Most of the human cancers are caused by certain chemical, physical and biological agents, collectively known as carcinogens, which damage DNA leading to mutations in the growth regulatory genes including Oncogenes and Tumour Suppressor Genes. To trace origin of cancer to a particular carcinogen is not an easy job as the latent period between exposure to a carcinogen and development of cancer is quite long. Moreover, the degree and nature of exposure is usually unclear or poorly documented.