Squamous cell carcinoma of the skin has multiple risk factors, but the greatest risk factor is sun exposure.
Sun: For those who have a long history of sun exposure, the risk of SCC is very high. Individuals who have lived in the Southern USA have a greater chance of developing SCC than those who live in the Northern parts of the country. Worldwide, SCC is again more common in the sunny areas like Australia and the Mediterranean.
The risk of cancer is greater with the longer duration of sun exposure. In most cases the skin cancers occur after decades of sun exposure.
Outdoors. Individuals who work in the sun like construction workers, gardeners, farmers and sports individuals are at high risk for skin cancer because of the prolonged duration of sun exposure
Fair skin. Individuals with fair skin complexion or those who sunburn easily are more likely to develop skin cancer. Skin cancers are generally more common in Caucasian than in dark skinned individuals. Individuals who live in Australia have the highest incidence of skin cancer because of the very high levels of UV rays and also the individuals are very fair skinned.
Gender: SCC is more common in males than females. The reason is that many more men have jobs outdoors and are more exposed to sun for long periods.
Family history: If there is a family history of skin cancer, the chances are high that someone in the family will develop a skin cancer
Immune suppressed: Individuals who are immunocompromised are also prone to developing skin cancers. This group includes individuals who have organ transplants, are receiving immunotherapy drugs or have inborn medical disorders which prevent proper development of the immune system
Genetic disorders: There are some rare disorders like xeroderma pigmentosum which make individuals prone to skin cancer. The disorder results in an impaired ability of the body to repair even the minor defects caused by sun damage.
Smoking: There is no doubt that smoking can increase the incidence of squamous cell cancers elsewhere in the body. Whether it also increases skin cancers is not really proven. However, tobacco smoking does contain a number of potent carcinogens and it is best avoided.
Chronic skin injury: Individuals who have a prolonged ulcer on the leg or a burn injury may develop a cancer in the area. This usually happens after decades of an unhealing ulcer (marjolin's ulcer).
Radiation: Radiation therapy is also a known risk factor for the development of skin cancer. The skin cancer usually occurs after several decades.
Metals: Exposure to heavy metals like arsenic is also a risk factor for skin cancer. Arsenic exposure related skin cancers were seen 3 decades ago when the metal was used as weed killer and insecticide. Growing awareness and environmental surveillance has limited exposure to this heavy metal.