Greener cities reduce dementia risks linked to air pollution

By Dr. Priyom Bose, Ph.D.Reviewed by Lauren HardakerJul 29 2025

An international study reveals that boosting greenery could be a powerful tool that protects the brain from the harmful effects of polluted air linked to Alzheimer’s and dementia worldwide.

Study: Greenness modified the association of PM_2.5 and ozone with global disease burden of Alzheimer’s disease and other dementias. Image credit: AMINSEN/Shutterstock.com

A recent study in Scientific Reports investigated whether greenness influences the association between particulate matter (PM) with a diameter of less than 2.5 micrometers (PM_2.5) and ozone, and whether this relationship affects the disease burden linked to Alzheimer’s disease (AD) and other dementias.

The association between dementia and air pollution

Dementia is a collection of symptoms associated with a decline in cognitive ability, such as memory loss, difficulty with thinking, and reduced problem-solving ability, impacting daily life. AD is a type of dementia, which has been identified as the seventh leading cause of death worldwide. Approximately 55 million people worldwide have been diagnosed with AD, and this number has been predicted to rise as high as 79 million by 2030 and 139 million by 2050.

Global healthcare costs have significantly increased due to the high prevalence of dementia. Considering the increased prevalence, it is crucial to identify modifiable risk factors, particularly to prevent AD and other dementias and reduce the economic burden of the disease. The World Health Organization (WHO) and the Lancet Commission have independently identified air pollution as an emerging risk factor of AD.

Air pollutants trigger oxidative stress and neuroinflammation in the brain, thereby increasing the risk of multiple diseases, including metabolic disorders, cardiovascular conditions, and dementia. Previous studies have established an association between PM_2.5 and AD. In 2015, approximately 28% of deaths and 30% of disability-adjusted life years (DALY) were linked to dementia caused by ambient PM_2.5 pollution.

Interestingly, a large-scale US-based study highlighted that higher neighbourhood greenness lowers the risk of AD and related dementias. This occurrence has been attributed to a greener environment, reducing stress by promoting physical activity and alleviating the adverse effects of air pollution. Not many studies have explored how much a greener environment impacts the association between air pollution and the global disease burden of AD and other dementias.

About the study

The current ecological study aimed to examine the relationship between air pollutants and AD burden worldwide and assess the potential to modify this association through a greener environment.

All relevant data on the incidence of AD and other dementias, related deaths, and DALY were obtained from the Global Burden of Disease (GBD) database. The current study's data were sourced from 162 countries, spanning 2010, 2011, 2014, 2015, 2016, and 2017.

PM2.5 and ozone, the two most extensively studied air pollutants in the GBD database, were considered. To evaluate the greenness exposure, this study used enhanced vegetation index (EVI) and country-level normalized difference vegetation index (NDVI) data from the Terra Moderate Resolution Imaging Spectroradiometer (MODIS). The NDVI and EVI values range from -0.2 to 1.0, where higher values approaching 1.0 represent greater greenness, while any negative values highlight cloud cover, snow, or water bodies.

Study findings

The current study estimated the global incidence rate of AD and other dementias, death rate, and DALY to be 55.52, 12.48, and 206.94 per 100,000 population, respectively. Global spatial distribution analysis revealed that the highest prevalence of AD, death, and DALY due to AD and other dementias occurred in Japan in 2017, followed by Italy. In contrast, Qatar and the United Arab Emirates exhibited the lowest incidence of these diseases.

During the study periods, the values of NDVI and EVI were estimated to be 0.56 and 0.33, respectively. Furthermore, PM_2.5 and ozone were estimated to be 23.13 μg/m³ and 39.96 ppb, respectively. Compared to the rest of the world, a higher greenness density was observed in America and Oceania.  

As per the global landscape, the average levels of PM_2.5 and ozone were higher in Asia and Africa, and lower in Europe, Oceania, and America. The Spearman correlations between air pollutants and greenness ranged between -0.36 and -0.40, indicating negative and moderate correlations.

A statistically significant association was observed between PM_2.5 and ozone and the disease burden of AD and other dementias. In a fully adjusted model, a 10-unit increment in PM_2.5 and ozone was associated with a 2.0% and 1.9% increase in the incidence rate of AD and other dementias, respectively. The same exposure levels were associated with a 2.8% and 9.5% increase in death rate, and a 2.2% and 6.7% increase in the DALY rate, respectively.

A robust negative association was observed between greenness and dementia, particularly at moderate levels of greenness, with weaker or non-significant associations at very high greenness levels, suggesting a non-linear relationship. The disease burden was significantly higher in countries with a low socio-demographic index (SDI) compared to middle or high SDI categories. Similarly, a higher AD and other dementia rate ratio was associated with countries with low gross national income (GNI).

The current study demonstrated that both PM_2.5 and ozone were associated with reduced harmful impacts on dementia burden (rather than an actual “protective” effect) in areas with high levels of greenness, and that in some of the highest greenness quartiles, the pollutant-disease associations became insignificant or even reversed.

Conclusions

The findings here showed a positive association between annual average concentrations of PM_2.5 and ozone exposure and the incidence, deaths, and DALYs of AD and other dementias. A greener environment could mitigate this association, particularly at moderate-to-high greenness levels. Therefore, increasing green space worldwide would positively impact health, although the protective effects of greenness may not increase linearly at the highest levels of vegetation.

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