Interestingly, the practice did not increase the overall risk of lung cancer.
"Given that our results are backed by numerous experimental studies showing that incense is a powerful producer of particulate matter and that incense smoke contains carcinogenic substances, I believe incense should be used with caution," said study author Dr. Jeppe Friborg, of the department of epidemiology research at Statens Serum Institute in Copenhagen, Denmark. "That is, frequent use in rooms where people live should be minimized, or at least sufficient ventilation should be secured. In our study, we find the increased risk of cancer to be present in individuals reporting frequent use of incense for many years, thus, repeated exposure for years should probably be avoided."
Others echoed the thought. "The American Lung Association is going to add it as a risk factor," said Dr. Norman Edelman, chief medical officer of the association. "It's not nearly the danger of smoking a pack a day for 20 years, but it's a danger."
Not only is incense burned regularly as part of daily life in large swaths of Asia, the practice is also popular among certain segments in the West.
Incense burning produces particulate matter and is known to contain possible carcinogens such as polyaromatic hyodrcarbons (PAHs), carbonyls and benzene.
There have also been reports linking the burning of incense with cancer but the results have been inconsistent.
For this study, researchers conducted face-to-face interviews with more than 61,000 Singapore Chinese aged 45 to 74 who were cancer-free at the beginning of the study.
Incense burning almost doubled the risk of developing squamous cell upper respiratory tract carcinomas including nasal/sinus, tongue, mouth, and laryngeal. There was an increased risk both in smokers and in nonsmokers, pointing to an independent effect of incense smoke.
There was no overall increased risk of lung cancer, but it did heighten the risk of squamous cell carcinoma of the lung.
Will incense go the way of tobacco? Not necessarily, said some experts.
The findings, which are in the Oct. 1 issue of Cancer, might also point researchers toward other household practices that should be investigated.
"It's a population-based study, which means that you can make an association but not necessarily a conclusion," said Dr. Erin Fleener, a clinical assistant professor in internal medicine at the Texas A&M Health Science Center College of Medicine and an oncologist at the Bryan-College Station Cancer Clinic. "It probably promotes more work in the area of routine household items and things we need to be looking at more prospectively to make a clear cause-and-effect relationship."
In general, though, it's not a bad idea to avoid environmental pollutants of various types.
"Anything that affects air quality negatively is not a good thing," said Dr. Len Horvitz, a pulmonary specialist at Lenox Hill Hospital in New York City. "Burning in general and the release of smoke, these things are certainly to be avoided. At the very least, chemical irritants will set off asthma, and that's reversible. Cancer is not reversible."
"This is not unlike the type of risk that one experiences from secondhand tobacco smoke," said Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society. "At the end of the day, people who use incense casually, I don't think that's a cause for major concern, but those cultures which embrace incense as part of their daily lifestyles have to consider this has a real potential risk for cancer."
Visit the National Cancer Institute for more on respiratory cancers.