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 histologic subtypes, incense was associated with
the risk of squamous cell carcinoma of the lung, but
not with the risk of nonsquamous cell carcinoma
(the majority of which are adenocarcinoma). The
association with squamous cell carcinoma 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 histologic subtype is unknown. Tobacco-specific nitrosamines have been found to induce mainly adenomas
and adenocarcinomas in rodents, whereas PAHs such
as benzopyrene primarily induce squamous cell carcinoma.
37
Although incense is known to be a significant contributor of PAH, the amount of nitrosamines
in incense smoke needs to be determined.