People get skin cancer primarily because ultraviolet radiation from the sun damages the DNA inside skin cells, and over time those damaged cells can grow out of control. Globally, about 1.2 million people are diagnosed with non-melanoma skin cancer each year, and another 330,000 are diagnosed with melanoma. The causes go beyond just “too much sun,” though. Genetics, immune function, chemical exposures, and the timing and pattern of sun exposure all play a role in who develops skin cancer and which type they get.
How UV Light Damages Your DNA
The core mechanism behind most skin cancers is surprisingly simple. When ultraviolet light hits your skin cells, it causes two neighboring DNA building blocks to fuse together, forming what scientists call a dimer. Think of it like two links in a chain getting welded shut. This distorts the DNA strand and, if your body’s repair machinery doesn’t catch it, the error gets copied every time that cell divides. Over years or decades, these errors can accumulate and eventually flip the switches that control cell growth.
UVB rays (the ones that cause sunburn) are the most efficient at creating these DNA fusions. UVA rays, which penetrate deeper into the skin and pass through clouds and windows, can cause the same kind of damage, but require a much higher dose to do so. UVA also causes a second type of harm: it generates reactive oxygen molecules inside your cells that chemically alter DNA bases in a different way. Between the two mechanisms, both UVA and UVB contribute to skin cancer risk, which is why broad-spectrum sun protection matters.
Why Sunburns in Childhood Are So Dangerous
Not all sun exposure carries equal risk. Blistering sunburns before age 15 appear to be especially harmful. A Mendelian randomization study found that childhood sunburn was associated with roughly a fivefold increase in melanoma risk, even after adjusting for skin color, hair color, vitamin D levels, BMI, and lifestyle factors like smoking and alcohol use. For melanoma in situ on the trunk (the earliest detectable stage), the risk jumped even higher, to about 28 times that of people without childhood sunburns.
The likely explanation is that children’s skin cells are dividing rapidly to support growth. A burst of DNA damage during that window means more mutated cells get locked into the body long-term, with decades ahead of them to accumulate additional mutations.
Intermittent Exposure vs. Steady Outdoor Work
One of the more counterintuitive findings in skin cancer research is that people who work outdoors all day don’t have the highest melanoma risk. Studies across the northern hemisphere consistently show that intermittent, intense sun exposure (beach vacations, weekend sports, holiday trips) is what drives melanoma rates up. Regular outdoor workers actually show no increased risk, and in some studies a decreased risk, compared to indoor workers who get intense bursts of sun.
The pattern makes biological sense. Skin that gradually adapts to UV exposure builds up protective pigment and thickens its outer layer. Skin that rarely sees the sun and then gets blasted on a two-week holiday has no such defenses. This distinction matters mainly for melanoma. Squamous cell carcinoma, by contrast, does follow a cumulative pattern, appearing most often on chronically sun-exposed areas like the face, ears, and backs of the hands.
Skin Type and Genetic Vulnerability
Your baseline skin color is one of the strongest predictors of skin cancer risk. The Fitzpatrick scale classifies skin into six types: Type I (very fair skin that always burns and never tans) through Type VI (dark brown or black skin that never burns). People with Type I and II skin produce less of the pigment that absorbs UV before it reaches DNA, so more radiation gets through to cause damage.
Beyond skin color, specific inherited gene variants shape your risk for each type of skin cancer. A gene called CDKN2A accounts for 35% to 40% of all familial melanomas. Variants in a gene called MC1R, which influences whether your body produces protective dark pigment or the less-protective reddish pigment found in people with red hair, nearly triple the risk of squamous cell carcinoma when two or more variants are present. For basal cell carcinoma, variants in a gene called PTCH1, which normally acts as a brake on cell growth, show up in about 30% of cases.
People with Li-Fraumeni syndrome, a rare inherited condition affecting a key tumor-suppressor gene, face a cumulative skin cancer risk of 36% to 44% by age 70 and a roughly sevenfold increase in melanoma risk compared to the general population.
When Your Immune System Can’t Keep Up
Your immune system catches and destroys abnormal skin cells constantly. When that surveillance system is weakened, skin cancer rates spike. Organ transplant recipients, who take medications to suppress their immune systems so their bodies don’t reject the new organ, develop non-melanoma skin cancers at rates of 16% to 22.5%. Skin cancers account for about 13.5% of all new cancers in transplant patients. The effect is strong enough that transplant dermatology has become its own specialty, with patients advised to get skin checks every six to twelve months.
This immune connection also helps explain why skin cancer risk rises with age. As you get older, your immune surveillance becomes less efficient, giving damaged cells more opportunity to grow unchecked.
Causes Beyond Sunlight
UV radiation drives the majority of skin cancers, but it isn’t the only cause. Chronic exposure to arsenic in drinking water is a well-established risk factor for non-melanoma skin cancer. In regions with naturally high arsenic levels, exposure alone raises risk roughly fourfold. When combined with human papillomavirus (HPV) infection, the risk climbs dramatically. One study in an arsenic-endemic region of Mexico found that people with both high arsenic exposure and HPV antibodies had a 16.5-fold increase in non-melanoma skin cancer risk.
Other non-UV causes include chronic wounds or scars that never fully heal (a type of squamous cell carcinoma called a Marjolin ulcer can develop in old burn scars), radiation therapy to the skin for other cancers, and certain inherited conditions that impair the body’s ability to repair DNA damage. Xeroderma pigmentosum, for example, leaves people unable to fix UV-induced DNA errors at all, resulting in extremely high skin cancer rates even with minimal sun exposure.
How Sunscreen Changes the Math
The strongest clinical evidence for sunscreen’s protective effect comes from a randomized trial in Queensland, Australia. People assigned to apply SPF 15+ broad-spectrum sunscreen daily had 40% fewer squamous cell carcinoma tumors compared to people who used sunscreen at their own discretion. That 40% reduction persisted for at least eight years after the trial ended. Basal cell carcinoma rates were also 25% lower in the daily sunscreen group during follow-up, though that result didn’t reach statistical significance.
Sunscreen works by absorbing or reflecting UV before it reaches the DNA in your skin cells, essentially reducing the number of DNA fusions that form with each exposure. It doesn’t eliminate risk entirely, because no sunscreen blocks 100% of UV, and most people apply far less than the tested amount. But combined with shade, protective clothing, and avoiding peak UV hours, it substantially reduces the cumulative DNA damage that leads to skin cancer over a lifetime.