Greyriverside: actually the link between passive smoking and lung cancer (and heart disease) has been proved.
The WHO International Agency for research on Cancer (IARC) is the global authority on carcinogens.
IARC's most recent review of the scientific evidence on tobacco smoke runs to 1500 pages - 200+ on passive smoking. This is a systematic review - looking at and reviewing the quality of all the individual studies on tobacco smoke. It was published in full in 2004.
Their Conclusion: "Involuntary smoking (exposure to secondhand or "environmental tobacco smoke) is carcinogenic to humans, (Group 1)"
Group 1 are classified as those substances known to cause cancer in humans.
Link to summary of the review here.
And yes IARC's view does matter - that's why Philip Morris spent millions of dollars on secretly infiltrating IARC in the 1990s to undermine IARC's work in this area. See the abstract of the Lancet article on what happenedhere.
The US Surgeon General's systematic Review on the health risks of secondhand smoke was published in 2006 and can be seen in full here
There is no debate about the science. The only controversy on this issue comes from the tobacco companies and its apologists.
By the 1990s, studies funded by the tobacco industry-funded were 90 times more likley to conclude that secondhand smoke does not harm health than independent research. Abstract here
(Tobacco company documents, incidentally, show that internal research by tobacco cos was showing that secondhand smoke was harmful as far back as the 1970s. They simply didn't publish it.)