During the course of a day, we may be exposed to things in our environment (external problem) that damage our DNA. This damage ordinarily would be repaired by the natural operation of our body. But sometimes these repairs do not happen (internal problem) and the damaged DNA can cause gene mutation, possibly leading to uncontrolled cell proliferation and ever cancer.
Americans commonly use more than 100,000 chemicals, and this doesn't take into account mixtures or combinations of chemicals.
Scientists have been working for several decades to identify substances that may cause cancer.