Do workplace smoking bans reduce the number of heart attacks?

Question

There is a lively debate on the health impact of passive smoking (other questions on skeptics.se are here Does passive smoking kill 600,000 people every year? and here Is secondhand smoke dangerous?).

But anti-smoking health campaigners made some startling claims when various towns, regions and countries started bring in legislation to ban smoking in the workplace over the last 15 years or so. The claims are that one major result of workplace bans is that the number of heart attacks is immediately reduced. One of the more lurid reports appeared in The Scotsman in 2009 (Scotland implemented its ban earlier than England):

SMOKING bans have dramatically reduced the number of heart attacks in Europe and North America, cutting rates by between a quarter and more than a third, two major studies have shown.

The evidence suggests anti-smoking laws have had a bigger impact on heart health than first thought.

But these are big public health effects and the size seems at least a little implausible. Anti-nanny-state activists have criticised the early reports. For example, this article by Christopher Snowdon in Spiked argues:

Tales of heart attacks being ‘slashed’ by smoking bans have appeared with such regularity in recent years that it is easy to forget that there is a conspicuous lack of reliable evidence to support them.

Criticising one high estimate of the effect thus:

...one must bear in mind that around 10 to 15 per cent of coronary heart disease cases are attributed to active smoking. That passive smoking could be responsible for a further 40 per cent strains all credibility.

Note that there are several plausible reasons why over eager campaigners might inadvertently exaggerate the effect. Failing to account for natural variation or long term trends in heart attack rates might easily lead to implausibly large estimates of the effect of smoking bans. So the key question here is: do smoking bans have a noticeable effect on heart attack rates? And, if there is an effect, how big is it?

Added clarification: since heart attacks are one of the prices smokers pay for their habit we expect to see reductions in heart attacks when they give up. And more people may quit when workplace and indoor smoking is banned. The interesting question, though, is what effect is seen in non-smokers. We care more about them as it is much easier to justify legislation to prevent harm to third parties than it it is to prevent self-harm. Great care with statistics is required to clearly distinguish the two groups.

NB Although the general question has been addressed here before, I'm asking this specific one as the effect on heart attacks has not, as far as I can tell, been covered in those questions.

Update

The claim that the smoking ban dramatically reduces heart attack rates is still very much alive. This May 2016 piece in The Times (paywalled) argues that it:

...has slashed heart attack rates by 42 per cent...

Answer 1

There is good evidence that smoking bans reduce the number of heart attacks. In the meta-analysis "Cardiovascular effect of bans on smoking in public places: a systematic review and meta-analysis" published in 2009 in the Journal of the American College of Cardiology the authors state in their conclusion

Using 11 reports from 10 study locations, AMI risk decreased by 17% overall (IRR: 0.83, 95% CI: 0.75 to 0.92), with the greatest effect among younger individuals and nonsmokers.

and

Smoking bans in public places and workplaces are significantly associated with a reduction in AMI incidence, particularly if enforced over several years.

AMI in this context means acute myocardial infarction, which is what is commonly known as heart attack.

They used 11 different studies in this meta-analysis. The authors further state

These studies include nearly 24 million people, observed 215,524 cardiac events, and suggest that community smoking bans are associated with a 17% reduction in AMI incidence. If this association represents a cause-andeffect relationship, and assuming approximately 920,000 incident AMIs each year in the U.S., a nationwide ban on public smoking might ultimately prevent as many as 156,400 new AMIs yearly.

From the INTERHEART study, a large study with 24767 people from 52 countries:

One of the most important risk factors for acute myocardial infarction in our study was smoking, which accounts for about 36% of the PAR of acute myocardial infarction worldwide (and about 44% in men)

So the claim that only 10-15% of coronary heart disease are attributed to smoking seems to be wrong.

Answer 2

Yes there is an effect but it may not be as large as early studies suggested with a result of reducing heart attacks by 3-4% in some population subgroups

The trouble with science in areas where there is a broad consensus is that many people don't check their results carefully when they agree with the consensus view. And when those results can be used as public health propaganda, there is a large does of overeager confirmation bias to account for. This effect may explain some of the results reported.

Normally to get a good view of multiple studies we would use a meta analysis (and Fabian reports the results of one in his answer). The best meta-analyses in medical science are those done by the Cochrane Collaboration. Their latest review of the effects of smoking bans was conducted in 2009. Their general conclusion is worth quoting (my emphasis):

Introduction of a legislative smoking ban does lead to a reduction in exposure to passive smoking. Hospitality workers experienced a greater reduction in exposure to SHS [second hand smoke] after implementing the ban compared to the general population. There is limited evidence about the impact on active smoking but the trend is downwards. There is some evidence of an improvement in health outcomes. The strongest evidence is the reduction seen in admissions for acute coronary syndrome.

But they also concluded:

More research on the impact of smoking bans on active smoking is warranted. Whilst more comprehensive bans are now being enacted in many jurisdictions, there is also a need for research into the extent to which exposure to SHS in non-smoking areas is a result of smoke coming from the outdoor smoking areas. Further studies should utilise larger sample sizes with common agreed measures for smoking behaviour and exposure to SHS of the study population. The lack of a reference group in many studies means that it is difficult to differentiate between secular trends and impact of the intervention...

The review was not, however, focussed on the heart attack issue (it covered many effects of legislation) and only looked at 12 studies where the impact on heart attacks were measured. Some of these were small.

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.

Their result is statistically reliable but much smaller than many of the earler reports. They report:

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 the quality of some previous studies is suspect for a variety of reasons:

There is some uncertainty around the extent to which some of these studies have effectively accounted for other factors that might influence patterns of admissions for myocardial infarction. Firstly, admissions for coronary heart disease have been declining across Europe, the US, and Canada, and failure to account for this might lead to an overestimation of impacts. Secondly, other factors such as season, flu, and temperature have all been shown to influence the incidence of myocardial infarction with, for example, peak admission rates in winter, in spring, and over the Christmas break, and seen in association with high flu rates and low temperatures.

The BMJ paper tried to address as many of these 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.