"Because nonmelanoma skin cancers generally occur in persons after 50, very little attention has been paid to their incidence in younger adults and children," explains lead investigator Leslie Christenson, MD, a Mayo Clinic dermatologist.

"We have discovered that these cancers are becoming increasingly prevalent in younger people," she says.

Most Common Cancer

Skin cancer is the most common form of cancer, with approximately 1 million new cases each year in the United States.

Of the two major categories of skin cancer -- melanoma and nonmelanoma -- the latter is far more prevalent.

There are two common types of nonmelanoma skin cancer: basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), the types evaluated in this study.

The incidence of BCC tumors increased over time; that is, there were more occurrences in the early 2000s than in the late 1970s, the Mayo Clinic investigators found. The number of diagnoses also rose with the age of the patient -- doubling during the study period.

The general increase in tumors relative to age was anticipated, but the overall increase over time was surprising. The researchers attribute it primarily to the exponential increase in BCC tumors in women, especially those in their late 30s.

Men often had larger BCC tumors than women -- probably because women are likely to seek medical attention sooner.

SCC diagnoses also increased significantly over time for both men and women -- incidence quadrupled -- with an especially large peak in men in their late 30s.

Likely Causes of Increase

Long-term or intense sun exposure and the use of tanning beds are probably the largest causes of the increase, but increased exposure to UV light, ozone depletion and increased detection are also likely contributors, say the researchers.

"This increase [in young adults] may lead to an exponential increase in the overall occurrence of nonmelanoma skin cancer over time as the population ages," they note.

They emphasize the need to focus on preventing skin cancer in the very young -- such as limiting sun exposure and always using sunscreen -- "so that the increasing incidence of a potentially preventable cancer can be halted."

The Mayo Clinic website has more information about cancer treatment and research programs.