Skin cancer develops when DNA in skin cells becomes damaged and the cells begin to grow uncontrollably. Ultraviolet radiation from sunlight is the most common cause, but it’s not the only one. Tanning beds, certain chemicals, viral infections, weakened immune systems, and your own genetics all play a role. Understanding each of these pathways helps you recognize which risks apply to you.
How UV Radiation Damages Skin Cells
The sun emits two types of ultraviolet light that reach your skin, and they cause damage in different ways. UVB rays (the ones that cause sunburn) penetrate only the outermost layer of skin, the epidermis. There, they directly alter the structure of DNA by fusing together building blocks in the genetic code. These fused segments, called pyrimidine dimers, become mutagenic when the cell tries to repair or copy them, introducing errors that can eventually trigger uncontrolled growth.
UVA rays penetrate much deeper, reaching the dermis beneath the surface. Rather than damaging DNA directly, UVA works through oxidative stress, generating reactive molecules that attack DNA indirectly. UVA also suppresses the skin’s local immune response about three times more than UVB does during moderate daily sun exposure, which matters because your immune system is one of the key defenses that catches and destroys abnormal cells before they become cancerous.
The type of UV exposure that matters depends on which skin cancer you’re looking at. Squamous cell carcinoma tracks closely with cumulative UVB exposure over a lifetime. Melanoma, the most dangerous form, correlates more strongly with UVA exposure patterns. A Scandinavian ecological study found that melanoma incidence mapped more closely to regional UVA levels (based on latitude), while squamous cell carcinoma tracked with UVB.
Sunburns vs. Steady Sun Exposure
Not all sun exposure carries the same type of risk. Intense, intermittent exposure (think a week-long beach vacation after months indoors) is consistently linked to melanoma. Sunburns during childhood and adolescence are especially dangerous. Five or more sunburns in your lifetime more than doubles your risk of developing melanoma, according to the Skin Cancer Foundation.
Chronic, steady sun exposure from outdoor work tells a different story. Studies of people with high occupational sun exposure often show a reduced risk of melanoma, which may partly reflect the fact that people who can tolerate long hours in the sun without burning tend to self-select into outdoor jobs. Researchers have proposed a dual-pathway model: one pathway involves the proliferation of moles and melanoma on the trunk (linked to intermittent exposure), while the other involves chronic sun damage and melanoma on the head and neck.
Interestingly, skin color modifies the relationship with cumulative exposure. In a study of an Iowa population, fair-skinned people showed little increased melanoma risk from total lifetime sun hours. But among people with medium to dark skin, the risk climbed significantly with more cumulative hours. Medium-to-dark-skinned men in the highest exposure group had more than five times the melanoma risk of those in the lowest group. This challenges the assumption that darker skin tones are broadly protected.
Tanning Beds
Indoor tanning delivers concentrated UV radiation, primarily UVA, directly to the skin. Using tanning beds before age 20 increases your chances of developing melanoma by 47%, and that risk grows with each session. Women younger than 30 who tan indoors are six times more likely to develop melanoma than those who don’t. These numbers are striking because tanning beds were once marketed as a “safer” alternative to natural sun, which they are not.
Your Skin Type and Genetic Risk
Dermatologists classify skin into six types based on how it responds to UV exposure. Type 1 (very pale, always burns, never tans) and Type 2 (burns easily, tans minimally) carry the highest skin cancer risk. Types 3 and 4 can tan but still sustain UV damage. Types 5 and 6, the most deeply pigmented, have more natural protection but are not immune. Skin cancer can develop in people of every skin type.
Beyond skin color, specific inherited traits raise your risk. Having many moles (especially atypical ones), red or blond hair, light eyes, and heavy freckling are all independent risk factors. A family history of melanoma significantly increases your personal risk, suggesting that some people inherit DNA repair mechanisms that are less effective at correcting UV damage before it becomes permanent.
Weakened Immune Systems
Your immune system constantly patrols for abnormal cells and destroys them. When that surveillance system is suppressed, skin cancers can take hold far more easily. Organ transplant recipients, who take immunosuppressive medications to prevent rejection, face a dramatic increase in risk. Their rate of squamous cell carcinoma is more than 50 times higher than the general population’s. Certain medications used in transplant regimens raise the risk further, with some associated with nearly a fivefold increase in the chance of developing multiple squamous cell cancers.
Other conditions that weaken immunity, including HIV/AIDS, certain blood cancers, and long-term use of immunosuppressive drugs for autoimmune diseases, carry similar concerns. The risk is highest for squamous cell carcinoma but extends to other skin cancer types as well.
Chemical and Environmental Exposures
Arsenic is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer, meaning there is sufficient evidence that it causes cancer in humans. Long-term exposure to arsenic through contaminated groundwater, certain industrial processes, mining, and pesticide use has been linked to skin cancers, both melanoma and non-melanoma types. In some parts of the world, arsenic in well water remains a significant public health concern.
Ionizing radiation from medical treatments (such as radiation therapy for other cancers) can also damage skin cell DNA and increase skin cancer risk in the treated area, sometimes years or decades later. Chronic wounds, burn scars, and areas of long-standing skin inflammation can occasionally give rise to squamous cell carcinoma, a phenomenon first described centuries ago in chimney sweeps and industrial workers.
Viral Infections
Certain strains of human papillomavirus (HPV) contribute to squamous cell carcinoma of the skin. The connection is strongest for cancers around the fingernails and toenails, where high-risk HPV (most commonly HPV type 16) is detected in roughly half of tumors. A separate family of HPV strains, the beta types, has been linked to squamous cell carcinoma on sun-exposed skin, with HPV-5, HPV-8, and HPV-24 showing the strongest associations. This viral contribution appears to work alongside UV damage rather than replacing it, meaning the virus may weaken the skin cell’s ability to repair sun-induced DNA errors.
How These Risks Add Up
Skin cancer rarely results from a single cause. In most cases, it develops from an accumulation of damage over time. Someone with fair skin who had several blistering sunburns as a teenager, used tanning beds in their twenties, and works near industrial chemicals carries compounding risks. Each factor adds another layer of DNA damage or reduces the body’s ability to repair it.
An estimated 112,000 new cases of melanoma alone are expected in the United States in 2026, with about 8,510 deaths. Non-melanoma skin cancers (basal cell and squamous cell carcinoma) are far more common, numbering in the millions each year. Most of these cases are driven by UV exposure that accumulated over years and decades, making the choices you make about sun protection today directly relevant to your future risk.