From an article by Sharon Lynn Campbell in a recent edition of News & Views, a publication of the American Council on Science and Health:

Radioactivity may be a key piece of the unsolved puzzle of exactly how cigarettes cause lung cancer and other diseases. There is increasing evidence that alpha radiation in tobacco causes cancer or sets the stage for further steps that could change normal cells into cancerous ones. . . .

Ionizing radiation consists of particles smaller than the atom that shoot through space at very high speeds, approximately 100,000 miles per second. Not all particles can penetrate equally well. Alpha particles can be stopped by a sheet of paper but can cause massive damage; beta particles are stopped by a thin sheet of foil. Gamma particles travel farther (thick layers of lead are required to stop these) but are less likely to cause massive damage. The damage done to a cell by any of these particles can cause a fatal cancer.

. . . The question of exactly how this is done is not yet resolved. It is possible that the radiation alone alters the genetic code of individual cells enough to make the cells simply grow without restraint, becoming cancerous. However, there is increasing evidence that the radiation in the tobacco smoke also sets the stage for further changes necessary to turn a normal cell into a cancerous one. Both ionizing radiation and chemical carcinogens are suspects in the additional steps.

While alpha particles can be stopped in their tracks by a single sheet of paper, there isn't even a paper-thin shield between tobacco smoke and the delicate cells of the human lung. And recent studies indicate that alpha radiation is a thousand times more carcinogenic than gamma radiation.