Before I comment on this, let me say that I loathe and detest tobacco smoke and consider those who light up in the presence of non-smokers to be pathetic and offensive addicts. So it would suit me if I could endorse the conclusions of the article below. But I cannot. It is one of a long line of attempts to portray secondhand smoke as harmful but the best research on the topic indicates that it is not . Existing research, however, has mainly looked at passive smoking as a cause of heart and lung disease. The study below takes a new tack. It tries to show that passive smoking makes you stupid.

The research below appears to have been done with unusual care but is still incapable of supporting its conclusions. It found that those who associated with smokers a lot had lower mental alertness. They were slower to process instructions that they were given. But we have known for years that smoking correlates with all indices of social disadvantage, including low IQ. I am delighted to note however that the researchers took extensive account of that and controlled for a whole range of social class indicators. That is rare sophistication in epidemiological research. They did NOT however control for IQ — which was arguably the most important thing to control for in the circumstances. IQ correlates strongly with mental speed.

So what was in fact found was that low IQ people tend to flock together. It was shown that people who associate with dummies (i.e. smokers) a lot also tend to be dummies (as measured by the tests used in the study below). The study tells us nothing about passive smoking.

Exposure to second-hand smoke boosts the risk of dementia and other cognitive problems, even among people who have never smoked, the largest study of its kind said. Ill effects on non-smokers of constant exposure to tobacco smoke include an increased risk of lung cancer, diabetes, cardiovascular disease, stroke and death, earlier research has shown. As for the impact on brain function, active smoking has been found to impair the mind but the evidence for passive smoking has until now been sketchy.

Using new methods in the largest clinical trial to date, a team led by Cambridge University professor David Llewellyn found that even people who had never smoked but kept constant company with smokers performed less well in cognitive tests. The investigation focused on nearly 5000 adults over the age of 50 who were former smokers or who had never smoked.

The volunteers were divided into four groups according to their exposure to passive smoking. This was determined by saliva samples, which were tested for a by-product of nicotine called cotinine. Cotinine lingers in the saliva for about 25 days. The higher the levels of cotinine, the higher the exposure to recent second-hand smoke.

The volunteers then took neuro-psychological tests that assessed brain function and cognitive abilities, focusing on memory and the ability to work with numbers and words. Using the lowest cotinine group as a benchmark, the researchers found a clear and progressively stronger link between impairment in brain function and exposure to second-hand smoke. In the most-exposed group, the risk of cognitive impairment was 44 per cent higher than the benchmark group.

Factors such as age and medical condition, including a history of heart disease, that could have skewed the outcome were all taken into account. “A similar pattern of associations was observed for never smokers and former smokers,” said the study, published in the British Medical Journal (BMJ). “Given the ongoing international policy debate on exposure to second-hand smoke, this is a topic of major public health significance.”

Governments in North America, Australia and Europe have progressively enacted “smoke-free” legislation for the workplaces, bars, restaurants and other public places over the last 15 years.


The original academic journal article is: Llewellyn, D.J. et al. (2009) “Exposure to secondhand smoke and cognitive impairment in non-smokers: national cross sectional study with cotinine measurement” BMJ 338:b462

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