can burn quickly at the beach or on a ski slope even when the air feels cool. Altitude matters too: the atmosphere filters less UV the higher you go, so a hike at elevation carries more UV risk than a walk at sea level.
Cloud cover is deceptive. Thin clouds block only a fraction of UV, so you can burn on overcast days without realizing it. Water is another trap. UV penetrates below the surface, so swimming doesn’t protect you, and the cooling effect of water masks the heat that would otherwise warn you to get out of the sun.
Medications That Lower Your Burn Threshold
Hundreds of common medications make your skin more sensitive to UV. A comprehensive review identified 393 drugs with photosensitizing potential across nearly every drug class. Some of the most common culprits include certain antibiotics (particularly tetracyclines and fluoroquinolones), anti-inflammatory painkillers like ibuprofen and naproxen, some blood pressure medications and diuretics, cholesterol-lowering statins, and several antidepressants and antipsychotics. If you’re taking any prescription medication and notice you’re burning far more easily than usual, the drug may be lowering your skin’s UV tolerance significantly.
Why Repeated Sunburns Raise Cancer Risk
Each sunburn leaves behind DNA mutations that your body couldn’t fully repair. Over time, these accumulate. A large meta-analysis found that just five sunburns during childhood nearly doubled the risk of developing melanoma (the most dangerous form of skin cancer), with an odds ratio of 1.8. Five sunburns during adolescence carried a similar risk increase of 1.7. Adult sunburns also mattered, with five burns raising melanoma risk by about 50%.
The childhood numbers are especially striking because young skin cells divide more rapidly, giving damaged DNA more chances to replicate its errors. But the data is clear that sunburns at any age contribute to cumulative risk. The mutations caused by UV, specifically the characteristic DNA defects where neighboring building blocks fuse together, are found in the vast majority of skin cancers. They’re considered more dangerous than other types of DNA damage because cells repair them slowly and incompletely.
The 4- to 6-Hour Window You Can’t Feel
One of the most practical things to understand about sunburn is the delay. Because the inflammatory response peaks 12 to 24 hours after exposure, your skin gives you almost no real-time feedback while the damage is happening. By the time you notice redness, the UV exposure that caused it happened hours earlier, and additional damage may still be developing. This is why time-based strategies (limiting direct sun exposure during peak UV hours, typically 10 a.m. to 4 p.m.) and preventive measures like sunscreen and protective clothing matter more than waiting until your skin “feels hot.” By then, the DNA damage is already done.