Actinic keratosis is caused by cumulative UV radiation damage to skin cells, specifically the keratinocytes that make up the outermost layer of your skin. Years of sun exposure create mutations in these cells’ DNA, eventually producing the rough, scaly patches that define the condition. It affects up to 38% of adults in some populations and becomes dramatically more common with age.
How UV Radiation Damages Skin Cells
The two types of ultraviolet radiation, UVA and UVB, both contribute to actinic keratosis but through different mechanisms. UVB rays are the primary driver. About 90% of UVB is absorbed by the epidermis (your skin’s outer layer), where it directly damages DNA by creating abnormal bonds between building blocks of the genetic code. These bonds, called thymine dimers, interrupt normal cell regulation and suppress the local immune response.
UVA rays penetrate deeper, with about 50% reaching the dermis below. Rather than damaging DNA directly, UVA generates free radicals that break DNA strands and cause mutations through oxidative stress. The combined effect of both types means that broad-spectrum UV exposure is the central cause of actinic keratosis, not just one wavelength.
What makes this process dangerous is that it’s cumulative. Each episode of UV exposure adds a small amount of DNA damage. Your cells have repair mechanisms that fix most of it, but over decades, errors slip through. These unrepaired mutations accumulate in sun-exposed skin cells, gradually pushing them toward abnormal growth.
The Role of the P53 Gene
One mutation matters more than the rest. The TP53 gene, often called the “guardian of the genome,” normally acts as a safety switch. When UV radiation damages a skin cell’s DNA beyond repair, TP53 triggers that cell to self-destruct, a process called apoptosis. You can actually see this happening after a sunburn: the individual dying keratinocytes are sometimes called “sunburn cells” and appear in the epidermis of overexposed skin.
When TP53 itself is mutated by UV radiation, this safety mechanism breaks down. Damaged keratinocytes no longer receive the signal to die. Instead, they survive and keep dividing, accumulating additional genetic abnormalities with each round of replication. Research in mice with transplanted human skin has confirmed this directly: UVB exposure produces actinic keratoses tied to specific TP53 mutations. This makes TP53 mutation the pivotal step that separates ordinary sun damage from the development of a precancerous lesion.
Cumulative Exposure, Not Just Sunburns
Actinic keratoses develop on areas of chronic, repeated sun exposure rather than from occasional intense burns. The lesions show up overwhelmingly on the head and face (about 92% of cases), followed by the arms (roughly 19%), with the trunk and legs accounting for less than 1% combined. Within the face, the forehead, temples, and cheeks are the most common sites. Men develop scalp lesions at far higher rates than women (41% vs. 10%), reflecting differences in hair coverage.
The pattern holds for both outdoor and indoor workers. Even people who work indoors develop actinic keratoses in the same facial locations as outdoor workers, though outdoor workers, particularly landscapers, have higher rates on the nose and other highly exposed areas. This tells you that routine, day-to-day UV exposure over years is enough to cause damage. You don’t need to work outside for a living to accumulate the mutations that lead to actinic keratosis.
Who Is Most at Risk
Fair skin is the strongest individual risk factor. People with lighter skin have less melanin to absorb UV before it reaches vulnerable cells, so more radiation penetrates to the DNA level. In Australia, where UV levels are high and a large share of the population has European ancestry, an estimated 40 to 60% of adults over 40 have at least one actinic keratosis. In the United States, the overall prevalence is estimated at 11 to 26%. A large Dutch study of adults with an average age of 72 found that 49% of men and 28% of women had at least one lesion.
Age is the other major factor, and it’s essentially a proxy for total lifetime UV exposure. The condition is roughly 16 times more common in people in their 80s than in their 40s. That steep rise reflects decades of accumulated DNA damage finally reaching a threshold where abnormal growths appear.
Immunosuppression as a Risk Factor
Your immune system plays an active role in identifying and destroying cells with abnormal DNA. When that surveillance is weakened, damaged keratinocytes are more likely to survive and multiply. This is why people who take long-term immune-suppressing medications, particularly organ transplant recipients, face a significantly elevated risk of actinic keratosis and subsequent skin cancer.
For transplant recipients, the risk is compounded by several factors: age over 50 at the time of transplant, any history of actinic keratosis or skin cancer before the transplant, infection with human papillomavirus, and having received a heart or lung transplant (which typically requires more aggressive immunosuppression than kidney or liver transplants). The immune suppression doesn’t cause the DNA mutations directly. Rather, it removes the body’s ability to catch and eliminate cells that have already been damaged by UV exposure.
From Precancerous Lesion to Skin Cancer
Actinic keratosis is classified as a precancerous condition because a small percentage of lesions progress to squamous cell carcinoma, an invasive skin cancer. The reported rate of this transformation varies widely, from as low as 0.025% to as high as 20% per lesion per year, depending on the study population and methodology. A recent systematic review placed the figure at the lower end, around 0.075% per lesion per year.
Those numbers may sound small for any single lesion, but many people have multiple actinic keratoses. In the Dutch study, 8% of participants had 10 or more. When you multiply even a modest per-lesion risk across a dozen spots over several years, the cumulative probability becomes meaningful. There’s currently no reliable way to predict which individual lesions will progress and which won’t, which is why dermatologists generally recommend treating them rather than monitoring.