Here's the definition:
"A large number of known carcinogens are found in cigarette smoke. Potent carcinogens found in cigarette smoke include polycyclic aromatic hydrocarbons (PAH, such as benzo(a)pyrene), benzene, and nitrosamine."
Some well known arcinogens are: asbestos, nickel, cadmium, radon, vinyl chloride, benzidene, and benzene
..20 pages!? Perhaps check if you..maybe..saw the number wrong?
But if you really need to end up doing 20 pages then do research. Write facts about benzene, nitrosamine..etc.
Make headings and try to fit in as much knowledge as you can.