The analysis, which the authors say is the first prospective investigation of incense and cancer risk, appears in the October 1, 2008 issue of CANCER, a peer-reviewed journal of the American Cancer Society.
Researchers have shown that burning incense-which is made of plant materials mixed with oils-produces a mixture of possible carcinogens, including polyaromatic hydrocarbons, carbonyls and benzene.
Because incense smoke is inhaled, a number of studies have looked at the possible link between incense burning and lung cancer, but results have been inconsistent. In addition, the possible association of incense use and other respiratory tract cancers has not been analyzed.
To investigate this, Dr. Jeppe Friborg of the Statens Serum Institut in Copenhagen, Denmark and colleagues in Singapore and the U.S. studied the associations between exposure to incense and the whole spectrum of respiratory tract cancers in a large population in Singapore.
The study involved 61,320 Singapore Chinese who were free of cancer and aged 45-74 years in 1993-1998. At that time, they completed a comprehensive interview on living conditions and dietary and lifestyle factors.
The investigators followed these individuals through 2005, noting which participants developed cancer during that time.
Dr. Friborg's team documented a total of 325 upper respiratory tract cancers (including nasal/sinus, tongue, mouth, laryngeal and other cancers) and 821 lung cancers during follow-up.
Incense use was associated with a significantly increased risk of upper respiratory tract cancer (other than nasopharyngeal), but there was no overall effect on lung cancer.
The researchers also noted that the duration and intensity of incense use were associated with an increased risk of squamous cell carcinomas in the entire respiratory tract.
Squamous cells cover the internal and external surfaces of the body.
According to the study data, incense use seemed to add to the increased risk of upper respiratory tract squamous cell carcinoma in smokers. It also considerably increased the risk in never smokers, which points to an independent effect of incense smoke.
"Given the widespread and sometimes involuntary exposure to smoke of burning incense, these findings carry significant public health implications," the researchers said.
"Besides initiatives to reduce incense smoke exposure, future studies should be undertaken to identify the least harmful types of incense," they added.