Indoor air pollution kills millions of people globally each year. Tsinghua University researchers are documenting the problem and coming up with simple solutions to save lives.
According to the World Health Organization, air pollution is the largest environmental risk to human health. Given that most people spend more than 80% of their time inside, indoor air pollution is an increasing area of concern, with numerous studies linking it to cardiovascular, pulmonary and neurological conditions, such as cancer and stroke. In 2020, indoor air pollution killed an estimated 3.2 million people worldwide, including more than 237,000 young children.
The World Health Organization reduced their recommended level of annual mean PM2.5 (as well as other air pollutants, such as NO2 and O3) concentrations specified in their air quality guidelines (AQG) in 2021. However, even if China urban areas meet these guidelines, Tsinghua modeling suggests that there would still be 485 thousand annual deaths attributable to indoor PM2.5 sources.
The problem is particularly pressing in China. “The rates of indoor air pollution here are generally higher compared to many parts of the world”, says Bin Zhao, a professor at Tsinghua University’s School of Architecture. “However, it’s a problem that affects many countries around the world, especially low-income and developing nations, where access to both clean energy and proper ventilation systems may be limited.”
In rural areas, burning solid fuels such as wood, crop waste and charcoal is a major source of indoor air pollution and generates large quantities of fine particulate matter (PM2.5 — particulate matter with aerodynamic diameter of 2.5 micrometers or less). In less wealthy areas, these materials provide fuel for heating and cooking as they are readily accessible and inexpensive, says Zhao.
Cooking and cigarettes
In urban areas, cooking is a major cause of indoor air pollution. Chinese cooking often involves frying in oil, which generates PM2.5 . Additionally, gas is a common cooking fuel in urban households, but it emits nitrogen dioxide (NO2). Research has shown that levels of PM2.5, which are tiny enough to penetrate deep into the lungs, can triple during cooking.
In 2022, Zhao and his PhD student, Ying Hu, created a model that allowed them to estimate the level of exposure to PM2.5 and NO2 for people in 333 Chinese cities, and from what sources it came from1. “We discovered that gas cooking is the most important indoor source of these pollutants to urban Chinese households,” says Zhao.
In addition to burning solid fuels and gas cooking, cigarette smoke is another major factor contributing to China’s indoor air pollution problem. As of 2018, there were 300 million smokers in China, with 44.9% of adults and 63.2% of adolescents exposed to second-hand smoke at home.
A deadly problem
Levels of pollution vary across the vast country, with the indoor concentrations of PM2.5, for example, highest in the north. “Colder climates can lead to more time spent indoors and less window ventilation, thus increasing exposure to indoor air pollutants,” explains Zhao. Industrial development is also denser in the north, which “leads to more severe outdoor pollution that can also affect indoor air quality.”
The impact of indoor air pollution can be severe. Gas cooking, for instance, accounted for 25% of total NO2-linked cases of asthma in children in 2019 in urban area, as Zhao’s team revealed in a modelling study published last year in The Lancet Regional Health-Western Pacific2. In a separate study, they determined that indoor PM2.5 contributed to 394,000 deaths and 1.53 trillion Chinese yuan (roughly US$222 billion) of economic losses due to premature deaths in 2019 in China urban areas3.
The Chinese government has taken steps to address the problem. In 2013, the Air Pollution Prevention and Control Action Plan helped improve outdoor air quality through restrictions on emissions from coal mining, transportation and other industries. In 2022 this was followed by a revision of government Standards for Indoor Air Quality, aligning it with the WHO’s new standards for a lower recommended mean annual PM2.5 concentration.
The Chinese government’s Poverty Alleviation Resettlement (PAR) program, which saw nearly 10 million rural residents being moved to urban centers between 2016 and 2020 has also helped reduce impacts of indoor air pollution as people adopted cleaner fuels for cooking and heating. Zhao’s team estimated that the move helped avoid 5,400 deaths due to reduction of indoor PM2.5 levels4.
People have become better aware of the health hazards associated with air pollution and have begun to take measures to protect themselves, for example using air purifiers5, says Zhao. These changes have helped reduce the problem over the years, but there is “still a long way to go as indoor levels of pollutants are still far from meeting the WHO standards.”
Towards cleaner air
There are many simple ways to further improve the situation, he says. For a start, ventilation can be enhanced by designing living and kitchen spaces to improve air flow. Optimizing the exhaust efficiency of cooker hoods — which currently average just 60% — could also help tremendously. His team also found that switching from gas to electric stoves can prevent nearly 300,000 new asthma cases in children every year due to a reduction in NO2 emissions.
Installing more air purifiers in homes could also help. Zhao suggests that government subsidies for purchasing and maintaining purifiers could help relieve economic pressure for lower income groups, particularly in less economically developed cities where high outdoor PM2.5 levels may be infiltrating homes and adding to indoor pollution5. Tighter regulations around smoking and availability of cigarettes, as well as public education campaigns could help reduce indoor air pollution even further.
Bin Zhao is a full professor at Tsinghua’s School of Architecture.
Zhao and his team are now looking to take their extensive research on indoor air pollution even further: they plan to conduct an interdisciplinary study alongside environmental epidemiologists, engineers and material scientists to look for new cost-effective solutions. “Overall, my hope is to build healthy indoor and built environments, and to raise everybody’s awareness of indoor air pollution,” he says.
1. Hu, Y. & Zhao, B. Indoor sources strongly contribute to exposure of Chinese urban residents to PM2.5 and NO2 Journal of Hazardous Materials 426, 127829 (2022)
2. Hu, Y., Ji, J. S., & Zhao, B. Restrictions on indoor and outdoor NO2 emissions to reduce disease burden for pediatric asthma in China: A modeling study The Lancet Regional Health - Western Pacific 24, 100463 (2022) doi: 10.1016/j.lanwpc.2022.100463
3. Hu, Y., Ji, J. & Zhao, B. Deaths Attributable to Indoor PM2.5 in Urban China When Outdoor Air Meets 2021 WHO Air Quality Guidelines Environmental Science & Technology 56(22), 15882–15891 (2022) doi: 10.1021/acs.est.2c03715
4. Liu, Y., Han, D., Ji, J.S. & Zhao, B. China's relocation policy for poverty reduction may save thousands of lives attributable to indoor PM2.5 Building and Environment 226, 109725 (2022) doi: 10.1016/j.buildenv.2022.109725.
5. Zhang, A., Liu, Y., Ji, J. & Zhao, B. Air Purifier Intervention to Remove Indoor PM2.5 in Urban China: A Cost-Effectiveness and Health Inequality Impact Study Environmental Science and Technology 57, 4492−4503 (2023)
Editor: Guo Lili