Second-hand smoke

Second-hand smoke is tobacco smoke blown into the air by a smoker and inhaled, indirectly, by a non-smoker.

Although direct smoking is more harmful, the claim that second-hand smoke is not damaging is just complete bullshit. Direct smoking or secondhand smoking, at the end of the day what matters is you inhale the smoke which is what harms you, regardless of whether you're exposed to it actively or passively.

Some people who say that second-hand smoke causes serious health problems

 * The International Agency for Research on Cancer (IARC)
 * WHO but what would they know, and who are they to know anything about it?
 * The U.S. EPA, as shown here, but they're a bunch of eco-nuts
 * U.S. National Institutes of Health
 * Centers for Disease Control
 * United States Surgeon General
 * U.S. National Cancer Institute
 * American medical association
 * American Heart Association, American Lung Association, and American Cancer Society
 * American Academy of pediatrics
 * The BMJ but they're in the pay of Big Pharma
 * The European Commission Public Health Section, and we all know where Hitler came from.
 * The City of San Rafael, California, even when it's not second hand.
 * The British Medical Association

Some people who deny that second-hand smoke causes serious health problems
Higher cancer risks have been found in eating mushrooms, drinking milk, using mouthwash, wearing a bra, and keeping pet birds. Statistically, you are much more likely to die in a bicycle accident, or from being left-handed and using right-handed things, than from exposure to smoke. (I swear I’m not making this up!)
 * The Tobacco Industry, (although they internally acknowledge the risk)
 * Frederick Seitz
 * Michael Fumento
 * S. Fred Singer
 * Steve Milloy
 * Some researchers at, via the Journal of the National Cancer Institute
 * Penn Jillette, though he later admitted he was wrong
 * Adam Carolla
 * Musician Joe Jackson (also argues that smoking in general is not really that harmful):

He swears he's not making it up, so it must be true! And since bicycle accidents also kill people, there's not really a problem anyway? In addition, it should be pointed out that wearing a bra does not cause cancer, although, to be on the safe side, it is recommended that females (specifically those aged 18 to 40) do not wear any upper-body garments. You can never be too safe, right?