75-80% of cancers are due to the action of external agents carcinogens or risk factors. In the other 20-25% of the cases, it is not known exactly the mechanisms by which it occurs and cancer develops.4

Among the risk factors are:

Genetic predisposition: in some cases people have a predisposition to the development of certain cancers, inherited genes since alterados4. However, possessing these mutations does not indicate that necessarily go on to develop cancer, although yes, there are more chances of getting it.
Age: aging is another fundamental factor in the development of cancer. The incidence of this disease increases with age, probably due to the accumulation of risk factors linked to certain types of cancer. This accumulation is combined with the tendency for cellular repair mechanisms lose efficiency with the edad5.

Chemical agents: most of the chemical carcinogens are related to industrial activities. Asbestos, arsenic, benzene, cadmium, mercury, nickel, lead, chlorinated hydrocarbons, naftilamina, are some of the agents with carcinogenic activity more usual.
Physical agents: radiation.
Some infections