Photograph: Anuruddha Lokuhapuarachchi/ Reuters
Along with the piles of dirty laundry, empty beer cans and Bob Marley posters they are a common feature of many student digs. But scientists have now found that regularly burning joss sticks increases the risk of certain mouth, throat and lung cancers.
The 12-year study backs up previous research showing that incense smoke contains cancer-causing chemicals such as polyaromatic hydrocarbons, carbonyls and benzene, which cause mutations to DNA in human cells. Another study found that the levels of airborne particulates from candles and incense burning in two Dutch churches were 20 times higher than they were next to a typical busy road.
"Incense is sold without a warning label, and given the high prevalence of use and the often involuntary nature of the exposure, clarifying the role of incense smoke as a carcinogen is important from a public health perspective," wrote Dr Jeppe Friborg of the Statens Serum Institute in Copenhagen and his colleagues in the latest issue of the journal Cancer.
The team interviewed over 60,000 ethnic Chinese people in Singapore aged between 45 and 78 between 1993 and 1998. They asked how much they used incense and collected detailed information on their lifestyle including their diet and how much they drank alcohol and smoked. All participants who had previously had cancer were excluded from the analysis.
In December 2005, the team checked the subjects' health by using the detailed records in Singapore's National Cancer Registry. Of the original group, 325 had developed cancer of the upper respiratory tract and 821 had developed lung cancer.
After adjusting for other lifestyle factors that are known to cause cancer, such as smoking, the team found that burning incense was associated with an increase in some types of lung cancer, and cancers of the upper respiratory tract, such as throat and mouth cancer.
The small risk of developing upper respiratory tract cancers nearly doubled in people who used incense regularly. They were also more common among women, which could be explained by the fact that women tend to spend more time in the smoky home environment.
Friborg said that the implications went beyond this particular study group. "It could be relevant for priests and others who are regularly exposed to incense," he said, "[but] I'm not sure if a short term effect would be measurable."
He added that it was too early to consider attaching health warnings to packets of joss sticks or incense. "I would suggest that people use incense with caution," he said. "If this study is confirmed I think regulation could be relevant."