Domestic Incense Burning and the Risk of Nasopharyngeal Carcinoma: A Case-Referent Study among Hong Kong Chinese

Background: The etiology of nasopharyngeal carcinoma (NPC) has not been completely understood. The unique spatial and ethnic clustering and a decrease in incidence since the 1970s-1980s in Chinese populations in Hong Kong, Taiwan, and Singapore suggest the roles of environmental exposures related to traditional lifestyles. Incense burning, a traditional daily practice in Chinese households, is a powerful producer of carcinogens and has been considered a risk factor for NPC.;Methods: A systematic review was performed with a comprehensive literature search, assessment of study quality and statistical syntheses of available evidence. A case-referent study was conducted with 352 incident cases of NPC and 410 referents recruited between June 2010 and December 2012. Information on lifelong exposures to incense smoke and potential confounders was collected via face-to-face interviews. Logistic regressions were performed to estimate the odds ratios (ORs) for NPC associated with incense burning. Multivariate regressions were performed to adjust for the effects of confounders. Interactions of incense burning with living conditions and exposures to other indoor inhalants were evaluated under multiplicative and additive assumptions.;Results: A total of 10 studies, including 1 cohort and 9 case-referent studies, were identified. Results of meta-analyses showed an increased risk of NPC associated with incense burning [pooled OR = 1.44, 95% confidence interval (CI): 1.07, 1.94] in spite of risk of bias in primary studies and possible publication bias.;In the present case-referent study, more cases had daily practice of incense burning during their lifetime than referents (crude OR = 1.41, 95% CI: 1.06, 1.88). However, the association was not statistically significant after adjustment for confounders. Daily burning for over 40 years was associated with an elevated risk of NPC (crude OR = 1.70, 95% CI: 1.15, 2.50) though the association was not significant in multivariate models. A significantly increased risk of NPC associated with incense burning demonstrating exposure-response relationships was observed in women but not in men. Comparing with those without daily burning, women with daily burning for over 40 years were at significantly elevated risk of NPC (adjusted OR = 4.33, 95% CI: 1.81, 10.36). Suggestive interactions of incense burning with ventilation habits and exposures to other indoor inhalants were observed. The adjusted OR for daily burning in poor ventilation conditions was 2.08 (95% CI: 1.02, 4.24), while that in good ventilation conditions was 1.35 (95% CI: 0.92, 1.98). The adjusted OR for combined exposures to incense smoke and at least two of other indoor inhalants was 4.34 (95% CI: 2.04, 9.24).;Conclusions: This study provided suggestive but inconclusive evidence for an overall association between domestic incense burning and NPC risk. Domestic incense burning, especially long-term daily practice may increase the risk of NPC. The observed effect of domestic incense burning on NPC risk was more pronounced in women. Poor ventilation conditions at home and combined exposures to other indoor inhalants may enhance the risk of NPC associated with incense burning. All these findings still need further confirmation in more large-scale studies with refined exposure assessment in the future.