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Passive smoking and the risk of coronary heart disease


Active cigarette smoking is one of the most important modifiable risk factors for coronary heart disease. What about passive smoking? This meta-analysis assesses the relationship between passive smoking and the risk of coronary heart disease among non-smokers.

Message

Overall, non-smokers exposed to passive smoking for a number of years have a 25% increased risk of coronary heart disease. Exposure to smoke at home is more dangerous than exposure at work.

Reference

J He et al. Passive smoking and the risk of coronary heart disease - a meta-analysis of epidemiologic studies. The New England Journal of Medicine 1999 340: 920-926.

Search

MEDLINE and Dissertation Abstracts Online databases were searched from 1966 to 1998. Bibliographies of identified articles were then reviewed. To be included papers had to be prospective cohort or case-control studies. Potential papers were independently reviewed by three investigators. Ten cohort and eight case-control studies were identified.

The majority of the cohort studies were very large, with participant numbers ranging from 513 to 479,680 (including both men and women). Eight were conducted in the United States. Participants were followed for a period of between 6 and 20 years and the outcome was myocardial infarction or death due to coronary heart disease. All studies adjusted for age and gender. Six adjusted for blood pressure/hypertension, body weight/body mass index and serum cholesterol/hyperlipidemia. Some studies also adjusted for a variety of other factors, e.g. exercise and alcohol consumption

The case-control studies were conduced in various countries. The number of patients ranged from 34 to 343 (with myocardial infarction or coronary heart disease) and the corresponding number of comparisons ranged from 68 to 825 (selected from the same hospital or community). All studies adjusted for age and gender and a variety of other risk factors.

All studies measured exposure to smoke at home, with four cohort and four case-control studies also measuring workplace exposure.

Results

Non-smokers exposed to passive smoking had an overall increased risk of coronary heart disease of 25% compared with non-smokers not exposed to smoke (relative risk 1.25, 95% confidence interval 1.17 to 1.32).

The analysis was repeated several times to exclude studies with different characteristics. The overall result hardly changed. For example, when the analysis was confined to studies that had adjusted for some of the risk factors for coronary heart disease, such as age, gender, blood pressure, body weight and serum cholesterol the relative risk was 1.26 (95% confidence interval 1.16 to 1.38). For men the relative risk was 1.22 (95% confidence interval 1.10 to 1.35) and for women, 1.24 (95% confidence interval 1.15 to 1.34).

The risk of coronary heart disease increased with exposure to a higher level or a longer duration of passive smoking.

Level of exposure

Compared with non-smokers not exposed to smoke, non-smokers exposed to 1 to 19 cigarettes per day had an increased risk of coronary heart disease of 23%, while those exposed to 20 or more cigarettes per day had an increased risk of 31% (relative risks and 95% confidence intervals 1.23, 1.13 to 1.34 and 1.31, 1.21 to 1.42, respectively).

Duration of exposure

Risk of coronary heart disease did not increase significantly until non-smokers had been exposed to smoke for 10 years, after which time, further exposure made no difference. Compared with non-smokers not exposed to smoke, non-smokers who were exposed to smoke for between 10 and 19 years and 20 years or more had increased risks of coronary heart disease of 31% and 29% respectively. (Relative risks and 95% confidence intervals: 1 to 9 years, 1.18, 0.98 to 1.42; 10 to 19 years, 1.31, 1.11 to 1.55; 20 years or more, 1.29, 1.16 to 1.43).

Place of exposure

Exposure to passive smoking at home was associated with an increased risk of 17% (relative risk 1.17, 95% confidence interval 1.11 to 1.24) whereas exposure at work was not associated with an increased risk (relative risk 1.11, 95% confidence interval 1.00 to 1.23).

Comment

This meta-analysis was performed carefully and thoroughly with some very large studies. It is also reassuring that the estimate was essentially the same when only studies adjusting for some of the risk factors for coronary heart disease were pooled. However, other lifestyle factors such as diet and exercise will also play a part in an individuals' risk.