Ultraviolet radiation from the sun is the primary cause of skin cancer, responsible for the vast majority of cases. But UV exposure isn’t the whole story. Genetics, chemical exposures, certain viruses, and a weakened immune system all play a role. In 2022, there were over 331,000 new cases of melanoma worldwide, and non-melanoma skin cancers number in the millions each year.
How UV Radiation Damages Your DNA
Sunlight contains two types of ultraviolet radiation that reach your skin: UVB and UVA. Each one damages cells in a different way, and together they account for most skin cancers.
UVB rays penetrate the outer layer of skin and directly damage DNA. When UVB photons hit your genetic material, they fuse neighboring molecules together, creating abnormal bonds that block cells from copying and reading their DNA correctly. About 75% of this direct damage takes the form of one specific defect, with the remaining 25% forming a second type. When cells try to repair these errors, they frequently swap in the wrong genetic letter, replacing cytosine with thymine. This particular “typo” is one of the signature mutations found in skin cancer cells.
UVA rays work differently. They penetrate deeper into the skin and generate reactive oxygen species, unstable molecules that cause oxidative stress. This damages DNA indirectly, accelerates aging, and suppresses the skin’s local immune defenses. UVA also interacts with melanin, the pigment that gives skin its color. Melanin, normally protective, can act as a photosensitizer under UVA exposure, actually amplifying DNA damage rather than preventing it.
Intermittent Burns vs. Chronic Exposure
Not all sun exposure carries the same type of risk, and this distinction matters. Intermittent, high-intensity exposure, the kind you get on a beach vacation or a weekend hike after months indoors, is consistently linked to melanoma, the most dangerous form of skin cancer. Several large studies show that short bursts of intense sun, especially in early adulthood, raise melanoma risk significantly. Recreational and vacation sun exposure show up repeatedly as risk factors.
Chronic, cumulative sun exposure over years tells a different story. It is more strongly associated with squamous cell carcinoma, a non-melanoma skin cancer that typically appears on areas of skin that get daily sun, like the face, ears, and hands. Interestingly, people who work outdoors regularly actually show a decreased risk of melanoma in some studies, likely because their skin adapts gradually rather than being overwhelmed by sudden UV doses. The takeaway: sunburns are particularly dangerous, not just total hours in the sun.
Skin Tone and Susceptibility
Darker skin contains more melanin, which absorbs and scatters UV radiation before it reaches vulnerable DNA. People with very light skin that always burns and never tans (Fitzpatrick type I) face the highest risk of both melanoma and non-melanoma skin cancers. Risk decreases as natural pigmentation increases, though the relationship isn’t perfectly linear. People with very dark skin (types V and VI) have a much lower incidence of skin cancer, but they are not immune to it, and their cancers are often diagnosed later because of lower suspicion.
It’s worth noting that red hair and fair skin are partly driven by variants in a gene called MC1R. People carrying two or more of these variants have roughly 2.5 times the risk of basal cell carcinoma and nearly 3 times the risk of squamous cell carcinoma compared to the general population. These MC1R variants also raise skin cancer risk independently of skin color, meaning even people with darker complexions who carry these genetic variants face elevated risk.
Indoor Tanning
Tanning beds deliver concentrated UV radiation, primarily UVA, directly to the skin. Using a tanning bed before age 35 increases the risk of melanoma by 75%. This is one of the clearest and most avoidable risk factors for the disease. The intensity of indoor tanning equipment can deliver UV doses several times higher than midday sun, and the damage accumulates with each session.
Inherited Genetic Risk
Some people inherit gene mutations that make their cells less able to repair DNA damage or control cell growth, raising their baseline skin cancer risk regardless of sun exposure.
For melanoma, the most significant inherited gene is CDKN2A, a tumor suppressor. Mutations in this gene account for an estimated 35% to 40% of all familial melanomas. Families carrying these mutations often see melanoma diagnosed across multiple generations, sometimes at younger ages. Other genes linked to hereditary melanoma include CDK4, which carries a similar risk level to CDKN2A but is much rarer, and BAP1, which is associated with both skin melanoma and a rare eye melanoma called uveal melanoma. BAP1 mutations follow an autosomal dominant pattern, meaning inheriting just one copy from either parent is enough to increase risk.
For basal cell carcinoma, the most well-known genetic link is PTCH1. Inherited mutations in this gene cause basal cell nevus syndrome, a condition where people develop dozens or even hundreds of basal cell carcinomas over their lifetime. Even outside this syndrome, up to 30% of sporadic basal cell carcinomas show PTCH1 mutations that arose on their own in skin cells.
Chemical and Environmental Carcinogens
Several substances cause skin cancer through mechanisms that have nothing to do with sunlight.
- Arsenic is classified as a top-tier carcinogen by both the EPA and the International Agency for Research on Cancer. Exposure comes primarily from contaminated drinking water and certain foods, including grain-based processed foods, dairy, and fish. Arsenic exposure is linked to both squamous cell and basal cell carcinomas.
- Cadmium is another group I carcinogen. Cigarette smoke is a major source, along with contaminated air, water, and green leafy vegetables grown in polluted soil.
- Hexavalent chromium, the toxic form of chromium (distinct from the harmless trivalent form found in food), is a known carcinogen primarily encountered in industrial settings like metalworking and manufacturing.
Trace elements like iron and copper aren’t carcinogens in the traditional sense, but elevated levels in the body may promote cancer growth. Iron catalyzes the formation of the same reactive oxygen species that UVA produces, while copper promotes the growth of new blood vessels that tumors need to expand.
Viruses and Skin Cancer
Human papillomavirus, best known for causing cervical cancer, also plays a role in skin cancer. More than 220 genotypes of HPV have been identified, and the beta strains are the ones most commonly found in skin tissue. Emerging evidence links beta-HPV to cutaneous squamous cell carcinoma, particularly in people with weakened immune systems. The connection is strongest in organ transplant recipients and others on long-term immunosuppressive therapy.
Immunosuppression
A weakened immune system is one of the most potent risk factors for skin cancer. Organ transplant recipients, who take medications to prevent their body from rejecting the new organ, face a 20- to 100-fold increased risk of non-melanoma skin cancer compared to the general population. This makes skin cancer one of the most common cancers in transplant patients.
The risk isn’t limited to transplant recipients. Anyone on long-term immunosuppressive therapy, including people with autoimmune diseases or HIV, faces elevated risk. The immune system normally identifies and destroys cells with damaged DNA before they can become cancerous. When that surveillance system is suppressed, damaged skin cells are far more likely to survive, multiply, and form tumors.
Ozone Depletion and Rising UV Levels
The ozone layer in the upper atmosphere filters out much of the sun’s UVB radiation before it reaches the ground. Where that layer thins, surface UV levels rise. In the Antarctic, UVB levels measured at the surface can double during the annual ozone hole. The Montreal Protocol, an international treaty that phased out ozone-depleting chemicals starting in the late 1980s, is expected to prevent roughly 443 million cases of skin cancer and 2.3 million skin cancer deaths among people in the United States alone over its effective lifespan. That figure illustrates just how directly ozone thickness translates to skin cancer rates at the population level.