Ka Hung Chan, Derrick Bennett, Kin Bong Hubert Lam and Zhengming Chen
Chronic liver diseases (CLD), such as liver cirrhosis and liver cancer, affect more than 800 million people worldwide, with a third of these living in China. Smoke from solid fuels (e.g. coal and wood) and tobacco contains high concentrations of thousands of toxic chemicals. When breathed in, these chemicals can reach and harm major internal organs, including the liver.
Despite improvements over the past few decades, more than 2.6 billion people worldwide still rely on solid fuels for cooking, while tobacco use has risen substantially in China and other low- and middle-income countries. China has 450 million users of solid fuel and 320 million smokers who consume more than 40% of the world’s tobacco. Although smoking has been linked to a higher risk of developing CLD in high-income countries, little is known about the situation in China, where the smoking epidemic has only emerged since the 1980s. Previous studies of solid fuel use have focused on cardiorespiratory diseases, and there is very little epidemiological evidence regarding its potential impact on CLD.
Using information from a cohort of half a million Chinese adults who were followed for 10 years, our study, recently published in the International Journal of Epidemiology, found that long-term use of solid fuels for cooking and tobacco smoking were each independently associated with an excess risk of death from CLD. Notably, those who had used solid fuels for a longer duration and those who started smoking regularly at an earlier age had a particularly high risk of dying from CLD.
Compared with outdoor air pollution — a ubiquitous problem that receives ample public attention — smoke from solid fuels or tobacco contains concentrations of harmful chemicals that are tens or hundreds of time higher. Just like other harmful substances, such as alcohol, toxins from solid fuel or tobacco smoke can overwhelm the detoxification capacity of the liver and cause harm.
In our study, participants who used solid fuels for cooking were exposed for 40 years on average, and smokers generally started smoking at the age of 20–25 years — common experiences of billions of people worldwide. Decades of regular inhalation of harmful pollutants might have caused chronic damage to the liver, both directly killing liver cells and indirectly causing chronic inflammation, thus resulting in the higher risk of death from CLD.
This is the world’s first population health study showing the potential harm of solid fuel use on the liver, and the largest of its kind showing the harm of smoking in contemporary China. These findings suggest that solid fuels and tobacco may be contributing to a greater public health impact than previously thought, although further research is warranted to verify this.
Encouragingly, we also found evidence suggesting that stopping the use of solid fuels or tobacco may mitigate the harm from previous exposure. In our study, participants who had switched from solid to clean fuels (for about 14 years) had a similar risk of death from CLD as those who had always used clean fuels. The same was true for ex-smokers who had stopped smoking for non-medical reasons (for about 10 years) when compared with those who had never smoked.
Neither solid fuels nor tobacco are new public health problems, but our findings offer yet another reason to further support the global effort to promote access to clean fuels and implement more stringent tobacco control, especially in developing countries where both hazards are common.
Chan KH, Bennett DA, Kurmi OP, et al. Solid fuels for cooking and tobacco use and risk of major chronic liver disease mortality: a prospective cohort study of 0.5 million Chinese adults. Int J Epidemiol 2019; Oct 25. doi: 10.1093/ije/dyz216.
Ka Hung Chan is a Nuffield Department of Population Health Early Career Research Fellow, Derrick Bennett is a Senior Statistical Epidemiologist, Kin Bong Hubert Lam is an Associate Professor, and Zhengming Chen is a Professor in Epidemiology at the University of Oxford.