Depending on particle size, smoke can be visible or invisible to the naked eye. This is best illustrated when toasting bread in a toaster. As the bread heats up, the products of combustion increase in size. The particles produced initially are invisible but become visible if the toast is burned or cooled rapidly.
What's always got me about the passive smoking argument is that when you walk down the street next to a road, kids and non-smokers are inhaling far more sh*t than you get in someone's second hand smoke.
Secondhand smoke is the combination of both sidestream and mainstream smoke emissions. These emissions contain more than 50 carcinogenic chemicals. According to the Surgeon General's latest report on the subject, "Short exposures to secondhand smoke can cause blood platelets to become stickier, damage the lining of blood vessels, decrease coronary flow velocity reserves, and reduce heart variability, potentially increasing the risk of a heart attack" [20] The American Cancer Society lists "heart disease, lung infections, increased asthma attacks, middle ear infections, and low birth weight" as ramifications of smoker's emission [21]