<human>:  Why do you think that cigarette smoke stinks?
<bot>: Cigarette smoke contains a number of harmful chemicals. They include carbon monoxide and tar, which have been shown to have negative health effects. Many people also find the smell of cigarette smoke unpleasant and avoid places where it is present. The Environmental Protection Agency considers cigarette smoke to be a class A carcinogen -- carcinogens are chemicals associated with cancer. Repairing damage to the lungs is difficult because cigarette smoke is breathed deep into the lungs every time you inhale.