Long-term exposure to burning incense could put people at risk of cancers of the respiratory tract, according to a new study published in the medical journal Cancer.

In a study of more than 61,000 ethnic Chinese ages 45 to 74 living in Singapore who were followed for up to 12 years, the researchers found that the practice of burning incense almost doubled the risk of developing squamous cell upper respiratory tract carcinomas including tongue, mouth, nasal/sinus and laryngeal.

They were all cancer-free when they were selected and they had to answer several question about their diet, lifestyle habits and to give details about their exposure to incense. They were asked how often they burned incense in their homes and for how long, and were asked details about their favourite moment – at night, all day and night – for this activity. Close to 75 percent of the participants were frequent incense users. Over the course of the study, 325 upper respiratory tract cancers and 821 lung cancers were reported. The incense use was associated with a significant higher risk of cancers of the upper respiratory tract.

Burning incense can produce potentially cancer-causing substances, including benzene and polyaromatic hydrocarbons.

“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 increased the risk of cancer in both smokers and nonsmokers. Investigators said further studies are needed to see whether different types of incense are associated with different degrees of cancer risks.