Why Do We Get Sunburn? What UV Light Does to Skin

Oct 14, 2026 · 4 min read · Sourced and fact-checked

The short answer: Sunburn is a mild burn caused mainly by UVB light, which damages skin cell DNA and triggers inflammation. A 2024 medical review says redness appears after 3 to 5 hours and peaks at 12 to 24 hours.

Sunburn feels like heat damage, but the sun's heat is not the main cause. A 2024 medical review explains what actually happens.

It is a burn from UV light

The review, published in Eplasty, says sunburn results from too much exposure to ultraviolet radiation from the sun. The main cause is UVB, which triggers an inflammatory response in the outer layer of skin, the epidermis. Even though there is little radiant heat involved, it is classed as a superficial, or first-degree, burn.

Ultraviolet radiation makes up about 5 percent of solar radiation. It is split into UVA, UVB and UVC. The review says the ozone layer blocks UVC, so ambient sunlight is roughly 90 to 95 percent UVA and 5 to 10 percent UVB.

What UVB does to skin cells

According to the review, UVB light is absorbed by DNA in skin cells, creating damaged DNA products. If the cell's repair systems fail, this can cause mutations that raise the risk of skin cancer. UVA can also contribute to cancer risk by producing reactive oxygen species that damage DNA and other molecules.

UVB also makes skin cells release signals that switch on inflammation, including the molecules IL-6 and TNF-alpha. At high doses it makes skin cells die, and these are known as sunburn cells.

The timeline

The skin is red, warm, swollen and tender. The review says this typically appears 3 to 5 hours after sun exposure and peaks at 12 to 24 hours. As the top layer peels, it usually heals without scarring within a week. Severe cases can blister and lead to dehydration or infection.

Why tanning happens

The same damage sets off a protective response. Skin produces melanin, a pigment that shields the nuclei of skin cells by absorbing ultraviolet light and releasing the energy as heat. The review says the brown-black form, eumelanin, protects well, while the red-yellow form, pheomelanin, protects less and can produce reactive oxygen species under UVA.

What helps

The review says treatment mainly aims at comfort: cool baths or compresses, loose clothing, moisturisers such as aloe vera, and plenty of fluids. It cites a review of 40 studies in which anti-inflammatory painkillers such as ibuprofen reduced redness for 24 to 36 hours, while corticosteroids, antihistamines, antioxidants, aloe vera and vitamins had little or no effect on recovery time. It concludes that prevention, through physical protection and sunscreen, remains the most effective approach.

A note

Blistering sunburn, fever or severe symptoms need medical care. This is general information, not medical advice.

Sources

Sunburn, Stephen M. Milner, Eplasty, June 11, 2024: https://pmc.ncbi.nlm.nih.gov/articles/PMC11374383/

Facts checked: October 14, 2026.