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Burning wood or coal to cook food is associated with higher risk of being hospitalised or dying from respiratory diseases, according to new research conducted in China and published online in the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine.

Around 3 billion people around the world live in households that regularly burn wood, coal or other biomass fuels (solid fuels), which emit very high levels of pollutants (especially fine particulates), to cook their food. Typically, these households are found in the rural areas of low- and middle-income countries. Although China is rapidly urbanising, one third of its population still rely on solid fuels.

Researchers from the Nuffield Department of Population Health at the University of Oxford and the Chinese Academy of Medical Sciences have studied the association of the long-term use of solid fuels for cooking with the risk of hospitalisations or death from chronic and acute respiratory diseases using data from the China Kadoorie Biobank (CKB).

The CKB is a prospective study of 0.5 million adults aged from 30 to 79 years who were recruited from 10 different areas of China during 2004-08. The participants included in the present report were never smokers and were free of respiratory and other major chronic disease upon enrolment. Their health status was monitored for 9 years through linkages to death registries and hospital records. During that time, 19,823 participants were either hospitalised or died from a major respiratory disease. Of these events, 10,553 were due to chronic lower respiratory disease (asthma or chronic obstructive pulmonary disease [COPD]), and 7,324 were due to acute lower respiratory infections, most often pneumonia.

The researchers found that the risk for chronic and acute respiratory disease hospitalisations or deaths was 36% higher among those who used wood or coal for cooking compared to those who used a clean-burning fuel (such as electricity or gas), after taking account of the effects of age, sex, socio-economic status, passive smoking, alcohol drinking, diet, physical activity, and adiposity. The study also showed clearly that the longer people used solid fuels, the higher the risk of hospitalisation or death from a respiratory disease; the risk was 54% higher among those who used either wood or coal for 40 years or longer compared to clean-fuel users.

Pollution arising from domestic burning of coal and wood for cooking and heating has been consistently shown to contribute to deaths from cardiovascular disease and lung cancer. While many previous studies have suggested a link between solid fuel use and COPD, most of them focused on changes in lung function rather than hospital admissions or deaths. The associations with other respiratory diseases have not been well described.

The analyses from this very large study provide important new evidence of the hazards of long-term exposure to solid fuel emissions related to both chronic and acute respiratory diseases. The study also found wood users and coal users were 37% and 22%, respectively, more likely to be hospitalised or die from respiratory disease than clean-fuel users. Those who burned wood or coal in ventilated stoves had a slightly lower risk of hospitalisation or death from respiratory disease, particularly for the milder conditions, than those who used stoves that were not ventilated.

The authors offered possible explanations for the relatively weak association between burning wood or coal in a cookstove and COPD (10% excess risk compared to up to 2-3 fold increase in previous studies based mainly on lung function deficit), including the fact that COPD is underdiagnosed in China. This is particularly true in rural areas because of the low rates of spirometry, an essential diagnostic tool in COPD. “It could also be due to concurrent exposure to wood or coal smoke among clean fuel users that our study was unable to measure, especially among those who lived in a community where solid fuel use is common,” said Hubert Lam, associate professor at the Nuffield Department of Population Health who co-led this study.

The researchers said that the most important finding of their study may be that the increased risk of major respiratory diseases posed by burning wood or coal can be significantly lowered by switching to a clean-burning fuel. Professor Zhengming Chen, from the Nuffield Department of Population Health, the lead principal investigator of the China Kadoorie Biobank, said: “Although we cannot infer a causal relationship from these observational findings, this still makes a compelling case to speed up the global implementation of universal access to affordable clean energy, one of the United Nations’ Sustainable Development Goals.”

The China Kadoorie Biobank is funded by: Wellcome; Medical Research Council; Hong Kong Kadoorie Charitable Foundation; Chinese Ministry of Science and Technology; National Natural Science Foundation of China.