Passive smoking and heart disease

Evidence of a link between passive smoking and heart disease began to be established in the mid 1980’s. The first qualitative reviews were included in the Report of the US Surgeon General, 1986 and the report of the US National Research Council, 1986. Both reviews concluded that an association between ETS and coronary heart disease (CHD) was biologically plausible but the epidemiological evidence was inconclusive.



Studies by Glantz and Parmley[13] [14] in the early 1990s estimated that heart disease caused by passive smoking was the third leading preventable cause of death in the United States, ranking behind active smoking and alcohol abuse, and that non-smokers living with smokers had an increased risk of heart disease of around 30%.



Analysis of a large sample in the United States also showed an elevated heart disease risk of around 20%[15]. Given how widespread heart disease is in non-smokers, a 20% additional risk is very significant. The authors concluded:



If true, ETS might account for an estimated 35 000 to 40 000 heart disease deaths per year in the United States.



Since then, studies have shown conclusively that not only does exposure to ETS increase the risk of heart disease in non-smokers but that the risks are non-linear. It would appear that even a small exposure to tobacco has a large effect on heart disease, with further exposure having a relatively small additional effect. This may be explained by the fact that exposure to ETS causes the blood to thicken - a phenomenon known as platelet aggregation. New research has shown that even half an hour’s exposure to environmental tobacco smoke by non-smokers is enough to adversely affect cells lining the coronary arteries. The dysfunction of these endothelial cells contributes towards the narrowing of arteries and a reduction in blood flow. [16]



Unlike the risk for lung cancer, where the risk is roughly in proportion to smoke exposure, passive smokers’ risk of heart disease may be as much as half that of someone smoking 20 cigarettes a day even though they only inhale about 1% of the smoke.



A review of 19 published studies of the risk of heart disease by Law et al[17] found that non-smokers have an overall 23 per cent increased risk of heart disease when living with a smoker, after adjusting for confounding factors such as diet. The authors also found that the immediate effect of a single environmental exposure was to increase risk by an estimated 34%. This compares with a risk of 39% from smoking one cigarette per day.



In a study by He et al[18] the authors reviewed 18 epidemiological studies and found that, overall, nonsmokers exposed to environmental tobacco smoke had a relative risk of coronary heart disease of 1.25 (ie a 25 per cent increased risk compared with nonsmokers not exposed). The relative risk for men was 1.22 and women 1.24. Non-smokers exposed to tobacco smoke at home had an overall risk of 1.17, while at work the risk was found to be 1.11.



While the risk of heart disease in non-smokers exposed to ETS is proportionally large, it would appear that some of the early damage to arteries caused by smoking may be reversible in healthy adults if further tobacco smoke exposure is avoided for at least a year.[19] The study by Raitakari et al in Australia found that most improvement in the former passive smokers was evident after 2 years of cessation of passive smoking.


Other circulatory diseases

Research in New Zealand by Bonita et al revealed that passive smoking as well as active smoking increases the risk of stroke.[20] The study found passive smoking exposure increased the risk of stroke in non-smokers by 82% ( odds ratio = 1.82; 95% confidence interval = 1.34-2.49). The risk was significant in men (OR = 2.10; 95% CI 1.33-3.32) and in women (OR = 1.66; 95% CI: 1.07-2.57). By comparison, active smokers had a fourfold risk of stroke compared with people who had never smoked or had stopped smoking more than 10 years earlier and who were not exposed to ETS (OR = 4.14; 95% CI 3.04-6.63.) Given that stroke is a common condition, this means that passive smoking is having a serious health impact on non-smokers.