Preventing cognitive impairment by reducing air pollution
Authors: Renjie Chen, Haidong Kan
Journal: The Lancet Healthy Longevity (2022)

Cognitive impairment has a profound effect on an individual's overall health and wellbeing. Dementia is considered one of the most pressing public health challenges because of the accelerated ageing of global populations and ever-increasing life expectancy. The situation might be worse in low-income and middle-income countries (LMICs) in which environmental quality and socioeconomic conditions are generally poor. Mounting epidemiological and mechanistic studies provide plausible evidence for air pollution as an emerging risk factor for dementia, but causality has not been established.1Delgado-Saborit JM Guercio V Gowers AM et al.A critical review of the epidemiological evidence of effects of air pollution on dementia, cognitive function and cognitive decline in adult population.Sci Total Environ. 2021; 757143734Crossref PubMed Scopus (26) Google Scholar Some countries, including China, have reported marked improvement in air quality due to strong, policy-driven, reductions in air pollutant emissions,2Geng G Zheng Y Zhang Q et al.Drivers of PM2.5 air pollution deaths in China 2002-2017.Nat Geosci. 2021; 14: 645-650Crossref Scopus (40) Google Scholar but there is a scarcity of epidemiological evidence regarding whether air pollution abatement could improve cognitive function. In The Lancet Healthy Longevity, Yao Yao and colleagues3Yao Y Lv X Qiu C et al.The effect of China's Clean Air Act on cognitive function in older adults: a population-based, quasi-experimental study.Lancet Healthy Longev. 2022; 3: e98-108Summary Full Text Full Text PDF Scopus (5) Google Scholar explored the associations between regulated air pollutants and cognitive function on the basis of a nationally representative follow-up prospective dataset of cognitive function measurements in people aged 60 years and older in China. The 2014 and 2018 waves of the Chinese Longitudinal Healthy Longevity Survey, accompanied by the implementation of China's Clean Air Act in 2013, offer a unique opportunity to address an innovative research question: could drastic air quality improvement mitigate ageing-related cognitive decline? The main advantage of this work is the quasi-experimental study design at an individual level, which contributes to establishing a casual association between long-term exposure to air pollution and cognitive impairment in a real-world setting. Establishing causality is necessary to inform the possible inclusion of dementia as an outcome of air pollution exposure in future assessments of global disease burden and for the primary prevention of dementia. The authors showed that air pollution reduction associated with rigorous air quality intervention programmes might meaningfully alleviate age-related cognitive decline. These findings contribute important knowledge to this area of research and are particularly valuable for LMICs with apparent air pollution problems. Despite the compelling evidence provided by Yao and colleagues,3Yao Y Lv X Qiu C et al.The effect of China's Clean Air Act on cognitive function in older adults: a population-based, quasi-experimental study.Lancet Healthy Longev. 2022; 3: e98-108Summary Full Text Full Text PDF Scopus (5) Google Scholar causality between air quality improvements and cognitive improvements is yet to be confirmed due to the difficulties in accurate exposure assessment for air pollutants and in controlling for a variety of confounders. The study by Yao and colleagues is an important step—especially from the perspective of causality inference—but several key questions remain. First, the authors only found significant effects of fine particles (particulate matter with an aerodynamic diameter of less than 2·5 μm) and sulfur dioxide, probably due to the absence of considerable changes in other air pollutants—such as nitrogen dioxide and ozone—during the study period and the larger uncertainty for these other pollutants. High-resolution exposure models need to be developed for all regulated air pollutants and traffic-related air pollutants (eg, black carbon, nitrogen dioxide, and carbon monoxide).1Delgado-Saborit JM Guercio V Gowers AM et al.A critical review of the epidemiological evidence of effects of air pollution on dementia, cognitive function and cognitive decline in adult population.Sci Total Environ. 2021; 757143734Crossref PubMed Scopus (26) Google Scholar Advanced statistical approaches to address multiple exposures and analysis of directed acyclic graphs are also useful for elucidating causality.4Barrera-Gomez J Agier L Portengen L et al.A systematic comparison of statistical methods to detect interactions in exposome-health associations.Environ Health. 2017; 16: 74Crossref PubMed Scopus (33) Google Scholar Second, ultrafine particles (ie, those with an aerodynamic diameter of less than 0·1 μm) might be particularly harmful to health because they can enter the brain and carry more toxic components,5Boda E Rigamonti AE Bollati V Understanding the effects of air pollution on neurogenesis and gliogenesis in the growing and adult brain.Curr Opin Pharmacol. 2020; 50: 61-66Crossref PubMed Scopus (23) Google Scholar but little epidemiological evidence is available. Third, although the present quasi-experimental study made a considerable progress in clarifying the causal association between air pollution and cognitive decline, additional epidemiological investigations are still warranted in terms of larger sample size, longer follow-up duration, and less residual confounding (especially from indoor air pollution and senility-related comorbidity and behavioural changes). Fourth, although several mechanisms have been proposed to account for the association,6Kritikos M Gandy SE Meliker JR Luft BJ Clouston SAP Acute versus chronic exposures to inhaled particulate matter and neurocognitive dysfunction: pathways to Alzheimer's disease or a related dementia.J Alzheimers Dis. 2020; 78: 871-886Crossref PubMed Scopus (6) Google Scholar the biological plausibility is not yet clear and a framework for integrating toxicological and epidemiological evidence is ultimately needed to confirm the causality.7Thiankhaw K Chattipakorn N Chattipakorn SC PM2.5 exposure in association with AD-related neuropathology and cognitive outcomes.Environ Pollut. 2022; 292118320Crossref PubMed Scopus (6) Google Scholar Fifth, in addition to the old age, early life (during pregancy and early childhood) is another sensitive period of cognitive impairment, and it remains to be determined whether air pollution abatement in early life could result in long-term improvement of cognitive function.8Morris RH Counsell SJ McGonnell IM Thornton C Early life exposure to air pollution impacts neuronal and glial cell function leading to impaired neurodevelopment.Bioessays. 2021; 43e2000288Crossref PubMed Scopus (5) Google Scholar Air pollution has been considered the single largest environmental threat for human health, and the disease burden related to air pollution has been estimated to be on a par with other common behavioural risk factors in predicting future health.9GBD 2019 Risk Factors CollaboratorsGlobal burden of 87 risk factors in 204 countries and territories, 1990-2019: a systematic analysis for the Global Burden of Disease Study 2019.Lancet. 2020; 396: 1223-1249Summary Full Text Full Text PDF PubMed Scopus (1221) Google Scholar The work by Yao and colleagues3Yao Y Lv X Qiu C et al.The effect of China's Clean Air Act on cognitive function in older adults: a population-based, quasi-experimental study.Lancet Healthy Longev. 2022; 3: e98-108Summary Full Text Full Text PDF Scopus (5) Google Scholar strengthens the scientific evidence for a link between air pollution and cognitive decline. WHO tightened the air quality guidelines aiming to prevent millions of premature deaths per year.10WHOWHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. World Health Organization, Geneva2021Google Scholar Yao and colleagues also provide robust support for continued endeavours to improve air quality to meet global targets global guidelines. Collectively, these findings highlight the great potential in improving cognitive function in regions with high levels of air pollution through the implementation of stringent air quality intervention campaigns and are also informative for strategies to mitigate the future burden of dementia, especially in an ageing world. We declare no competing interests. This work was supported by the National Natural Science Foundation of China (92043301). The effect of China's Clean Air Act on cognitive function in older adults: a population-based, quasi-experimental studyImplementing stringent clean air policies might mitigate the risk of air pollutant-associated cognitive ageing in older people. Full-Text PDF Open Access