Is Chinese Incense Smoke Hazardous to Respiratory Health?

Abstract

Incense is a common source of indoor air pollution, especially in Asian homes where it is burned for religious reasons. In previous studies in Hong Kong, it was found to be the major source of airborne carcinogens in the home, and a significant contributor to personal exposures to nitrogen dioxide among women. To evaluate its effects on respiratory health, data from an air pollution cross-sectional study of 346 primary school children and their 293 non-smoking mothers, and a lung cancer case-control study of 189 female patients and 197 district matched controls who had ever been married were analysed. No association was found between exposure to incense burning and respiratory symptoms like chronic cough, chronic sputum, chronic bronchitis, runny nose, wheezing, asthma, allergic rhinitis, or pneumonia among the three populations studied; i.e. primary school children, their non-smoking mothers, or a group of older non-smoking female controls. Incense burning did not affect lung cancer risk among non-smokers, but it significantly reduced risk among smokers, even after adjusting for lifetime smoking amount. A possible explanation for this unexpected finding is that incense burning was associated with certain dietary habits, i.e. more fresh fish, more retinol, and less alcohol, which have been associated with lower lung cancer risk in this population. These results indicate that diet can be a significant confounder of epidemiological studies on air pollution and respiratory health.