Skin cancer starts when ultraviolet (UV) radiation damages the DNA inside skin cells, and the body’s normal repair systems fail to fix that damage before the cell divides. Over time, these mutations accumulate, disabling the genes that control cell growth, and a single cell begins multiplying without the usual checks. With roughly 330,000 new melanoma cases diagnosed worldwide in 2022 alone, and non-melanoma skin cancers numbering in the millions, it’s the most common form of cancer in humans.
How UV Light Damages Your DNA
The process begins at a molecular level, inside the nucleus of a single skin cell. When UV-B rays (the shorter, more energetic wavelengths in sunlight) hit your skin, they are absorbed directly by DNA. This energy forces neighboring building blocks of the DNA strand to fuse together abnormally, creating clumps called pyrimidine dimers. These clumps distort the shape of the DNA helix, like a kink in a zipper, and prevent the cell from reading its genetic instructions correctly.
Your cells have repair enzymes that scan for these distortions and snip them out. Most of the time, repairs happen successfully. But when UV exposure is intense or repeated, the repair machinery gets overwhelmed, and some damaged segments slip through. When a cell with unrepaired damage divides, the kink gets copied as a permanent mutation. One of the most common results is a specific type of error in the p53 gene, a critical tumor suppressor that normally forces damaged cells to stop dividing or self-destruct. Once p53 is knocked out, the cell loses one of its most important brakes on growth.
UV Also Suppresses Your Skin’s Immune Defense
DNA damage is only half the story. UV radiation simultaneously weakens the immune surveillance in your skin, making it harder for your body to catch and destroy abnormal cells before they become dangerous.
Your skin contains specialized immune cells called Langerhans cells, which act as sentinels. They detect foreign or abnormal material and alert the broader immune system. UV exposure directly reduces the number of these cells and impairs their ability to present threats to the immune system. Normally, Langerhans cells activate a type of immune response that attacks abnormal cells. After UV damage, they instead trigger a suppressive response, essentially telling the immune system to stand down. This tolerance is reinforced by signaling molecules released in UV-damaged skin that further dampen the immune reaction. The result: mutated cells that would normally be flagged and destroyed can survive, divide, and eventually form a tumor.
Three Types From Three Cell Origins
Not all skin cancers are the same, because they arise from different cell types in the skin, each with its own vulnerabilities.
Basal Cell Carcinoma
Basal cell carcinoma (BCC) is the most common skin cancer. It starts in basal cells, the small round cells at the base of the outermost skin layer that continuously divide to replace the skin you shed. BCC typically involves mutations in a growth-control system called the Hedgehog signaling pathway. In healthy skin, a protein called PTCH acts as a gatekeeper, keeping cell growth signals turned off. When UV damage inactivates the gene that produces PTCH, the pathway switches on permanently, and the cell receives a constant “grow and divide” signal. BCC tends to grow slowly and rarely spreads to distant organs, but it can invade surrounding tissue if left untreated.
Squamous Cell Carcinoma
Squamous cell carcinoma (SCC) develops in keratinocytes, the cells that make up most of the skin’s surface. As keratinocytes mature, they migrate upward and flatten into tough, protective squamous cells. SCC is strongly linked to cumulative, long-term sun exposure, the kind that comes from years of outdoor work or recreation. Mutations in p53 are especially common in SCC. Unlike BCC, squamous cell carcinoma has a meaningful risk of spreading to lymph nodes and other organs if it reaches an advanced stage, though most cases are caught and treated before that happens.
Melanoma
Melanoma originates in melanocytes, the cells that produce the pigment melanin and give skin its color. It is the least common of the three major types but the most dangerous. About 40 to 50 percent of melanomas carry a mutation in the BRAF gene, which controls a signaling chain that tells cells when to grow. A mutated BRAF gene gets stuck in the “on” position, driving relentless cell proliferation. Unlike the other two types, melanoma is strongly associated with intermittent, intense UV exposure, particularly severe sunburns, rather than steady cumulative exposure. Sunburns during childhood carry an especially strong association with melanoma risk later in life, though a higher total number of sunburns at any age is the more important factor overall.
Why Sun Exposure Patterns Matter
The relationship between sun exposure and skin cancer is not as simple as “more sun equals more cancer.” The pattern of exposure matters, and it differs by cancer type.
Chronic, cumulative exposure, the kind outdoor workers accumulate over decades, is most closely tied to squamous cell carcinoma and basal cell carcinoma. These cancers tend to appear on the face, ears, neck, and hands, the areas that get the most consistent sun over a lifetime.
Melanoma follows a different pattern. It is more consistently linked to intermittent sun exposure: intense bursts followed by periods of little sun, like a week-long beach vacation after months indoors. This pattern causes acute DNA damage in cells that haven’t built up any protective adaptation. Interestingly, research on an Iowa population found that for people with fair skin, cumulative sun hours didn’t strongly predict melanoma risk. But for people with medium or dark skin, who are generally at lower risk, high cumulative sun exposure did increase melanoma risk. This suggests that people who don’t burn easily may still accumulate significant DNA damage over time, just without the visible warning signal of a sunburn.
Tanning Beds Concentrate the Risk
Indoor tanning beds emit primarily UVA rays, the longer wavelengths that penetrate deeper into the skin. A 2025 study on the molecular effects of indoor tanning found that tanning bed users had more DNA damage in their skin cells than people twice their age who had never used tanning beds. In another study of 63 women diagnosed with melanoma before age 30, 61 of them (97 percent) had used tanning beds. Even a single tanning session causes measurable DNA damage and increases cancer risk.
Skin Type and Individual Risk
Your baseline risk depends heavily on how much natural melanin your skin produces. The Fitzpatrick scale classifies skin into six types based on how it responds to UV exposure. People with Type 1 and Type 2 skin (who always burn easily and rarely or never tan) are at the highest risk. Types 3 and 4 can tan but still sustain UV damage with each exposure, and the tanning itself is a sign of that damage. Types 5 and 6, with deeply pigmented skin, have significantly more natural protection from UV, but skin cancer can still occur, and melanomas in darker skin are more likely to be diagnosed at a later stage simply because they’re less expected.
Beyond skin type, other factors compound risk. Having a large number of moles, a family history of melanoma, a weakened immune system (from organ transplant medications, for example), or certain inherited gene variants all increase susceptibility. Some of these genetic variants, particularly in the MC1R gene associated with red hair and fair skin, raise the risk of all three major skin cancer types.
What to Watch For on Your Skin
Melanoma is the type most important to catch early, and the ABCDE criteria from the National Cancer Institute remain the standard self-check guide:
- Asymmetry: one half of the mole doesn’t match the other.
- Border: edges are ragged, notched, or blurred, sometimes with pigment spreading into surrounding skin.
- Color: uneven shades of brown, black, or tan, possibly with areas of white, red, pink, or blue.
- Diameter: larger than about 6 millimeters (roughly the size of a pencil eraser), though melanomas can be smaller.
- Evolving: any change in size, shape, or color over weeks or months.
Basal cell carcinomas often appear as pearly or waxy bumps, sometimes with visible blood vessels, or as flat, flesh-colored lesions. Squamous cell carcinomas typically look like firm red nodules or flat lesions with a scaly, crusted surface. Any sore that doesn’t heal within a few weeks, or any spot that bleeds, crusts over, and then bleeds again, warrants attention.