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One consistent theme of SBM is that the application of science to medicine is not easy. We are often dealing with a complex set of conflicting information about a complex system that is difficult to predict. That is precisely why we need to take a thorough and rigorous approach to information in order to make reliable decisions.

The same is true when applied to an individual patient. Often times we cannot make a single confident diagnosis based upon objective information. We have to be content with a diagnosis that is based partly on probability or on ruling out other possibilities. Sometimes we rely upon a so-called “therapeutic trial” to help confirm a diagnosis. If, for example, it is my clinical impression that a patient is probably having seizures, but I have no objective information to verify that (EEG and MRI scans are normal, which is often the case) I can help confirm the diagnosis by giving the patient an anti-seizure medication to see if that makes the episodes stop, or at least become less frequent. Placebo effects make therapeutic trials problematic, but if you have an objective outcome measure and a fairly dramatic response to treatment, that at least raises your confidence in the diagnosis.

We can apply the same basic principle on the population level. If a public health intervention is addressing the actual cause of one or more diseases, then we should see some objective markers of disease frequency or severity decrease over time. Putting fluoride in the public water supply decreased the incidence of tooth decay. Adding iodine to salt decreased the incidence of goiter. Fortifying milk with vitamin D decreased the incidence of rickets. However, removing thimerosal from the childhood vaccine schedule did not reduce the incidence of autism (or the rate of increase in autism diagnosis). That is because calcium deficiency causes rickets, but thimerosal (or the mercury it contains) does not cause autism.

In public health there is also the equivalent of placebo effects – confounding factors in epidemiological studies. So studies need to be interpreted with caution. But if we see a consistent signal – a consistent association between a treatment and a decrease in disease incidence or severity, then our conclusion becomes more and more confident.

We are beginning to see this consistent signal with anti-smoking laws and a decrease in diseases that previous evidence suggests is increased by smoking or exposure to second-hand smoke. A recent study published in PLOS Medicine looked at the incidence of preterm birth and low birth weight in Scotland following legislation that came into effect on March 26, 2006 banning smoking in public places. Prior to the legislation preterm and low birth weight were trending up. The study found a statistically significant drop of about 10% beginning January 1, 2006, with a slight reversal two years later. They interpret these results as an anticipatory effect – smokers trying to quit in anticipation of the legislation, with some smokers failing and going back to smoking over the next two years. This conclusion is supported by the spike in nicotine patch prescriptions in January of 2006.

Further, the reduction in preterm birth and low birth weight was found among current smokers as well as never smokers. The decrease among never smokers suggests a second-hand smoke effect.

Epidemiological studies are always difficult to interpret, as I stated above, and this study is no exception. The strength of this study is that it was very thorough, looking at all pregnancies in Scotland over the study period. But there are many confounding factors, one of which pointed out by the researchers is the fact that smoking status was self-reported, and the introduction of legislation may have affected willingness to self-report smoking. Since all pregnancies were looked at, however, this would not have affected the overall decrease in these outcomes reported.

The significance of this one study is enhanced by the fact that it is part of a trend in studies showing a decrease in diseases that prior evidence suggests are worsened by smoking, following smoking bans. A study of acute myocardial infarction (AMI) in Massachusetts found a decrease of 7.4% following a statewide ban on smoking in public places. It is also significant that the decrease occurred in towns that did not have a prior local smoking ban, but not in towns that did. The effect was therefore also likely attenuated by the prior existence of local smoking bans.

A 2009 review and meta-analysis concluded:

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. The IRR incrementally decreased 26% for each year of observation after ban implementation.

Using the same Scotland cohort following the 2006 ban, a study has also found a decrease in hospital admissions for asthma:

After implementation of the legislation, there was a mean reduction in the rate of admissions of 18.2% per year relative to the rate on March 26, 2006 (95% CI, 14.7 to 21.8; P<0.001). The reduction was apparent among both preschool and school-age children.

Overall the data show that smoking bans reduce second hand smoke exposure, smoking, and adverse health outcomes associated with smoking. There is a fair degree of consistency in the data, and this consistency is growing as more studies are being published. We have appeared to cross the fuzzy threshold where we can conclude with a fair degree of confidence that banning smoking in public places works. In the interest of public health, reducing health care costs, and child safety, the totality of evidence strongly suggests that we should strengthen bans on smoking in public places and apply them universally.

Meanwhile, concerns that such bans would economically harm bar and restaurant owners have not been born out in the evidence. A Minnesota study, for example, found a slight increase in revenue following smoking ban laws. It seems more people are willing to go to bars if the experience does not necessarily involve being exposed to intense levels of second-hand smoke.

Smoking ban laws have passed the test of the “therapeutic trial”. Combined with the totality of evidence for the risks of smoking and second-hand smoke, this strongly supports such bans as effective public health measures.