NEW YORK (Reuters Health) - Burning incense may create a sweet scent, but regularly inhaling the smoke could put people at risk of cancers of the respiratory tract, researchers reported Monday.

A man prays with incense sticks at the more than 300-year-old Yong He Gong, also known as the Tibetan Lama Temple, in Beijing July 7, 2008. REUTERS/David Gray

In a study of more than 61,000 ethnic Chinese living in Singapore who were followed for up to 12 years, the investigators found a link between heavy incense use and various respiratory cancers.

The findings are published in the medical journal Cancer.

Incense has been used for millennia in many cultures’ religious and spiritual ceremonies. In Asia, people commonly burn incense in their homes -- a practice that is becoming more popular in Western countries as well.

Incense is usually derived from fragrant plant materials, like tree bark, resins, roots, flowers and essential oils. Past research has found that burning these materials can produce potentially cancer-causing substances, including benzene and polyaromatic hydrocarbons.

However, no studies until now had linked the practice of burning incense to an increased cancer risk over time, according to the researchers, led by Dr. Jeppe T. Friborg of the Statens Serum Institute in Copenhagen.

For their study, the researchers followed 61,320 Singapore Chinese men and women between the ages of 45 and 74 from the Hokkien or Cantonese dialect group. All of the subjects were cancer-free at the outset.

Participants reported on their typical incense use, including how often they burned it in their homes and for how long -- only at night, for instance, or all day and night.

Over the next 12 years, 325 men and women developed cancer of the upper respiratory tract, such as nasal, oral or throat cancer. Another 821 developed lung cancer.

The researchers found that incense use was associated with a statistically significant higher risk of cancers of the upper respiratory tract, with the exception of nasopharyngeal cancer. However, they observed no overall effect on lung cancer risk.

Those who used incense heavily also had higher rates of a type of cancer called squamous cell carcinoma, which refers to tumors that arise in the cells lining the internal and external surfaces of the body. The risk was seen in smokers and nonsmokers.

Study participants who used incense in their homes all day or throughout the day and night were 80 percent more likely than non-users to develop squamous cell carcinoma of the entire respiratory tract.

The link between incense use and increased cancer risk held when the researchers weighed other factors, including cigarette smoking, diet and drinking habits.

“This association is consistent with a large number of studies identifying carcinogens in incense smoke,” Friborg’s team writes, “and given the widespread and sometimes involuntary exposure to smoke from burning incense, these findings carry significant public health implications.”

They say further studies are needed to see whether different types of incense are associated with different degrees of cancer risk. In Singapore, the researchers note, most people burn long sticks or coils of incense that burn slowly over an extended period.

SOURCE: Cancer, October 1, 2008.