Does second hand smoke cause cancer?

Question

Related: Does smoking cigarettes cause lung cancer?

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A friend of mine related today, as shocked as I was to hear it, that he heard that every single study that had ever been conducted on second hand smoke effects was flawed. While I was fairly shocked to hear this (if cigarette smoke causes cancer, why would second hand cigarette smoke not cause cancer?), apparently there's others who think this.

For example the site: Second hand smoke studies: the hype and deceit

"Secondhand smoke kills, according to a new study!," is frantically shouted from the bold type headlines of newspapers and magazine articles. Mostly it is uneducated and/or issue biased newspaper reporters who add the hype to the already deceitful study. Either they don't bother to read the study for themselves (or they do and don't understand the scientific implications), relying on misleading statements from the researchers themselves, or they are far more interested in the fear mongering factor that makes for good reading and do not bother to check with anyone how accurate or reliable a study is. Not to mention it is more politically correct to nod their head yes and insert their own views and conclusions based on the scary sounding research title and bits of information that they have heard.

Whether it is a reporter with no concept of what it is they are actually reporting, simply relaying the study du jour, or a health organization or researcher submitting a report, none of what they say can be taken at face value. News reports are manipulated and then hyped, as are practically all of the studies on secondhand smoke.

The site appears to go through about 10 various studies on second hand smoke, proceeding to illustrate why they don't show what they intend to show: that second hand smoke, does, indeed, cause cancer.

For once and for all: Is there current good, non-flawed evidence from studies showing that second-hand smoke does cause cancer?

Answer 1

When there is doubt about individual studies, the place to go is a meta-study - one that looks at many different experiments, evaluates the quality according to a strict methodology, and pools the results to get stronger statistical findings.

Here's one from 2007:

• Meta-analysis of studies of passive smoking and lung cancer: effects of study type and continent. Taylor R, Najafi F, Dobson A. Int J Epidemiol. 2007 Oct;36(5):1048-59. Epub 2007 Aug 9.

It extracted 55 high quality studies from a pool of 101 that looked at never-smoking women with smoking spouses.

They found an increased risk of lung cancer, across each continent, with no evidence of publication bias.

The abundance of evidence, consistency of finding across continent and study type, dose-response relationship and biological plausibility, overwhelmingly support the existence of a causal relationship between passive smoking and lung cancer.

But wait, that's only in the home. Why would we ban tobacco in the work-place? Oh, because there is another meta-analysis that shows the effect can be even stronger there!

• Stayner L, Bena J, Sasco AJ, Smith R, Steenland K, Kreuzer M, Straif K. Lung cancer risk and workplace exposure to environmental tobacco smoke. Am J Public Health. 2007 Mar;97(3):545-51. Epub 2007 Jan 31.

They looked at 22 studies from around the world.

A strong relationship was observed between lung cancer and duration of exposure to environmental tobacco smoke.

The findings from this investigation provide the strongest evidence to date that exposure to environmental tobacco smoke in the workplace is associated with an increased risk of lung cancer.

Extra Reading:

• Cancer Research UK give an easier-to-read explanation of the science.

• This paper reflects the claims of bias straight back. It shows that the bias, in this not-really-but-presented-as controversy, comes from the tobacco companies.

Answer 2

In addition to the meta-analyses Oddthinking reported on, there’s also the US Surgeon General’s report from 2006:

… the debate is over. The science is clear: secondhand smoke is … a serious health hazard that causes premature death and disease in children and nonsmoking adults.

[…]

Since [the 1986 Surgeon General’s Report] was published, hundreds of peer-reviewed studies and several additional major reports on the health effects of secondhand smoke have been published, and the evidence on these health effects has become even stronger.

And also:

• Secondhand smoke exposure causes heart disease and lung cancer in adults and sudden infant death syndrome and respiratory problems in children.
• There is NO risk-free level of secondhand smoke exposure, with even brief exposure adversely affecting the cardiovascular and respiratory system.

It’s worth pointing out that the report is uncharacteristically clear and drastic. No allowance is made for reasonable doubt.

It’s also worth noting that the source cited in the question disqualifies itself with its name-calling, scientific ignorance (such as reference to “the mysterious SIDS” – there’s nothing at all mysterious about it) and by its routine use of Godwin’s law.

Answer 3

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.

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Update There has been some new debate on the subject since the original answer was written.

One author at reasons.com argues that it is becoming easier to challenge the evidence as the political significance of the position you take is now less significant since many countries now have widely accepted indoor smoking bans. The debate is summarised thus (my emphasis):

Several years ago I was talking to an epidemiologist who is skeptical of the idea that smokers pose a mortal threat to people in their vicinity. Although he supported workplace smoking bans, he was frustrated by the willingness of so many anti-tobacco activists and public health officials to overlook or minimize the weakness of the scientific case that secondhand smoke causes fatal illnesses such as lung cancer and heart disease. He wondered when it would be possible to have a calm, rational discussion of the issue, one in which skeptics would not be automatically dismissed as tools of the tobacco industry.

The opinion piece references a new recent study that doesn't suffer from many of the biases that affect many other studies. The bias is explained here:

...the study has advantages over most of the research commonly cited as evidence that secondhand smoke causes lung cancer. "To our knowledge," the authors say, "this is the first study to examine both active and passive smoking in relation to lung cancer incidence in a complete prospective cohort of US women." The prospective design avoids a weakness of studies that start with lung cancer cases and "match" them to controls. "Many studies that showed the strongest links between secondhand smoke and lung cancer were case-control studies, which can suffer from recall bias," notes the JNCI article, since "people who develop a disease that might be related to passive smoking are more likely to recall being exposed to passive smoking."

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.