Skin cancer starts when cells in your skin grow abnormally and multiply without stopping

Skin cancer occurs when the DNA inside skin cells becomes damaged, usually from ultraviolet (UV) radiation from the sun or tanning beds. That damage causes the cells to grow and divide in ways the body cannot control. Unlike normal skin cells that follow a predictable life cycle, cancer cells keep reproducing and can spread to other parts of your body if not caught early.

Your skin has three main layers: the epidermis (the outer layer you see), the dermis (the thick layer underneath), and the subcutaneous tissue (the deepest layer of fat). Skin cancer almost always begins in the epidermis, where most of the cells that produce skin color live. The type of cancer that develops depends on which cells become abnormal.

Key Takeaways

  • Skin cancer develops when UV radiation damages DNA in skin cells, causing them to multiply uncontrollably.
  • The three main types are basal cell carcinoma (most common and slowest to spread), squamous cell carcinoma (more likely to spread than basal cell), and melanoma (rarest but most dangerous).
  • Melanoma can develop from existing moles or appear as new growths, and it spreads faster than other skin cancers.
  • Early detection makes a major difference in treatment outcomes, which is why regular skin checks matter.

The three main types of skin cancer and how they differ

Basal cell carcinoma is the most common type, accounting for roughly 80 percent of all skin cancers. It starts in the basal cells at the base of the epidermis. These cancers grow slowly and rarely spread to other parts of the body, but they can damage surrounding skin and tissue if left untreated. They often appear as a shiny bump, a scar-like patch, or a sore that does not heal.

Squamous cell carcinoma is the second most common type and starts in the flat cells in the upper part of the epidermis. It grows faster than basal cell carcinoma and is more likely to spread to lymph nodes or other organs, though this is still uncommon if caught early. Squamous cell cancers often look like a scaly patch, a wart-like bump, or an open sore.

Melanoma is the rarest type but the most dangerous. It starts in melanocytes, the cells that produce melanin (the pigment that gives skin its color). Melanoma can develop from an existing mole or appear as a new growth. It grows quickly and spreads to other organs more readily than the other types. Melanoma accounts for only about 1 percent of skin cancers but causes the majority of skin cancer deaths.

Why UV radiation causes skin cell damage

Ultraviolet radiation comes in two main forms that reach Earth: UVA and UVB. Both can damage the DNA inside skin cells. UVB rays are strongest during midday hours and cause the sunburns you feel immediately. UVA rays penetrate deeper into the skin and can damage cells even on cloudy days or through windows. Tanning beds emit both types of UV radiation and carry the same cancer risk as sun exposure.

The damage happens at the cellular level. When UV rays hit skin, they can break the bonds in DNA molecules. Your body has repair systems to fix this damage, but repeated exposure or intense exposure can overwhelm those systems. Over time, mutations accumulate in skin cells. Most of these mutations cause no harm, but occasionally one occurs in a gene that controls cell growth, and that cell begins to divide uncontrollably.

This is why skin cancer risk increases with cumulative sun exposure over a lifetime, not just from occasional sunburns. Someone who spends years working outdoors faces higher risk than someone who gets one severe burn. However, a single intense sunburn, especially during childhood, also raises the risk of melanoma later in life.

How skin cancer grows and spreads

Skin cancers grow outward from where they start. In the early stages, cancer cells remain in the epidermis. This stage is called carcinoma in situ or non-invasive cancer. At this point, the cancer has not yet broken through the basement membrane—the boundary between the epidermis and the dermis below.

As the cancer grows, it can push through that boundary and invade the dermis and deeper layers. Once it reaches the dermis, it gains access to blood vessels and lymph vessels, which are the pathways cancer cells use to travel to other parts of the body. The deeper a skin cancer grows into the skin, the higher the risk it has already spread or will spread.

Melanoma is more likely to spread than basal or squamous cell cancers. It can travel to nearby lymph nodes first, then to distant organs like the lungs, liver, or brain. Basal cell carcinoma rarely spreads even when it grows deep, which is one reason it has such a good outlook. Squamous cell carcinoma spreads more often than basal cell but less often than melanoma.

