© 2004 by Oxford University Press
© 2004 Oxford University Press
CORRESPONDENCE |
Re: Active Smoking, Household Passive Smoking, and Breast Cancer: Evidence From the California Teachers Study
Correspondence to: Kenneth C. Johnson, PhD, Surveillance and Risk Assessment Division, Centre for Chronic Disease Prevention and Control, Population and Public Health Branch, Health Canada, 120 Colonnade Rd., PL 6702A, Ottawa, ON, Canada K1A 0K9 (e-mail: ken_lcdc_johnson{at}hc-sc.gc.ca)
Reynolds et al. are to be congratulated on a careful and thorough prospective study of active smoking and breast cancer (1). They found higher breast cancer risks among women who smoked for 5 or more years pre-partum (odds ratio [OR] = 1.13, 95% confidence interval [CI] = 1.00 to 1.28) or smoked more than 30 pack-years (OR = 1.25, 95% CI = 1.06 to 1.47). Current research from my group (2) suggests that combining these factors may result in higher risks. It would therefore be useful if the authors could provide breast cancer risk estimates by pack-years and years of smoking, stratified by the number of years of pre-partum smoking. It would also be helpful to present risks for women who smoked but never had a full-term pregnancy, because such women would have had the potentially critical pre-partum exposure for the entire time they smoked.
The analysis of passive smoking is inadequate. Although Reynolds et al. (1) collected detailed lifetime passive smoking measures, the authors note that their published analysis does not include degree of exposure or occupational exposure, being based only on a binary evaluation of household passive smoking exposure (yes/no) and the timing of that exposure (childhood, adult, or both). They report that, in their data, spousal exposure was the primary source of all passive smoking exposures until the 1980s. They imply, because residential sources were the primary source of passive exposure, that their analysis based on nonquantitative, residential-only exposure is likely sound (1). If passive exposure to smoke were rare, affecting only a small percentage of the population, then their argument for the soundness of their analysis would be more compelling. However, with an exposure like passive smoking, which affects a large percentage of the population, exposure misclassification (in this case from ignoring occupational exposure) has the potential to strongly dilute observed risks, primarily because of the degree of contamination of the referent "unexposed" group (3–5).
Although other passive smoking studies have used similarly incomplete measures of passive exposure, studies with the most thorough measures of passive exposure have observed substantially increased breast cancer risk associated with both passive and active smoking (6,7). With less complete passive exposure measures, the risks have varied.
As with the other American cohort study of breast cancer and passive smoking based on a profession (in that case, nursing) (5), it is possible that most of the study subjects in the Reynolds et al. study (1) were regularly exposed as a result of their work. Indeed, it seems likely that historically many teachers would have had long-term, daily, occupational exposure to passive smoking in their teachers’ staff rooms, before smoking restrictions were imposed in the 1980s or early 1990s.
In conclusion, the authors are to be commended for having decided to collect more detailed passive smoking information than originally planned for the study. It would be prudent to wait for the authors’ complete analysis of all sources and of the degree of passive smoke exposure before passing judgment as to what this study has to say about the relationship of passive smoking exposure to breast cancer or the magnitude of the effect of active smoking, after controlling for passive smoking.
REFERENCES
1 Reynolds P, Hurley S, Goldberg DE, Anton-Culver H, Bernstein L, Deapen D, et al. Active smoking, household passive smoking, and breast cancer: evidence from the California Teachers Study. J Natl Cancer Inst 2004;96:29–37.
2 Johnson K, Pan S, Ugnat AM, Mao Y; Canadian Cancer Registries Epidemiology Research Group. Smoking before the first full-term pregnancy: a critical window for breast cancer risk? [Abstract]. Health Canada Research Forum: From Science to Policy, 2003, Oct 20–21. Ottawa, ON Canada: Health Canada; 2003. p. 4.25. Publ No. 1317.
3 Johnson KC. Re: Passive smoking exposure and female breast cancer mortality. J Natl Cancer Inst 2001;93:719–20.
4 Rothman KJ, Greenland S. Modern epidemiology. 2nd ed. Philadelphia (PA): Lippincott-Raven; 1998. p. 127–30.
5 Johnson KC, Wells AJ. Active and passive smoking in breast cancer. Epidemiology 2002;13:745–6.[CrossRef][ISI][Medline]
6 Morabia A, Bernstein M, Heritier S, Khatchatrian N. Relation of breast cancer with passive and active exposure to tobacco smoke. Am J Epidemiol 1996;143:918–28.
7 Johnson KC, Hu J, Mao Y. Passive and active smoking and breast cancer risk in Canada, 1994–97. The Canadian Cancer Registries Epidemiology Research Group. Cancer Causes Control 2000;11:211–21.[CrossRef][ISI][Medline]
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J Natl Cancer Inst 2004 96: 1042-1043.
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