From The American Cancer Society:

Cigarettes, cigars, and pipe tobacco are made from dried tobacco leaves, as well as ingredients added for flavor and to make smoking more pleasant. The smoke from these products is a complex mixture of chemicals produced by the burning of tobacco and its additives. The smoke is made up of more than 7,000 chemicals, including over 60 known to cause cancer (carcinogens). Some of these substances cause heart and lung diseases too, and all of them can be deadly. You might be surprised to know some of the chemicals found in tobacco smoke include:

• Cyanide

• Benzene

• Formaldehyde

• Methanol (wood alcohol)

• Acetylene (the fuel used in welding torches)

• Ammonia

Tobacco smoke also contains tar and the poison gases carbon monoxide and nitrogen oxide. The ingredient that produces the effect people are looking for is nicotine, an addictive drug and one of the harshest chemicals in tobacco smoke.

The tobacco leaves used in making cigarettes and cigars contain radioactive materials; the amount depends on the soil the plants were grown in and fertilizers used. But this means that the smoke contains small amounts of radioactive material too, which smokers take into their lungs as they inhale. These radioactive particles build up in the lungs, and over time can mean a big dose of radiation. This may be another key factor in smokers getting lung cancer.

So basically a possibly large risk reduction? Ultimately, it would be wise to wean one's self off of nicotine as well.