Epidemiologists, physicians, and laboratory scientists all play a role in identifying agents that contribute to causation of cancer in humans. Two hundred and twenty-eight years ago, the British physician, Percivall Pott, determined that there was an association between occupational exposure of chimney sweeps to soot in the chimneys they cleaned and the induction of scrotal cancer in these workers. Since that time, epidemiological studies have shown associations between exposure to cigarette smoking and lung cancer, exposure to asbestos and mesothelioma, exposure to β-naphthylamine and several other aromatic amines and urinary cancer, and exposure to aflatoxin B1 and hepatocellular carcinoma [1-6]. There are therefore a variety of chemicals in various occupational settings and in the environment that are human and animal carcinogens. To determine whether an agent is a potential human carcinogen, the strongest tools to use are epidemiological studies. However, these studies are difficult, since they must rely on natural, not experimental, conditions [1]. Further, epidemiological

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studies are somewhat insensitive. It took many decades for epidemiologists to determine that cigarette smoke, a very strong carcinogenic mixture, was a cause of human lung cancer.