MINNEAPOLIS, Aug. 25 -- Long-term exposure to incense may increase the risk of respiratory-tract cancers, according to data from a large Chinese cohort study. Incense users had an 80% increased risk of squamous-cell respiratory-tract cancers compared with nonusers, Jeppe T. Friborg, M.D., Ph.D., of Statens Serum Institut in Copenhagen, Denmark, and formerly of the University of Minnesota, and colleagues reported in the October issue of Cancer.
The risk increased with intensity and duration of exposure to incense and was greatest in the upper respiratory tract, the researchers said.
"This association is consistent with a large number of studies identifying carcinogens in incense smoke, and given the widespread and sometimes involuntary exposure to smoke from burning incense, these findings carry significant public health implications," they added.
Incense smoke has long been known to contain multiple potentially carcinogenic substances, including polyaromatic hydrocarbons, carbonyls, and benzene. Additionally, incense smoke condensates have been found to have mutagenic and genotoxic activity, some of which exceeds that of tobacco smoke in mammalian cells, the authors said.
Incense use in the home has been associated with childhood leukemia and brain tumors. However, studies of potential associations between the smoke and lung cancer have yielded inconsistent results, the authors continued.
So, to examine potential associations between incense smoke and respiratory-tract cancers, investigators reviewed data from a population-based cohort study of 61,320 Chinese residents of Singapore. From 1993 through 1998, each completed a comprehensive interview regarding living conditions and dietary and lifestyle factors.
The study participants were ages 45 to 74 when interviewed, and all were cancer free. About three-fourths of participants were current users of incense.
Among current users, 92.7% used incense daily and 83.9% had used incense for more than 40 years.
During follow-up through 2005, 325 upper respiratory tract cancers and 821 lung cancers were diagnosed in the cohort.
Nasopharyngeal cancers accounted for more than half of all upper respiratory-tract cancers (175 of 325).
Nasopharyngeal cancers were predominately undifferentiated carcinomas (89%), whereas squamous histology predominated in the non-nasopharyngeal cancers (88%).
Adenocarcinoma accounted for 42% of lung cancers, followed by squamous-cell carcinomas (24%).
Exposure to incense smoke was associated with a significantly increased risk of non-nasopharyngeal upper respiratory-tract cancers (P=0.04).
Among current users, the hazard ratios were:
1.4 for less than daily use
1.3 for daily use for 40 years or less
1.7 for daily use for 41+ years
Nightly or intermittent use of incense increased the risk of non-nasopharyngeal cancers by 50% compared with noncurrent users, and daily or continuous use increased the risk by more than two-fold (P=0.02 for trend).
Duration and intensity of incense use combined significantly increased the risk of squamous-cell carcinomas throughout the respiratory tract (HR 1.8, P=0.004).
Incense use did not affect the risk of nasopharyngeal cancers or the risk of lung cancer.
Limitations of the study included the small size of the group that had never been exposed to incense and the fact that incense use was based on a questionnaire at the baseline evaluation, which did not ensure accuracy of lifetime exposure.
Source: Friborg JT, et al "Incense use and respiratory tract carcinomas. A prospective cohort study" Cancer 2008; DOI: 10.1002/cncr.23788.
