Cigarettes have been known<unk> s for at least 65 years. A<unk>  is any substance, radionuclide, or radiation that promotes<unk> esis, the formation of cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substances are considered<unk> s, but their<unk> ic activity is attributed to the radiation, for example gamma rays and alpha particles, which they emit. Common examples of non-radioactive<unk> s are inhaled asbestos, certain dioxins, and tobacco smoke. Although the public generally associates<unk> icity with synthetic chemicals, it is equally likely to arise from both natural and synthetic substances.<unk> s are not necessarily immediately toxic; thus, their effect can be insidious. Cancer is any disease in which normal cells are damaged and do not undergo programmed cell death as fast as they divide via mitosis.<unk> s may increase the risk of cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with biological processes, and induces the uncontrolled, malignant division, ultimately leading to the formation of tumors. Usually, severe DNA damage leads to programmed cell death, but if the programmed cell death pathway is damaged, then the cell cannot prevent itself from becoming a cancer cell. There are many natural<unk> s. Aflatoxin B1, which is produced by the fungus Aspergillus flavus growing on stored grains, nuts and peanut butter, is an example of a potent, naturally occurring microbial<unk> . Certain viruses such as hepatitis B and human papilloma virus have been found to cause cancer in humans. The first one shown to cause cancer in animals is Rous sarcoma virus, discovered in 1910 by Peyton Rous. Other infectious organisms which cause cancer in humans include some bacteria (e.g. Helicobacter pylori ) and helminths (e.g. Opisthorchis viverrini and Clonorchis sinensis). Dioxins and dioxin-like compounds, benzene, kepone, EDB, and asbestos have all been classified as<unk> ic. As far back as the 1930s, industrial smoke and tobacco smoke were identified as sources of dozens of<unk> s,benzoapyrene, tobacco-specific nitrosamines such as nitrosonornicotine, and reactive aldehydes such as formaldehyde, which is also a hazard in embalming and making plastics. Vinyl chloride, from which PVC is manufactured, is a<unk>  and thus a hazard in PVC production. Co-carcinogens are chemicals that do not necessarily cause cancer on their own, but promote the activity of other<unk> s in causing cancer. After the<unk>  enters the body, the body makes an attempt to eliminate it through a process called biotransformation. The purpose of these reactions is to make the<unk>  more water-soluble so that it can be removed from the body. However, in some cases, these reactions can also convert a less toxic<unk>  into a more toxic<unk> . DNA is nucleophilic; therefore, soluble carbon electrophiles are<unk> ic, because DNA attacks them. For example, some alkenes are toxicated by human enzymes to produce an electrophilic epoxide. DNA attacks the epoxide, and is bound permanently to it.benzoapyrene in tobacco smoke, other aromatics, aflatoxin and mustard gas.
Carcinogen
Toxicant
Co-carcinogen
Drug
Drug 
Biological agent
Pollutant
Pathogen