Tobacco smoke from cigarettes contains over 4000 harmful chemicals at least 43 of these are carcinogenic (causing cancer). That's 43 reasons to stop smoking cigarettes!
There might potentially be 4000 additional reasons to quit smoking as well.

Some of the most toxic chemicals contained in cigarette smoke are Benzen,formaldehyde,acetone,ammonia tar,arsenic and carbon monoxide.

Read more about these chemicals and their harmful effects here