Carcinogens are substances that can cause cancer, either by damaging people's DNA or other means, such as causing cells to divide more quickly than usual, according to the American Cancer Society. It's often difficult for scientists to determine if a particular substance is carcinogenic for humans.
Agencies around the world use lab tests and population studies to determine whether or not something causes or contributes to cancer. Some carcinogens, such as tobacco and certain types of radiation, are relatively well understood, while other substances are considered probable causes of cancer. The American Cancer Society notes that carcinogens are often associated with specific types of cancer and that some carcinogens, such as radiation from the sun, are natural and difficult to avoid.