Air pollution and dementia

Due to the ageing global population, the number of people with dementia worldwide is predicted to rise from over 50 million in 2020 to more than 150 million by 2050,1 with associated higher economic, social, and individual costs.2 As such, identification of factors that can be targeted to reduce dementia’s impact is urgently needed. The plausible effect of ambient air pollutants on brain pathology, via cerebrovascular disease or inflammation,3 has contributed to an increasing volume of research exploring whether air pollution is a risk factor for dementia. In a linked BMJ paper,4 Wilker and colleagues (doi:10.1136/bmj-2022-071620) examined the association between ambient air pollution and risk of all-cause dementia in a systematic review of 51 longitudinal studies from North America, Europe, Asia, and Australia. They found that higher environmental exposure to fine particulate pollution—particulate matter of less than 2.5 microns in diameter (PM_2.5)—was associated with an increased risk of dementia: for every 2 µg/m³ increase in average annual PM_2.5 concentration, overall risk of dementia rose by 4% (hazard ratio 1.04 (95% confidence interval 0.99 to 1.09)). Current estimates suggest that PM_2.5 concentrations in major cities vary considerably from below 10 µg/m³ in some cities (eg, Toronto, Canada) to more than 100 µg/m³ in others (eg, Delhi, India),5 therefore, air pollution has the potential to substantially affect dementia risk globally.

Wilker and colleagues’ findings are consistent with, and build on, evidence from earlier systematic reviews.67 The authors included the latest evidence in this rapidly increasing research area and evaluated study quality using the Risk of Bias in Non-Randomised Studies of Exposures (ROBINS-E) tool, which facilitates a detailed evaluation of bias in environmental studies. This appraisal established the methodological limitations affecting numerous earlier studies. The new systematic review found, for example, that studies using active case ascertainment reported a stronger association between dementia risk and air pollution than studies using passive surveillance methods, such as electronic health records. This difference might reflect a higher rate of misclassification of dementia status in health records; diagnosis is more frequently missed in certain sociodemographic populations, such as ethnic minority groups,8 who might also be more likely to have been exposed to air pollution, so associations might be obscured in health record studies.

The new systematic review highlights many remaining uncertainties about the association between air pollution and dementia. Risk factors for dementia cluster together, many of which are driven by socioeconomic status, and the interactions between risk factors are likely to be complex. Many studies have attempted to resolve this issue by adjusting for potential confounders, but this adjustment might not be adequate because the manipulation could obscure the complex inter-relations between socioeconomic status, ethnic group, air pollution, and dementia.9

Evidence that a reduction in air pollution is associated with a lower incidence of dementia is only just emerging.1011 Most studies so far have been conducted in Europe and North America. Few studies are from lower-income and middle-income countries in sub-Saharan Africa and Asia, where PM_2.5 concentrations are highest and increasing,12 and individuals are also more likely to be exposed to indoor air pollution.13 Which pollution constituents are most harmful and how these interact to influence dementia risk also is unclear.

Ambient air pollution is also associated with other health conditions and with mortality. The Lancet Commission on Pollution and Health14 estimated that more than 6.5 million deaths globally are attributable to air pollution each year and that this number is increasing, driven by industrialisation, urbanisation, population growth, and rising use of fossil fuels. Exposure to air pollution is often lifelong with potential negative effects on cognitive health even in utero.15 Although individuals can take steps to reduce their own personal exposure, for example, by remaining indoors on high air pollution days,16 this solution is impractical in the long term, so for many people, the risk is inescapable. Wilker and colleagues’ findings therefore add urgency to the need for effective policy measures to reduce air pollution globally.

Effective measures will likely require global legislation, not relying on individual countries’ efforts. Around 40% of countries still have no published standards for air pollution, supporting the need for the World Health Organization’s global air quality guidelines that proposed increasingly strict targets, ultimately aiming for average annual PM_2.5 concentration of less than 5 µg/m³.17 Reductions in air pollution are theoretically achievable through global environmental policy programmes that focus on transition to clean and renewable energy sources, reduced energy consumption, and changes in agriculture.18 Any positive effect on dementia and general health would be accompanied by an important impact on climate change and biodiversity, therefore, reducing air pollution should be a global health and humanitarian priority.