Incense smoke and cancer risk

Wednesday August 27 2008

"Incense smoke may increase the risk of developing some cancers" reports The Guardian today. It said the findings of a 12-year study of 60,000 Chinese people backed up previous research showing that incense smoke contains cancer-causing chemicals. Those who used it had an increased risk of some types of lung cancer, and cancers of the upper respiratory tract, such as throat and mouth cancer.

The researchers in this study found an increased risk of squamous cell respiratory cancer in long-term, regular incense users. However, they only found this when they split the group into smaller subgroups and the results were of borderline significance. The study’s limitations must also be considered, including its methods of categorising incense use. Yet the findings do suggest a possible link between incense and cancer and given the associations between respiratory cancer and tobacco smoke and other inhaled substances, this seems plausible. Further follow-up studies will be needed to confirm the results.

Where did the story come from?

Dr Jeppe Friborg and colleagues from The Masonic Cancer Center, University of Minnesota, the Statens Serum Institute, Copenhagen, and the National University of Singapore, carried out the research. The study was funded by the National Cancer Institute, Maryland, US. The study was published in the (peer-reviewed) medical journal: Cancer.

What kind of scientific study was this?

This was a prospective cohort study in which the researchers aimed to investigate the relationship between incense use and risk of respiratory tract cancer.

The researchers used data from another study, the Singapore Chinese Health Study, in their assessment. The first study was set up in 1993 to investigate the role of diet and nutrition in the development of cancer and enrolled 63,257 Chinese men and women between the ages of 45 and 74. After excluding anyone who already had cancer when they enrolled, the researchers were left with  61,320 people. On enrolment, the participants were asked questions on medical history, social status, physical activity, dietary intake, smoking and alcohol use, and exposure to other inhalants and to incense. Incense questions asked participants if they had ever used it (with the options of yes or no), the approximate number of years they had used it (split into 10 year categories), their current frequency of use (from never, a few times per year, few times per month, few times per week, or daily), where in the house the incense was burned, and the times of the day when incense was burnt.

To identify new cancer patients during the follow-up period until 2005, the researchers linked their database with the Singapore Registry of Births and Deaths and the Singapore Cancer Registry. Using this data, they were able to assess the association between type of cancer diagnosis and incense use.

What were the results of the study?

At the time of enrolment to the study, incense use was high and similar for men and women (78 and 77% respectively). Overall, 93% of the participants were using incense on a daily basis and 84% had used incense for over 40 years.

The researchers say that only 0.03% of the group was lost to follow-up, and this was due to emigration from Singapore. At the end of 12 years of follow-up, 1,304 respiratory cancers had been diagnosed (including those of the upper respiratory tract, i.e. nose, throat and larynx, and lower respiratory tract, i.e. the lung). The final analysis included the 1,146 cases for which there was a histological diagnosis (microscopic analysis). Of these respiratory cancers, 89% of the nose and throat cancers were undifferentiated cells, 88% of the non-nasopharyngeal upper respiratory cancers (hereafter written as laryngeal cancers) were squamous cell, and of the lung cancers, 24% were squamous cell (usually the most common type of lung cancer) and 42% adenocarcinoma.

Participants who currently or formerly used incense did not have an increased risk of respiratory cancer compared to people who had never used it. There was a borderline significant increase in risk of cancer of the larynx in current users who had been using incense daily for 41 years or more (hazard ratio 1.7, 95% confidence interval 1.0 to 2.8) compared to noncurrent users, and also those considered to have high use (day rather than night use, and all times of the day) compared to noncurrent use (hazard ratio 2.1, 95% confidence interval 1.1 to 3.8). No other significant relationships were found between site of respiratory cancer and incense use.

When the researchers looked at the participants’ incense use and if they smoked or not, they found a borderline increased risk of laryngeal cancer in non-smokers who currently had a high incense use. However, numbers of people in these analyses were very small, There were no other relationships between smoking status and any other site of cancer.

When type of cancer (by histological diagnosis) and frequency of incense use were examined, the researchers found a borderline increase in risk of squamous cell cancer at any site in the respiratory tract in those who currently had high incense use compared to those who had never used it (hazard ratio 1.8, 95% confidence interval 1.2 to 2.6). The risk was also of borderline significance for squamous cell cancer of both the upper and lower respiratory tract. There was no association between non-squamous cell cancer and incense use.

What interpretations did the researchers draw from these results?

The researchers conclude that long-term use of incense is associated with an increased risk of squamous cell carcinoma of the respiratory tract.

What does the NHS Knowledge Service make of this study?

The findings do suggest a link between squamous cell respiratory cancer and incense use, particularly of laryngeal cancers, and this requires further study and investigation. The findings seem plausible given the associations between respiratory cancer and tobacco smoke and other inhaled substances; however, there are limitations to consider:

  • The participants that currently or formerly used incense had no higher increased risk of any type of respiratory cancer compared to those who had never used it. In other analyses where an increase in risk was found (when current use was split into duration and frequency) this was only of borderline significance.
  • Further analyses according to smoking categorisation found some association between laryngeal cancer in non-smokers with high current use of incense; however, there was only a small number of people in these analyses which means it is subject to a high degree of error.
  • The participants incense use was only assessed at one point in time and errors are likely to have been introduced as this was based on the individuals own ability to recall their incense use over a long period. Additionally, errors could have been introduced by the researchers categorisation of use, for example, high use was considered as daytime and use at all times of the day, compared to low use being night-time or intermittent use.
  • Although smoking, alcohol, dietary intake and weight have been taken into account in the analysis, other environmental or occupational exposures to inhalants were not available for analysis.
  • Incense use in this Chinese population was very high and is likely to be much higher than use in the general Western population. Additionally, the small number of people who never used incense compared to the high numbers of those who currently or formerly used it raises the possibility of error when comparing risks in the two groups.

Further follow-up studies will be needed to confirm the results.

Sir Muir Gray adds...

Smoke is smoke, and cigarette smoke is not the only type of smoke that is harmful. Clean air is as important for health as clean water.

Analysis by Bazian
Edited by NHS Choices