Take a minute to go through the process of LUNG ATTACK presented by  Air Info Now.
http://www.airinfonow.org/html/lungattack/lungplay.htm

In contrast to smoke particles, gases emitted by the cigarette such as benzene and carbon monoxide (CO) are invisible to the eye. Particles smaller than 2.5 micrometers (PM_2.5) are major components of cigarette smoke and can enter deep into the lung where they can cause serious health problems.