Why Do Some People Argue That Smoking Isn't That Dangerous?

It's very hard to credily argue that smoking is anything but dangerous (long-term) for the person actually doing the smoking.  The evidence on this is overwhelming. So the real controversy is over passive smoking (when you don't smoke but somebody you spend time with often does smoke).  Is it unsafe to spend time with someone who smokes a lot?

Several studies have noticed that passive smokers seem to have a higher risk of lung cancer, asthma and heart disease.  The question is, would these people be at elevated risk of these conditions for other reasons, or is it just down to the smoky atmosphere?  This is an extremely tricky thing to measure, because there are so many variables that can come into understanding the development of conditions as complicated and varied as asthma and heart disease.  And many passive smokers are ex-smokers themselves, so would tend to be at risk of developing lung cancer, anyway.