Predisposing factors associated with squamous cell carcinoma include overexposure to the sun’s ultraviolet rays and the presence of premalignant lesions (such as actinic keratosis or Bowen’s disease).

Other predisposing factors include X-ray therapy, ingestion of herbicides containing arsenic, chronic skin irritation and inflammation, burns or scars,
exposure to local carcinogens (such as tar and oil), and hereditary diseases (such as xeroderma pigmentosum and albinism). Rarely, squamous cell carcinoma may develop on the site of smallpox vaccination, psoriasis, or chronic discoid lupus erythematosus.

Squamous cell carcinoma commonly develops on the skin of the face, the ears, the dorsa of the hands and forearms, and other sun-damaged areas. Early lesions appear as opaque, firm nodules with indistinct borders, scaling, and ulceration. Later lesions become scaly in appearance from keratinization and are most common on the face and hands (sun-exposed areas). Lesions on sun-damaged skin tend to be less invasive and less likely to metastasize than lesions on unexposed skin.

Notable exceptions to this tendency are squamous cell lesions on the lower lip and the ears. These are almost invariably markedly invasive metastastic lesions with a generally poor prognosis.