Secondhand Smoke May Increase Risk of COPD

Key Point

Passive smoking could increase the risk of COPD and respiratory symptoms.

Although active smoking has been well established as the predominant risk factor for COPD, passive smoking has so far only been implicated as a potential COPD risk factor. A study published in the September 1 Lancet has found a significant association between passive smoking during adulthood and prevalence of COPD and respiratory symptoms.

“Previous studies have shown mixed results [regarding the link between passive smoking and COPD], and most have examined lung function rather than COPD as the outcome,” Peng Yin, MSc, of the University of Birmingham in the United Kingdom, and colleagues stated. “In this study, older adults who were exposed to [environmental tobacco smoke] at work and at home, particularly higher level exposure, [had a] higher prevalence of COPD than those not exposed.”

INCREASED COPD RISK

The investigators used baseline data of 20,430 Chinese men and women older than 50 who are participating in the ongoing Guangzhou Biobank Cohort Study. Participants answered questions in an interview that assessed active and passive smoking at home or at work. A total of 15,379 people who had never smoked were included for analysis; 6,497 of the group had valid spirometry.

Of the participants with valid spirometry, 13.7% reported more than five years of passive smoking during 40 hours a week at work, while 14.5% reported the same amount of exposure at home. Significant but weak associations were found between dust and indoor pollutant exposure and COPD risk. When measured by the number of smokers present while indoors, there was no significant association between COPD risk and passive smoking during childhood or adulthood.

However, when measured by the duration of passive smoking, there was a significantly increased risk of COPD during adulthood. “This finding might mean that duration of exposure is a more accurate indicator of true total exposure,” the research team noted.
COPD prevalence among participants who had never smoked and who had high-level exposure to environmental tobacco smoke was significantly greater than the prevalence among people with low-level exposure (odds ratio for home exposure, 1.6; for work exposure, 1.5). Increased COPD risk remained among those who had high exposure to environmental tobacco smoke at home and at work (odds ratio, 1.48) for the longest duration of total adulthood exposure.

RESPIRATORY REVIEWS

Any reported respiratory symptom—such as cough and shortness of breath—and passive smoking were significantly associated with each other. The prevalence of any respiratory symptom rose from 43.3% for no childhood exposure in the home to 46.9% for childhood exposure to two or more smokers in the household. Similarly, the prevalence of any respiratory symptoms rose from 42.3% for no exposure in the home during adulthood to 48.7% for adulthood exposure to two or more smokers in the household.

Less than two years of passive smoking at home 40 hours per week among adults was linked to a 43.1% prevalence of any respiratory symptoms, which rose to 46.8% when exposure lasted between two and five years, and to 47.6% when passive smoking lasted more than five years. Furthermore, shortness of breath was significantly associated with passive smoking during adulthood, the investigators noted.

POTENTIAL MECHANISMS

“Active smoking causes COPD,” K. K. Cheng, PhD, Professor of Epidemiology at the University of Birmingham and one of the investigators in the study, told Pulmonary Reviews. “Since people engaged in passive smoking also inhale a large amount of toxic agents, the association is therefore entirely biologically plausible. Some potential mechanisms include lung injury arising from imbalance of proteases and antiproteases and tissue damage by oxidative stress. The processes contributing to obstruction in the small conducting airways include disruption of the epithelial barrier, infiltration of the airway walls by inflammatory cells, and deposition of connective tissue in the airway walls.”

Dr. Cheng emphasized the “crucial” need to educate smokers about the risks of smoking, so that passive smoking is also reduced. He noted that 57.7% of people in the present study who never smoked reported exposure to environmental tobacco smoke at home. “Although a number of potentially important new treatments for COPD are in clinical development, no currently available treatments have been shown to slow the progression of COPD,” Dr. Cheng said. “Smoking cessation is by far the most important method to prevent COPD.”           

—John Merriman

Suggested Reading
Yin P, Jiang CQ, Cheng KK, et al. Passive smoking exposure and risk of COPD among adults in China: the Guangzhou Biobank Cohort Study. Lancet. 2007;370(9589):751-757.

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