Researchers reported Monday they had discovered a direct link between heavy incense use and various respiratory cancers. The findings will be published in the October 1st medical journal Cancer, a journal of the American Cancer Society.

Researchers from Statens Serum Institute in Copenhagen, Denmark conducted interviews with more than 61,000 Singapore Chinese. They were aged between 45 and 74 and were all clinically healthy at the beginning of the study.

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.

Over the course of the study, 325 upper respiratory tract cancers and 821 lung cancers were reported. Burning incense almost doubles the risk of developing squamous cell upper respiratory tract carcinomas such as nasal or sinus, tongue, mouth and laryngeal. There is no connection between incense inhaled into the lungs and lung cancer whatsoever.

"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 the author of the study, Dr. Jeppe Friborg, of the department of epidemiology research at Statens Serum Institute.

Exposure to incense also considerably increased the risk in non smokers, which leads to an independent effect of incense smoke.

"Besides initiatives to reduce incense smoke exposure, future studies should be undertaken to identify the least harmful types of incense," the scientists also explained.