According to the literature, cigarette mainstream smoke (MS) essentially increases the risk of two major groups of life threatening diseases: Cancer and Cardiovascular diseases.
The assessment that Environmental Tobacco Smoke (ETS) exposure should cause similar effects is a logic deduction, albeit theoric but epidemiologic data is controversial, at best.
Since Cardiovascular Dieses are multifactorial and poor targets for comparison, most studies use Cancer (Lung Carcinoma specially) for comparison.
From the papers I had the opportunity to review, most are biased or are statistically weak.
Here's some excerpts from Review Articles in the subject with the respective source:
A significant part of an association between lung cancer and exposure
to ETS would disappear, if, on the average, 1 patient out of 20
nonsmoking cases had failed to tell the interviewer that he had, in
fact, recently stopped smoking.
The fact that the mutation spectrum of the p53 tumor suppressor gene
in lung tumors of ETS-exposed nonsmokers generally differs from that
found in tumors of active smokers lends additional support to the
notion that the majority of tumors found in ETS-exposed nonsmokers
have nothing to do with tobacco smoke.
The average intake of toxic and genotoxic compounds due to ETS
exposure is that low that it is difficult, if not impossible, to
explain the increased risk of lung cancer as found in epidemiological
studies. The uncertainty is further increased because the validity of
epidemiological studies on passive smoking is limited severely by
numerous bias and confounding factors which cannot be controlled for
reliability. The question of whether or not ETS exposure is high
enough to induce and/or promote the carcinogenic effects observed in
epidemiological studies thus remains open, and the assumption of an
increased risk of lung cancer due to ETS exposure is, at present, more
a matter of opinion than of firm scientific evidence.
My point is: If the claim that ETS increases death risk is controversial and no large scale studies could have possibly been conducted, this number is, quite possibly at best, an abusive statistical deduction.
SIDENOTE: Here's, for comparison, a interesting study regarding Air Pollution exposure: