Prolonged use of solid fuels such as coal and wood for cooking, and tobacco smoking are associated with higher risk of dying from chronic liver diseases (CLD) including liver cirrhosis and liver cancer, according to new research published in the International Journal of Epidemiology.

Despite improvements over the past few decades, over 2.6 billion individuals worldwide still rely on solid fuels for cooking. Tobacco use has also risen substantially in China and other low- and middle-income countries (LMICs). Currently, China has 450 million solid fuel users and 320 million smokers who consume more than 40% of the world’s tobacco. Solid fuels and tobacco smoke contain high concentrations of thousands of toxic chemicals which when breathed in, can get into the blood stream, reach the liver and lead to harm. Despite this plausible link, little contemporary epidemiological evidence exists on the potential impact of these risk factors on chronic lung disease in low and middle income countries.

Researchers from NDPH, the Chinese Academy of Medical Sciences, and the Peking University investigated the independent associations of long-term use of solid fuels for cooking and smoking with CLD deaths using data from the China Kadoorie Biobank (CKB).

The study, led by Drs Ka Hung Chan, Derrick Bennett and Hubert Lam, recruited 501,104 adults with an average age of 52, and no history of cancer, from 10 diverse areas of China during 2004-08. Cooking fuel use patterns, over a 40 year period and current and past smoking behaviour were assessed using questionnaires. The occurrence and causes of death were monitored for 10 years via linking to death registries, during which 2,461 deaths from major CLD were recorded.

After accounting for potential confounders, such as age, sex, socioeconomic status, alcohol intake, adiposity, diabetes, and hepatitis B infection status, those who had always used coal or wood for cooking had 26% higher risk of dying from CLD compared to those who had always used gas or electricity. Notably, those who had used solid fuels for 40 years or longer (32%) and those who primarily used wood (33%) had higher risks of CLD death.

Similarly analyses of smoking indicated that current-smokers had 28% excess risk of CLD death compared to those who never smoked, with higher risk among those who started smoking at an earlier age (under 20 years: 37%) and those who smoked more cigarettes per day (25 or more cigarettes: 44%).

This is one of the first and largest epidemiological studies demonstrating the potential harm of solid fuels and tobacco use on the liver in an LMIC setting. “Our findings provided new evidence to suggest that the public health impact of household air pollution and smoking may be larger than we initially thought.” said Dr Derrick Bennett, Senior Statistical Epidemiologist.

The researchers also highlighted that the excess risk of CLD death associated with use of solid fuels and tobacco may be mitigated by switching to clean-burning fuels such as gas or electricity and stopping smoking. Professor Zhengming Chen, the principal investigator of the China Kadoorie Biobank, said: “The risk of CLD death in those who had switched from solid to clean fuels for about 14 years and those who stopped smoking for 10 years were similar to the long-term clean fuel users and never-smokers. Although such observational evidence cannot confirm causality, it calls for greater efforts to promote universal access to affordable clean energy and smoking cessation worldwide”.