Ultraviolet radiation is the dominant cause of skin cancer, responsible for more than 80% of melanoma cases worldwide. A 2025 analysis from the International Agency for Research on Cancer found that 83% of all new melanomas diagnosed in 2022 were caused by UV exposure. In regions like Australia, New Zealand, northern Europe, and North America, that figure climbs above 95%. But UV light isn’t the only factor. Genetics, immune function, chemical exposures, and certain medical conditions all play a role.
How UV Light Damages Skin Cells
Sunlight contains two types of ultraviolet radiation that reach your skin: UVA and UVB. Both damage the DNA inside skin cells, but they do it in slightly different ways. UVB rays cause direct hits to DNA, forcing the chemical “letters” of your genetic code to pair up incorrectly. Specifically, certain DNA bases bond with each other instead of their correct partners, creating bulges that bend the DNA strand out of shape.
Your body has a built-in repair system for this kind of damage. Specialized proteins scan DNA for these defects, latch onto the damaged section, and replace it. The problem is that other proteins involved in reading your DNA sometimes compete for the same spot. When a repair protein can’t access the damaged area, the fix doesn’t happen. That unrepaired damage becomes a permanent mutation, and if it occurs in a gene that controls cell growth, a cell can begin dividing uncontrollably. This is how a sunburn at the molecular level becomes cancer years or decades later.
UVA rays penetrate deeper into the skin and contribute to damage through a different route, generating reactive molecules that indirectly harm DNA and break down the structural proteins in your skin. The cumulative effect of both UVA and UVB exposure over a lifetime is what drives most skin cancer risk.
Indoor Tanning and Concentrated UV Exposure
Tanning beds deliver concentrated UV radiation, and the data on their risk is striking. Indoor tanning can more than double the risk of melanoma overall. Using tanning beds before age 20 increases melanoma risk by 47%, and that risk grows with each additional session. Women younger than 30 who tan indoors are six times more likely to develop melanoma compared to those who don’t.
Higher melanoma rates among young women compared to young men are partly attributed to more widespread indoor tanning among females. People who start tanning as teenagers are also more likely to continue the habit into adulthood, compounding their lifetime UV dose during the years their skin cells are most vulnerable to lasting damage.
Genetics and Inherited Risk
Some people inherit gene variants that make their skin cells less able to repair UV damage or more prone to uncontrolled growth. For melanoma, the most significant inherited risk comes from mutations in genes that regulate cell division. Families with a strong history of melanoma often carry changes in these growth-control genes, which can be passed from parent to child.
A gene called MC1R, which influences skin and hair pigmentation, acts as a modifier across all types of skin cancer. Variants of this gene are common in people with red hair, fair skin, and freckles, and they increase susceptibility to UV damage beyond what skin tone alone would predict. This means two people with similarly light skin can have meaningfully different cancer risks based on the version of this gene they carry.
Rare inherited conditions amplify risk even further. Xeroderma pigmentosum, for instance, disables the DNA repair system almost entirely, making even minimal sun exposure extremely dangerous. People with this condition can develop skin cancers in childhood. Other inherited syndromes affect basal cell and squamous cell carcinoma risk through different genetic pathways, though these are uncommon enough that most people will never encounter them.
A Weakened Immune System
Your immune system does more than fight infections. It also identifies and destroys abnormal cells before they can grow into tumors. When that surveillance system is suppressed, skin cancer risk rises dramatically.
The clearest example comes from organ transplant recipients, who take powerful immune-suppressing medications to prevent their body from rejecting the new organ. These patients face a 65- to 250-fold higher risk of squamous cell carcinoma, a 10-fold higher risk of basal cell carcinoma, and up to an 8-fold higher risk of melanoma. The medications not only weaken the immune response to abnormal cells but may also directly promote cancer growth through separate biological pathways.
Long-term use of certain immune-suppressing drugs for autoimmune conditions can carry similar, though typically lower, risks. Extended courses of systemic corticosteroids may also increase skin cancer risk. People with HIV or other conditions that weaken immune function face elevated risk as well, even in areas of skin that rarely see the sun.
Chemical and Environmental Exposures
Arsenic is the best-documented chemical cause of skin cancer. Prolonged exposure through contaminated drinking water, which still affects millions of people worldwide, is associated with increased rates of both skin cancer and bladder cancer. Historically, arsenic also appeared in certain pesticides and medical treatments, creating occupational and iatrogenic exposure. Well water in some regions can contain naturally elevated arsenic levels, making it a relevant concern even today.
Ionizing radiation from medical treatments or occupational exposure is another non-solar cause. People who received radiation therapy, particularly in childhood, have a higher lifetime risk of developing skin cancer in the treated area. Chronic skin inflammation, long-standing scars, and non-healing ulcers can also give rise to squamous cell carcinoma over time, as the repeated cycle of tissue damage and repair increases the chance of a cancerous mutation taking hold.
Skin Cancer in Darker Skin Tones
Higher melanin levels provide some protection against UV-driven skin cancers, but they don’t eliminate the risk entirely, and non-UV causes become proportionally more important. The most common form of melanoma in people with dark skin is acral lentiginous melanoma, which appears on the palms, soles of the feet, fingers, toes, and under the nails. These are areas with little melanin protection regardless of overall skin tone.
Because skin cancer in darker skin often occurs in unexpected locations, it’s frequently diagnosed at a later stage. A dark streak under a fingernail or toenail, a persistent dark patch on the sole of the foot, or a sore that won’t heal in a non-sun-exposed area can all be signs. The causes in these cases are less clearly tied to sun exposure and may involve genetic factors, immune suppression, or chronic irritation rather than UV damage alone.
How Multiple Risk Factors Combine
Skin cancer rarely results from a single event. It develops from the accumulation of DNA damage over time, combined with your body’s ability (or inability) to repair that damage and catch abnormal cells before they multiply. A person with fair skin who tans indoors and carries certain gene variants faces a compounding of risks that is greater than any one factor alone.
Age plays a role simply because more time means more accumulated damage. Certain medications, including some antibiotics and hormones, can make skin more sensitive to UV radiation, amplifying the effect of everyday sun exposure. The interplay of these factors explains why skin cancer can appear in people who don’t fit the typical profile, including younger adults, people with darker skin, and those with limited sun exposure histories.