Is secondhand smoke really bad for you?

In a word "yes". The weight of evidence shows that exposure to secondhand tobacco smoke (SHS) can lead to a variety of maladies some very severe, such as lung cancer and heart disease mortality.

See this description of the 2006 US Surgeon General's Report on the health effects of secondhand smoke.

The U.S. Environmental Protection Agency published a seminal report on respiratory illnesses due to SHS exposure in 1993. You may want to read an updated assessment of that report.

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