Risk factors that increase your chances of developing skin cancer

Fair skin, red or blonde hair, and light-colored eyes increase risk because these traits mean less melanin in the skin, which provides some natural UV protection. People with darker skin can still develop skin cancer, but it occurs less frequently. However, when skin cancer does develop in people with darker skin, it is often diagnosed at a later stage, which affects outcomes.

A personal history of skin cancer or a family history of melanoma raises your risk. People with many moles (more than 50) or with atypical moles (moles that look irregular or have unusual colors) face higher risk. Age matters too—skin cancer becomes more common as you get older, though melanoma can occur at any age, including in young adults and children.

Certain medical conditions and medications increase risk. People with weakened immune systems (from HIV, organ transplants, or immunosuppressive drugs) develop skin cancer more often. Some medications make skin more sensitive to UV damage. A history of severe sunburns, especially in childhood, raises melanoma risk significantly.

How doctors detect and diagnose skin cancer

Skin cancer is usually found through visual inspection. A doctor or dermatologist examines your skin with the naked eye or with a magnifying tool called a dermoscope. They look for growths that have changed in size, shape, or color, or that have unusual features like irregular borders, multiple colors, or asymmetry.

If a spot looks suspicious, the doctor removes a sample of tissue (a biopsy) and sends it to a lab. A pathologist examines the cells under a microscope to determine whether cancer is present and, if so, what type and how deep it goes. The biopsy result guides treatment decisions and helps predict how the cancer is likely to behave.

For melanoma, doctors also assess the thickness of the cancer and whether it has spread. Thickness is measured in millimeters and is one of the most important factors in predicting outcomes. If there is concern the cancer has spread, imaging tests like CT scans or blood tests may be ordered.

Why early detection changes outcomes

Skin cancer caught in its earliest stages—when it is still confined to the epidermis—has a much higher chance of being cured with simple removal. Basal cell carcinoma found early has a cure rate above 95 percent. Squamous cell carcinoma caught early also has excellent outcomes. Even melanoma, when found before it spreads to lymph nodes or distant organs, has significantly better survival rates than melanoma found at later stages.

This is why dermatologists recommend regular skin checks, especially for people with risk factors. Self-examination using the ABCDE rule (Asymmetry, Border irregularity, Color variation, Diameter larger than a pencil eraser, Evolving or changing) can help catch changes early. Any spot that is new, changing, or looks different from other moles warrants a doctor's evaluation.

Frequently Asked Questions

Can you get skin cancer from a sunburn that happened years ago?

Yes. Intense sunburns, especially in childhood, increase melanoma risk even decades later. The DNA damage from that burn can persist in skin cells and eventually lead to cancer. This is why sun protection matters at every age, not just when you are young.

Does sunscreen prevent all skin cancer?

Sunscreen reduces risk significantly when used correctly and reapplied every two hours, but it does not prevent all skin cancer. UV rays can still reach skin through gaps in application or from reflection off water and sand. Sunscreen works best combined with other protection like shade, protective clothing, and avoiding peak sun hours.

If I have a mole, will it definitely turn into cancer?

No. Most moles never become cancer. However, moles can change over time, and any mole that changes in size, shape, color, or texture should be checked by a doctor. People with many moles or atypical moles have higher risk, which is why regular monitoring matters for them.

Can skin cancer appear on parts of your body that do not get sun?

Skin cancer is much rarer in areas with little sun exposure, but it can occur. Melanoma occasionally develops on the soles of feet, palms, or under nails—areas that receive little UV exposure. This is why full-body skin checks by a dermatologist are important, not just checks of sun-exposed areas.

Is melanoma always fatal if it spreads?

No. Melanoma that has spread to lymph nodes or distant organs is more serious than early-stage melanoma, but treatment options exist and some people survive for years or longer. Outcomes depend on how far it has spread, the thickness of the original tumor, and other factors. Early detection remains the strongest predictor of survival.