y Toxicological evaluations provide robust biological evidence that exposure to key ambient air pollutants – particularly PM2.5 and O3 – induces oxidative stress, neuroinflammation, systemic inflammatory responses, vascular dysfunction and disruptions in cardiopulmonary physiology (232,250). These mechanistic pathways are consistent with epidemiological findings, and they support the plausibility that long-term exposure to classic air pollutants contributes to increased dementia risk. y Although everyone can contribute to reducing air pollution, ensuring clean air is ultimately a societal responsibility rather than an individual one. Health workers play a crucial role in educating individuals about the health risks associated with air pollution. y Interventions to reduce exposure (e.g. improved urban planning, traffic emission controls and clean energy policies) can have widespread impact (231,251). However, owing to ethical and practical concerns, it is not feasible to control environmental exposures in studies, making it challenging to evaluate air pollution reduction interventions through RCTs and thus affecting the certainty of evidence available for decision-makers. y Air pollution reduction policies offer a win-win for public health and climate change mitigation (in both the short and long term). y Air pollution disproportionately affects marginalized and low-income populations, who may also be at elevated risk for dementia. However, perceptions of the value of clean air vary globally, influenced by economic, cultural and environmental factors. This underscores the importance of equity considerations when implementing clean air policies and these dementia risk reduction guidelines. y Risk reduction efforts should be aligned with national and international air quality standards, as provided in the WHO global air quality guidelines (231). [The World Health Organization (WHO) Air Quality Guidelines recommend that the annual mean concentration of fine particulate matter (PM2.5) should not exceed 5 µg/m³, with a 24-hour mean of no more than 15 µg/m³. For coarse particulate matter (PM10), the recommended limits are an annual mean of 15 µg/m³ and a 24-hour mean of 45 µg/m³. For ozone (O₃), the guidelines recommend a peak-season mean of 60 µg/m³ and a maximum 8-hour mean of 100 µg/m³. The recommended levels for nitrogen dioxide (NO₂) are an annual mean of 10 µg/m³ and a 24-hour mean of 25 µg/m³. For sulfur dioxide (SO₂), the WHO recommends a 24-hour mean of 40 µg/m³, while for carbon monoxide (CO), the recommended 24-hour mean is 4 µg/m³. PM2.5 refers to particles with an aerodynamic diameter of 2.5 µm or less, whereas PM10 refers to particles with an aerodynamic diameter of 10 µm or less.]

y Effective implementation requires multisectoral collaboration among various sectors: public health, environment, transport and energy. By addressing air pollution through integrated health and environmental policies, significant progress can be made in reducing dementia risk while also achieving broader health and sustainability goals. y Public education campaigns may increase awareness of cognitive risks related to air pollution and support advocacy for cleaner environments. y Policy-makers should consider targeted interventions in high-risk areas (e.g. urban centres and regions with industrial emissions) and for vulnerable groups (e.g. older adults and socioeconomically disadvantaged communities). y Surveillance systems that monitor trends in both air quality and cognitive health may help in evaluating the impact of environmental interventions over time.

Air pollution is toxic and contributes substantially to the global burden of communicable and noncommunicable diseases, including dementia. Reducing exposure to air pollution, both indoors and outdoors, is critical to improving health and wellbeing for all people, across the entire life course, in line with guidance from WHO and other organizations: Public health and policy: y WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide (62) y Integrated science assessment (ISA) for particulate matter (final report, Dec 2019) (63)