The present study is the first prospective study addressing use of incense and cancer risk, and the results indicate an association between long-term incense use and development of squamous cell carcinomas of the respiratory tract.
Traditional incense burning produces a considerable amount of smoke, and indoor concentrations of particulate matter during incense burning have been found to far exceed outdoor air quality standards (28
). The smoke contains a multitude of harmful constituents, including polycyclic aromatic hydrocarbons (PAHs), benzene and carbonyls, and incense is found to be the major contributor to PAHs in Hong Kong homes (29
). While the carcinogenic properties of incense smoke is well-established (7
), no epidemiological studies have so far succeeded in establishing an association between use of incense and risk of cancer in the respiratory tract. Upper respiratory tract cancers have not been studied, but a number of case-control studies in Chinese populations investigating the effect of incense burning on the risk of lung cancer have produced conflicting results. Some studies found that incense burning was associated with an increased risk of lung cancer in women (3
), while other studies either have reported lack of an overall association or an inverse association between incense burning and adenocarcinoma of the lung (17
We found the influence of incense on the risk of cancer to be dependent on the histological subtype, and that use of incense seemed to increase the risk of squamous cell carcinomas of the respiratory tract in a dose-dependent manner. The influence was most pronounced for carcinomas of the upper respiratory tract, which is consistent with studies showing that particulate matter from incense burning to a large degree deposits in the upper airways (30
). Incense use not only seem to add to the increased risk of upper respiratory tract squamous cell carcinoma in smokers, but the considerably increased risk in never smokers points to an independent effect of incense smoke.
Nasopharyngeal carcinoma (NPC) is rare in most populations, but the undifferentiated type is prevalent in areas of South East Asia, North Africa and among Inuit in the Arctic (23
). The pathogenesis of NPC differ substantially from that of other pharyngeal carcinomas, and undifferentiated NPC is believed to be the result of environmental factors, among these Epstein-Barr virus and preserved foods, acting on genetically susceptible individuals (31
). A single study have found a higher risk of NPC among individuals with a burning alter at home (32
), while other studies have found no association between incense exposure and risk of NPC (33
). In contrast to the noticeable effect on non-NPC upper respiratory tract carcinomas, use of incense did not influence the risk of NPC in our study. This finding is consistent with the overall evidence based on a large number of studies that undifferentiated NPC is only marginally affected by smoking, and not affected at all by environmental tobacco smoke (23
We also did not observe any influence of incense on the overall risk of lung carcinoma. This may represent a true lack of influence of incense smoke on the risk of lung cancer, However, when stratified by histological subtypes, incense was associated with risk of squamous cell carcinomas of the lung, but not with risk of non-squamous cell carcinomas (majority of them are adenocarcinoma). The association with squamous cell carcinomas of the lung is consistent with the observed association between incense and non-NPC carcinomas in the upper respiratory tract, the majority of which are squamous cell carcinomas. The reason for a possible differential association between incense and histological subtype is unknown. Tobacco-specific nitrosamines have been found to induce mainly adenomas and adenocarcinomas in rodents, whereas polycyclic aromatic hydrocarbons (PAH) such as benzopyrene primarily induce squamous cell carcinoma (37
). While incense is known to be a significant contributor of PAH, the amount of nitrosamines in incense smoke needs to be determined.
The risk of squamous cell carcinomas associated with incense use was higher among women compared to men, which is consistent with Chinese women spending more time at home. Exposure to oil vapors from high temperature wok cooking has been postulated to be a risk factor for lung cancer in Chinese women, especially among never smokers (38
). We did not specifically ask women about their cooking practices and frequencies of wok frying during the baseline interview. Frequency of consuming deep- or stir-fried foods at home could be viewed as a crude surrogate marker for exposure to cooking fumes in women of our study cohort. We did not observe any association between consumption of deep- or stir-fry foods and risk of respiratory tract cancer by subsite, gender or histological subtype. Coal burning is another indoor air pollutant linked to risk of lung cancer in Chinese (38
). This latter exposure is irrelevant in Singapore, a city-state one degree north of the equator.
Environmental tobacco smoke (ETS) has been associated with an increased risk of lung cancer (39
), but information on baseline ETS exposure is unavailable in our cohort study. Thus, there is a legitimate concern that ETS may influence our observed incense-cancer associations. We think this is an unlikely scenario, given that ETS has been shown to exert comparable effects on non-squamous cell carcinomas and squamous cell carcinomas of the lung (39
). It is interesting to note that incense burning can potentially generate much higher quantities of particulate matter than side-stream cigarette smoke (40
The importance of incense use as a risk factor for respiratory tract carcinomas extends beyond the Chinese populations. Incense is used on a daily basis in both temples and homes in many non-Chinese, Asian communities, including those in Southeast Asia and the Indian subcontinent. Regular use also has been reported in the West. Among non-Asian, minority women in New York, 28% reported burning incense during pregnancy, which was identified as a significant source of exposure to polyaromatic hydrocarbons in this group (41
). The present study suggests that incense use was associated with one-third of upper respiratory tract cancers other than NPC occurring among Singapore Chinese. The cancer burden associated with incense burning may be more significant in other Asian populations with a higher underlying rate of non-NPC pharyngeal cancer, such as Asian Indians whose rate is 3-4 times higher than the Singapore Chinese (21
One limitation of our study is the relatively low proportion of individuals that had never been exposed to incense. Nonetheless, we were able to observe a dose-dependent, statistically significant association between incense exposure and upper respiratory tract carcinomas. Another limitation of our study is that we measured subject's use of incense with history of use at baseline only, which could result in misclassification of subjects in term of incense use over lifetime. However, given the prospective nature of the study, such misclassification, if ever occurred, would be more likely to be non-differential, which could lead to attenuating, but rather than creating, the observed positive association between incense use and cancer risk. Moreover, it would be ever more difficult to imagine such misclassification would resulted in a spurious relation for incense use with squamous cell carcinomas but not with adenocarcinoma.
Despite these limitations, the present study represents the first prospective investigation on incense and cancer risk, and results indicate that incense use is associated with increased risk of squamous cell carcinomas in the respiratory tract, especially in the upper part. 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 of burning incense, these findings carry significant public health implications. Besides initiatives to reduce incense smoke exposure, future studies should be undertaken to identify the least harmful types of incense.