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Chinese woman cooking using solid fuel

Air pollution and non-optimal ambient temperature are the top two environmental factors affecting human health and well-being worldwide. It is estimated that 5-9 million deaths each year are linked to air pollution, and 4-6 million to non-optimal ambient temperature. However, substantial uncertainty persists about their relevance, both qualitatively and quantitatively, for specific diseases in diverse populations. Decade-long research in CKB has helped address some of the key knowledge gaps. Using a prospective cohort approach, CKB has produced new evidence on the health impact of household air pollution from domestic solid fuel use on risks of a wide range of diseases including those affecting the cardiovascular and respiratory systems, the liver, and eyes. We have also clarified the role of ambient temperature on blood pressure, diagnosis and management of hypertension, and risks of cardiovascular disease in China.

In the past few decades China has undergone rapid economic development and environmental changes. Worsening ambient air pollution (AAP) in urban areas has caused immense public concern, but household air pollution (HAP), a much more intense, longstanding risk exposure, has been largely overlooked despite China still having ~500 million users of solid fuel during the 2010s.

By collecting detailed cooking and heating fuel use data from all study participants at various time periods, CKB was able to demonstrate the gradual transition from using coal and wood as cooking fuel to cleaner gas and electricity from the 1960s to the 2010s. However, the rate of transition was markedly slower among rural residents, elderly, and those with lower education or income  (Int J hyg Environ Health 2017). There was also a major lag of fuel modernisation for heating, with up to 75% of rural residents still relying on solid fuels for heating by 2014, and just under 50% using solid fuels for cooking.

CKB showed that long-term solid fuel users have 10-20% higher risks of death, particularly from major cardiovascular and respiratory diseases, compared to clean fuel users (JAMA 2018; AJRCCM 2019). Importantly, these studies suggest that the potential harm of household air pollution could be at least partially mitigated through improved ventilation and use of clean fuels. CKB was the first epidemiological study to extend the harmful effects of air pollutants from cardio-respiratory systems to the liver, given its pivotal role of detoxification, showing that long-term solid fuel users had over 25% higher risk of death from major chronic liver disease (e.g. liver cancer and cirrhosis) (IJE 2020).

The eyes are very exposed to air pollution and most previous studies only investigated mild eye symptoms or cataract, however CKB demonstrated for the first time the increased risks of hospitalisation from a wide range of eye diseases, including cataract, conjunctiva disorders such as conjunctivitis (inflammation of the conjunctiva), and disorders of the sclera, cornea, iris and ciliary body such as keratitis (inflammation of the cornea) (PLOS Med 2021).

The 10 CKB study sites cover a diverse range of climate zones, with, for example, average winter temperature ranging from -14°C to 19°C. We found a strong inverse relationship between ambient temperature and blood pressure (J Hypertens 2012). Overall, a 10°C drop in ambient temperature was associated with 6-7 mmHg rise in systolic blood pressure, with greater effects in older participants and those with low body mass index. Notably, people with pre-existing cardiovascular disease had a significant higher risk of dying in the winter months (Eur heart J 2015). CKB also identified clearly how temperature and seasons may influence the detection, diagnosis, and management of hypertension, indicating an under-diagnosis in warmer seasons and poorer management during colder seasons (J Hypertens 2012; IJE 2014). This research has major implications for the clinical management of hypertension and epidemiological research into the condition. Our findings have informed the design of periodic national surveys on blood pressure in China in recent years. 

The widely different heating fuel use patterns across China enabled CKB to explore how various heating approaches may mitigate against the health effects of cold temperatures. Notably, individuals with well-established and universal central heating system in Harbin in northern China  tended to have modest elevation of blood pressure during the winter, despite the region having the coldest winter of all 10 CKB areas [J Hypertens 2012; Eur Heart J 2015]. This offers an important insight into the significance of access to affordable clean energy and the increasing need for appropriate environmental and energy policies to reduce the threat to public health from increasingly extreme weather associated with climate change.

Impact of research

The global health impact of air pollution and non-optimal ambient temperature will likely grow with the worsening global climate change and rapid industrialisation in emerging economies, unless strong action is taken to control them. CKB has produced novel evidence to enable more accurate estimation of the disease burden linked to these exposures, inform clinical management of hypertension and cardiovascular disease, and encourage better access to clean energy. To improve the exposure assessment at personal level, further research will involve development of better methods to understand the health effects of ambient air pollution and temperature, including the use of real-time measurements of personal exposure to these factors (Environ Int 2021).