Inhaling incense fumes over long periods increases the risk of developing cancers of the respiratory tract, a finding that applies to Asian populations worldwide, researchers warn.

Burning incense — a mix of plant materials and oils — is an integral part of daily life in large parts of Asia, as well as in North Africa and among Inuit populations.

Incense releases large amounts of smoke containing particulate matter that gets caught in the lungs, as well as possible carcinogens such as polyaromatic hydrocarbons (PAHs), carbonyls and benzene.

Indoor concentrations of particulate matter from burning incense has been found to far exceed outdoor air quality standards, and can potentially produce more particulate matter than second-hand tobacco smoke, previous studies have shown.

Burning incense almost doubled the risk of squamous cell carcinomas in the upper respiratory tract, such as the nose and sinuses, tongue, mouth and larynx, said the researchers led by Dr. Jeppe Friborg of the epidemiology department at Statens Serum Institut in Copenhagen, Denmark.

Their findings are published in the Oct. 1 issue of the journal Cancer.

"Given the widespread and sometimes involuntary exposure to smoke of burning incense, these findings carry significant public health implications," the study's authors concluded.

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

While a cause-and-effect relationship could not be established in this type of study, experts suggest burning incense less often and improving ventilation to minimize the long-term risks.

The risk increased in both smokers and non-smokers, the study of more than 61,000 Singaporean Chinese found. The intensity and duration of incense use were also both linked to the levels of risk, Friborg's team found.

The overall risk of lung cancer did not appear to increase with incense use, but the study did suggest a link to increased risk of a specific type of disease, squamous cell carcinoma of the lung.

Participants were aged 45 to 74 and were free of cancer when they were first interviewed in 1993-1998. They were followed until 2005.

Over that time, a total of 325 upper respiratory tract cancers (including nasal/sinus, tongue, mouth, laryngeal and other cancers) and 821 lung cancers were found.