I present to you "The Health Consequences of Involuntary Exposure to Tobacco Smoke: A Report of the Surgeon General"

http://www.surgeongeneral.gov/library/secondhandsmoke/report/

Here is what it says about lung cancer in the conclusions section:

"Lung Cancer
1. The evidence is sufficient to infer a causal relationship between secondhand smoke exposure and lung cancer among lifetime nonsmokers. This conclusion extends to all secondhand smoke exposure, regardless of location.

2. The pooled evidence indicates a 20 to 30 percent increase in the risk of lung cancer from secondhand smoke exposure"

If you want the scientific, nitty gritty details about how they came to that conclusion, see chapter 7, which includes data from studies conducted in the USA, Canada, Europe and Asia from 1984 onward.

And that's just lung cancer. The report has sections on numerous other ill-effects of second-hand smoke.