In this post I am going to summarise and comment on an article recently published in the journal CA: A Cancer Journal for Clinicians. The article “Cancer Statistics in China, 2015” uses data from the National Cancer Registry of China to estimate the prevalence and mortality from cancer in China in 2015. The data from this paper is a little scary although not really that surprising.

Cancer is the leading cause of death in China, with 12 000 new diagnoses and more than 7500 deaths per day. Due to China’s large population size, almost 22% of global new cancer cases and 27% of cancer deaths worldwide occur in China. Given that China has approximately 20% of the world’s population these figures are slightly higher than would be expected all things being equal. However we know all things are not equal and given China’s legacy and development status it is not surprising that China has more new cases of cancer and worse outcome for those suffering with cancer than would be expected just based on its population size.

Lung Cancer

Lung cancer is the most common cancer and the leading cause of cancer deaths. Due to the high rates and because of my interest in the negative health effects of air pollution I am going to focus on lung cancer. Of course the major known cause of lung cancer is smoking. In China the proportion of the population who smoke is high, over half of adult men smoke and smoking rates in adolescents and young adults are still rising (the rates in women is much lower). This is reflected by the fact that the 12 cancers which are formally established as being caused by smoking account for 75% of all cancers in China, with tobacco accounting for approximately 25% of cancer deaths in China. The impact of smoking related disease become evident 20 to 30 years after the onset of smoking, so the burden of cancer is likely to continue to rise irrespective to changes in tobacco control programs.

The link between the exposure of the tissue in the respiratory system to chemicals in tobacco smoke and cancer is well known. These chemicals damage the DNA in lung cells; the build up of this damage in the same cell can cause cancer to develop. It usually takes many years for the DNA damage from smoking to cause cancer. Our bodies are designed to deal with a bit of damage but it is hard for the body to cope with the number of harmful chemicals in tobacco smoke or the duration of the exposure.

Air pollution and Lung Cancer

So what about the link between air pollution and lung cancer? Contamination of the environment is strongly associated with cancer. Factors that have been directly implicated in the risk of developing cancer include radiation, carcinogenic chemicals, and biological carcinogens (e.g, some viruses and bacteria that are know to cause changes to DNA within cells they effect that can lead to cancer). The factors we are mostly considering when we think about air pollution are the carcinogenic chemicals found in smog. In the same way that the carcinogens in tobacco smoke cause DNA damage in the cells in the lungs leading to lung cancer developing after many years, exposure for long periods to these chemicals in air pollution will have the same effects. The massive amounts of outdoor and indoor air pollution, together with contamination of soil and drinking water, have increased exposure of the Chinese population to many environmental carcinogens. It has been estimated that 60% of cancer deaths can be reduced by reducing exposure to modifiable risk factors — this includes smoking and exposure to air pollution.

One interesting statistic in the article is that the measured attributable risk (of causing cancer) for environmental pollution is low (<1.0%). What this means is that less than 1.0% of cancers can be directly related to environmental pollution. This however does not mean that environmental pollution is not having a causative effect in many more cancers than this. It is often difficult to attribute one particular cancer to a specific cause. As I have said cancer is caused by changes to DNA, these changes often occur after decades of insults to the DNA, eventually overwhelming the bodies repair mechanisms. The insults can be from different sources for example living in a polluted environment and being a smoker, the effects would be additive. The WHO gives a much higher estimate of the percentage of cancers worldwide attributable to the environment (including work environment), putting it at 19% with 1·3 million deaths annually. It is difficult to directly compare these figures from different sources, as we are not clearly told what is or is not included in both. The data complied by the WHO states that air pollution caused 165 000 lung cancer deaths globally in 2004, approximately 65% of which were attributed to outdoor air pollution.

How do you separate out the cancers caused by smoking from those caused by air pollution in a population with high exposure to both? And does it really matter? We know the chemicals in both are harmful and can cause the type of damage to DNA that can lead to cancer. In China, the increase in cancer rates has been strongly linked to environmental pollutants, as noted in the so called “Cancer Villages” where the proportion of the population suffering with particular types of cancer make an environmental cause very likely. The increased risk of lung cancer in eastern China compared with western China is probably attributable to both higher rates of smoking and worse air pollution; these risks are particularly high in some rural communities.

What is really required is for tobacco control measures to be put in place and for measures to reduce air pollution to be enacted. Lung cancer mortality has increased rapidly since the 1970’s, mirroring with a time-lag the rise in the prevalence of smoking, and this rise also mirrors the rise in air pollution as China has become more and more industrialised.

One last point with reference to the article is the data source. As I mentioned at the start of the article the data is taken from the National Cancer Registry of China. The authors analysed data from 72 local, population-based cancer registries, representing 6.5% of the population to estimate number of new cases and cancer deaths for 2015. They comment that previous studies used even smaller percentages of the population for their estimates. However we should also consider that 6.5% is not a large percentage of the population, and that the centres are generally concentrated in east of the country with several provinces not having even one registry. There consequently remains an unknown level of uncertainty in their estimates. This uncertainty is almost certainly going to be an underestimate of the actual numbers of new cancer diagnoses and cancer deaths.

There is a long history of governments having a greater incentive to act when there is clear evidence demonstrating need. This paper goes some way towards providing this quantitative evidence and demonstrating the huge challenge in managing the burden of cancer now and in the future. Part of this management strategy must be through prevention, even if that prevention is aiming to stop cancers developing in 20-30 years time. It must remain a priority to continue to push tobacco control policies and to ensure that existing pollution control legislation is adequately implemented and continues to be strengthened to ensure real improvements occur in practice.

Glossary

Prevalence — the number of people with a condition at a certain time, how common it is.

Mortality — the numbers of deaths from a condition.

DNA — deoxyribonucleic acid, a self-replicating material which is present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information.

References

Chen W, Zheng R, Baade P et al. Cancer Statistics in China, 2015, CA Cancer J Clin. 2016. Available from: cacancerjournal.com

Cancer Research UK. How smoking causes cancer. Website. Available from:

http://www.cancerresearchuk.org/about-cancer/causes-of-cancer/smoking-and-cancer/how-smoking-causes-cancer [Accessed 17th February 2016]

Goss P, Strasser-Weipl K, Lee-Bychovsky et al. Challenges to effective cancer control in China, India and Russia. Lancet Oncol. 2014; 15:489- 538.

WHO. Environmental and Occupational Cancers, Factsheet No 350, 2011. Available from: http://www.who.int/mediacentre/factsheets/fs350/en/ [Accessed 17th February 2016]

Wang J, Jiang Y, Liang H et al. Attributable causes of cancer in China. Ann Oncol. 2012; 23:2983- 2989.