"Incense may need a health warning over 'toxic' smoke, claims research," The Daily Telegraph reports. Analysis of incense smoke, used in both western and Asian religious ceremonies for possibly thousands of years, found it contains many chemicals, some of which may be harmful.

The researchers – two of whom worked for a tobacco company – tested the residue of tobacco and incense smoke directly on animal and bacteria cells in a laboratory. They did this to see whether they could induce mutations in the DNA and if the smoke was toxic to the cells.

They found the effect of some of the incense smoke tested on the cells was greater than that of the tobacco smoke. However, only four incense sticks and one cigarette were tested, so we have to be cautious about these results.

But incense isn't smoked and so is not drawn directly into the lungs in the way tobacco smoke is, so the effects on lung cells may be very different. It is also unclear how exposure to incense smoke compares to the health risks associated with passive smoking.

Still, the study is a reminder that burning anything – whether it's incense, coal or tobacco – produces smoke that can irritate and damage the lungs.

Where did the story come from?

The study was carried out by researchers from the South China University of Technology and the China Tobacco Guangdong Industrial Company.

No information was given about funding. However, the lead researcher worked for the tobacco company, which raises questions about the impartiality of the research.

The study was published in the peer-reviewed science journal Environmental Chemistry Letters, and is available on an open access basis to read online or download as a PDF file.

It was covered cautiously by the Mail Online and The Daily Telegraph, both of which included warnings about the study's links to the tobacco industry.

What kind of research was this?

This laboratory research used instruments to measure and identify the types of particles and chemicals given off by burning incense.

After measuring the chemicals, the researchers did in vitro studies of the effects of the smoke on bacteria and animal cells.

What did the research involve?

The researchers burned four incense sticks and one cigarette in a machine that collected particles of smoke through a series of filters. They graded the size of the particles collected, and performed chemical analysis by gas chromatography and mass spectrometry on the contents of the filters. They then tested the smoke residues on cells in petri dishes.

The first test, on salmonella cells, was to see whether the samples prompted mutations in the DNA of the cells. Mutations in DNA can sometimes lead to cancer. The second test used cells from the ovaries of Chinese hamsters to see whether the samples had toxic effects on the cells.

What were the basic results?

Smoke from burning incense created a mixture of fine and ultrafine particles, which are known to be bad for lung health. The chemical analysis found 64 compounds, taking into account all the components of all four incense sticks.

These included chemical components of essential oils and lignin wood, which is commonly used in incense. The compounds were mostly "irritants", although some toxic compounds were found. The paper did not give the equivalent results on particle size and chemical compounds found in the cigarette tested.

The four incense smoke samples and one cigarette smoke sample caused varying degrees of mutation in the salmonella cells. The incense and cigarette smoke was toxic for the hamster ovary cells. Toxicity was maintained at all different levels for the different samples. The incense smoke was toxic at lower concentrations than the cigarette smoke.

How did the researchers interpret the results?

The researchers showed smoke from some incense samples was "higher than for the reference cigarette sample with the same dose", and said their findings suggest that, "incense smoke was more cytotoxic against Chinese hamster ovary cells" than cigarette smoke.

However, they added: "We cannot simply conclude that cigarette smoke is less cytotoxic than incense smoke, firstly because of the small sample size analysed in this study, and secondly because of huge variability in the consumption of incense sticks and cigarettes."

Conclusion

This laboratory study found smoke from burning incense can produce fine particles and chemical compounds of a type that may irritate the lungs and damage health. This is not surprising, as most types of smoke indoors produces fine particles that are likely to have this effect, whether from cooking, smoking tobacco, or burning incense.

The suggestion that incense smoke might be more harmful than cigarette smoke needs to be treated with caution. The four incense stick samples had different effects when tested for the ability to mutate cell DNA and toxicity to cells. These were compared with just one cigarette.

This means we cannot draw conclusions about whether most incense sticks produce smoke that is more or less toxic than most cigarettes. Also, research using animal cells in the laboratory is not the same as research on living humans. Adding substances to cells in a petri dish can cause very different effects from what happens when people come across these substances in a dilute form in the environment.

The way we use incense and tobacco is different. Cigarette smoke is drawn directly into the lungs and held there before being exhaled. Incense smoke is burned into the environment and inhaled from the surrounding air. The amount of smoke that gets into the lungs will depend on how much incense is burned, for how long, and on the size and ventilation of the room.

The association of the lead researcher with a tobacco company raises another point of concern. While the researchers stop short of saying incense is more dangerous than cigarettes, it is in the interests of the tobacco company for people to think cigarette smoking and incense burning are on a par – which is not the case.

It seems sensible that people who have lung conditions should avoid using incense, and the rest of us should limit its use for personal reasons, such as improving the smell of your home.

Smoking tobacco, which is known to cause illness and death from conditions including heart disease, lung cancer and stroke, is something everyone should stop altogether.

Read more advice on how the NHS can help you quit smoking.

Analysis by Bazian

Edited by NHS Website