Since ancient Egyptian times, individuals have sought ways to shield their skin from the sun, as over a century ago, we recognized the link between ultraviolet (UV) light and skin injuries, including burns and cancers. Yet, there remains some uncertainty regarding the most effective methods to evade sunburn, how to remedy it, and whether each occurrence escalates the chances of developing cancer. It’s beneficial to grasp the cellular dynamics of tanning.

“Sunburn is an inflammatory response,” explains Leslie Rhodes from the University of Manchester, UK. UV rays inflict damage to proteins, fats, and DNA in skin cells located in the epidermis, triggering a cascade of inflammatory reactions resulting in redness, swelling, pain, and peeling.

Though UVB radiation is chiefly responsible for this damage, UVA rays, which have longer wavelengths, penetrate the skin more deeply. “Typically, UVB is approximately 1,000 times more effective than UVA for sunburning,” states Antony Young from King’s College London.

In reaction to UV injury, skin cells emit inflammatory molecules that enlarge blood vessels in the dermis, the layer of skin beneath the epidermis. Within hours, this increased blood circulation facilitates the influx of immune cells from the bloodstream into the skin, heightening inflammation.

For individuals with lighter skin tones, this augmented blood flow may cause sun-damaged skin to appear pink or red, while those with darker skin might notice skin changes in various shades including red, gray, brown, and black. The enhanced blood supply also results in greater fluid leakage from blood vessels to the skin, leading to swelling. Both swelling and inflammatory molecules activate the nerves, rendering the tanned skin hot and painful.

In extreme cases, blisters may form if patches of epidermal cells become severely damaged and begin self-destructing. As these dead patches detach from the underlying layer, the resulting spaces fill with liquid, creating a foamy structure within the skin.

How does tanned skin heal?

According to Rhodes, “A mild tan will diminish more rapidly than a severe burn if the effects persist beyond 72 hours.”

Healing initiates when skin immune cells start generating anti-inflammatory molecules a few days post UV exposure. “It’s a self-resolving inflammation,” Rhodes notes. “The various molecules and cells transition over time from pro-inflammatory to anti-inflammatory states.” Consequently, blood vessels cease to dilate, and the redness, swelling, and pain gradually subside.

Stem cells situated at the base of the epidermis accelerate healing by producing new skin cells at an increased rate. These cells replace the damaged ones, often shedding or peeling off in large sheets to facilitate growth. “You always shed skin, but UV damage accelerates the conversion of those cells,” says Young.

Regrettably, there is insufficient evidence to suggest that applying after-sun or aloe vera gels can hasten healing of tanned skin, according to Rhodes. Most of these lotions aim to alleviate pain by providing a cooling sensation. Cold showers, cool compresses, and over-the-counter pain relievers like paracetamol (acetaminophen) and ibuprofen may also be beneficial.

What are the long-term effects?

The sunburn subsides as inflammation lessens and damaged surface cells slough off. However, DNA damage to deeper stem cells in the epidermis may leave a lasting legacy.

“DNA damage occurs, and while cells attempt to repair it, their efforts are not flawless,” notes Young. This leads to genetic mutations that accumulate over time in genes governing cell growth and division, resulting in uncontrolled skin cell proliferation, heightening cancer risks.

Numerous studies indicate that experiencing five sunburns within a decade more than doubles the risk of melanoma, a type of skin cancer. However, these findings often rely on individuals’ recollections of their sunburn occurrences, which may not be precise, complicating the accurate assessment of how a single sunburn contributes to skin cancer risk.

What is the best method to prevent sunburn?

The skin pigment melanin encircles skin cell DNA, offering some level of protection from UV damage. Consequently, individuals with darker skin tones face a significantly lower risk of skin cancer compared to those with lighter complexions, though they are not immune to sunburn or DNA damage.

To assess the risk of burning on any given day, monitor the UV index, which measures ultraviolet radiation levels. Rachel Abbott from the University Hospital of Wales, Cardiff, advises applying sunscreen if the index reaches 3 or higher. Typically, UV indexes seldom exceed 3 early in the morning, evening, or between October and March in the UK. Nonetheless, UV rays are more intense near the equator and may necessitate sunscreen application at any time. Fortunately, free apps provide local UV index information. “I use one daily,” Abbott shares.

Most individuals don’t apply sunscreen with the thickness utilized in testing—2 milligrams per square centimeter—making an SPF 50 sunscreen a wise default choice, according to Young.

Nevertheless, one of the most effective strategies to prevent sunburn is to avoid direct sunlight when it is highest in the sky. In the UK, this window is between 11 a.m. and 3 p.m., while in the US, it generally extends from 10 a.m. to 4 p.m. During this time, sunlight takes a shorter route through the atmosphere, allowing more UVB radiation to reach the skin. When outdoors, donning a hat and long-sleeved clothing can further diminish the risk.