There seems to be little doubt in public opinion that second hand smoke is dangerous, and I can see why. But many smokers have claimed to me that it is not dangerous at all and it is all media hype. What is the scientific consensus? I have heard that second hand smoking is almost equivalent to smoking a cigarette directly.
The Surgeon General of the United states issued a report in 2006 about second-hand smoke. The six major conclusions were:
Many millions of Americans, both children and adults, are still exposed to secondhand smoke in their homes and workplaces despite substantial progress in tobacco control.
Secondhand smoke exposure causes disease and premature death in children and adults who do not smoke.
Children exposed to secondhand smoke are at an increased risk for sudden infant death syndrome (SIDS), acute respiratory infections, ear problems, and more severe asthma. Smoking by parents causes respiratory symptoms and slows lung growth in their children.
Exposure of adults to secondhand smoke has immediate adverse effects on the cardiovascular system and causes coronary heart disease and lung cancer.
The scientific evidence indicates that there is no risk-free level of exposure to secondhand smoke.
Eliminating smoking in indoor spaces fully protects nonsmokers from exposure to secondhand smoke. Separating smokers from nonsmokers, cleaning the air, and ventilating buildings cannot eliminate exposures of nonsmokers to secondhand smoke.
Similar information can be obtained from the Centers for Disease Control (also see this Morbidity and Mortality Weekly Report), the Institute of Medicine, the journal Environmental Health Perspectives, the National Cancer Institute (part of the National Institutes of Health), and the Mayo Clinic.
A full list of research can be found at the MedLine, and it includes research from the American Heart Association, North Carolina Medical Journal, Current Opinions in Pulmonary Medicine, British Medical Journal, and Neurotoxicology and Teratology. (This is all in the first 15 articles.)
I'm not a pro-smoking crusader. (Can't stand the things) But in the interest of balance, there has been at least one study which somewhat contradicts the commonly accepted theme on second hand smoke.
http://jnci.oxfordjournals.org/content/90/19/1440.full.pdf is a largish WHO study that described an apparently (and surprising) protective effect of second hand smoke on children of smokers. From the abstract:
Our results indicate no association between childhood exposure to ETS and lung cancer risk. We did find weak evidence of a dose-response relationship between risk of lung cancer and exposure to spousal and workplace ETS. There was no detectable risk after cessation of exposure.
Note that in this particular study, with "a total of 650 patients with lung cancer and 1542 control subjects", found the reduced chance of lung cancer among children of smokers to be statistically significant, but the "weak evidence" in this case is code for "not what would normally be called statistically significant".
Despite the consensus, there are good quality studies that don't find a strong link
I think the scientific consensus is clear: almost everyone thinks secondhand smoke is bad. But skeptics should carefully consider, especially in areas where emotions run as high as this, the alternative views.
Here is a large scale study of the long term effects of secondhand smoke that finds little impact on all cause mortality. The BMJ paper is here. The conclusion:
Conclusions. The results do not support a causal relation between environmental tobacco smoke and tobacco related mortality, although they do not rule out a small effect. The association between exposure to environmental tobacco smoke and coronary heart disease and lung cancer may be considerably weaker than generally believed.
For the sake of balance it is worth reading the follow-up debate. My reading is that there are few direct criticisms of the science but many attacks on the scientists (who have some relationship with the "evil" tobacco industry). The intemperate nature of many responses suggest that it is becoming hard to conduct and publish any research on the topic that does not agree with the consensus. I'd recommend any skeptic to read the paper and then the responses and make their own mind up while trying to consider only criticism of the results and not their funding.
Both poor quality studies agreeing with the consensus and entirely unreferenced claims are widely repeated to justify legislation. One example is the (entirely unreferenced) claim that secondhand smoke kills 600,000 people a year. The claim comes from a WHO factsheet which does not reference any sources. Yet this is repeated as an accurate assessment even by people who should know better such as David Nutt (it is in his, mostly sensible, book *Drugs-without the hot air"). This is not a small number of deaths and, if the claim were true, we would expect strong epidemiological evidence. This is not obviously there in the literature.
The best and most recent studies cast doubt on any relationship between passive smoking and lung cancer. But there may be a small affect on heart-attack rates though this could be an overestimate because it is hard to distinguish the effect on non-smokers from the effect of smoking who quit smoking as a result of the bans.
NB There are several skeptics.SE questions on passive smoking and this section summarises some results explored in more depth on other, more specific, questions. Specifically: Does second hand smoke cause cancer? Do workplace smoking bans reduce the number of heart attacks?
There may be some duplication until I get through a few more edit cycles.
Passive smoking and cancer There has been some new debate on the subject since the original answer was written.
On the subject of Lung cancer and second hand smoke, a new study using rather better methods than many previous studies occluded that no relationship could be shown a significant levels. The [referenced study] is from the Journal of the National Cancer Institute. Its key conclusions are (my emphasis):
A large prospective cohort study of more than 76,000 women confirmed a strong association between cigarette smoking and lung cancer but found no link between the disease and secondhand smoke.
Passive smoking and cardiovascular disease.
In the early days of workplace smoking bans a number of studies reported large reductions in the number of heart attacks (the quote if from the BMJ paper referenced below).
The largest impacts have been reported in smaller studies in the United States, with reported reductions in the range of 27-40%, while larger studies have reported more modest reductions: 8% in the state of New York, 13% in four Italian regions, and 17% in Scotland.
But many of these were fairly slipshod in statistical terms (often small, often ignoring long term trends, often ignoring the possibility of random fluctuations in numbers).
More recently a high quality study has appeared in the BMJ which overcomes some of the issues with previous studies partially because comprehensive population-wide data is accessible from the English NHS hospital activity statistics which are collected centrally. They conclude:
After adjustment for secular and seasonal trends and variation in population size, there was a small but significant reduction in the number of emergency admissions for myocardial infarction after the implementation of smoke-free legislation (−2.4%, 95% confidence interval −4.06% to −0.66%, P=0.007). This equates to 1200 fewer emergency admissions for myocardial infarction (1600 including readmissions) in the first year after legislation. The reduction in admissions was significant in men (3.1%, P=0.001) and women (3.8%, P=0.007) aged 60 and over, and men (3.5%, P<0.01) but not women (2.5% P=0.38) aged under 60.
The BMJ paper tried to address as many of the defects of previous studies as possible by careful design and analysis. The biggest remaining confounding factor is the effect derived from smoking quitters which is hard to separate as English hospital records don't include smoking status.
The conclusion seems to be that careful statistical analysis shows a real effect on the heart attack rate (AMI (acute myocardial infarction) admissions down by a few percent), but nothing like the large gains claimed by public health activists in the early days of legislation.