Air Pollution and Incidence of Dementia
A Systematic Review and Meta-analysis
Abstract
Background and Objectives
Studies of association between air pollution and incidence of dementia have shown discrepant results. The aim of this study was to evaluate the association between air pollution and dementia.
Methods
In this systematic review and meta-analysis, PubMed, MEDLINE, EMBASE, PsycINFO, Scopus, and Web of Science were searched and updated in August 2021. Population-based cohort studies that reported on hazard ratio (HR) of dementia in association with exposure to fine particulate matter (PM2·5), nitrogen oxides (NOX), nitrogen dioxide (NO2), or ozone (O3) in those aged >40 years were included. Data were extracted by 2 independent investigators. The main outcome was the pooled HR for dementia per increment of pollutant, calculated using a random-effects model. Results were reported in accordance with PRISMA guidelines. The protocol was registered in PROSPERO (registration number: CRD42020219036).
Results
A total of 20 studies were included in the systematic review, and 17 provided data for the meta-analysis. The total included population was 91,391,296, with 5,521,111 (6%) being diagnosed with dementia. A total of 12, 5, 6, and 4 studies were included in the meta-analyses of PM2·5, NOX, NO2, and O3, respectively. The risk of dementia increased by 3% per 1 μg/m3 increment in PM2·5 (HR, 1.03; 95% CI [1.02–1.05]; I2 = 100%). The association between dementia per 10 μg/m3 increment in NOX (HR, 1.05; 95% CI [0.99–1.13]; I2 = 61%), NO2 (HR, 1.03; 95% CI [1.00–1.07]; I2 = 94%), and O3 levels (HR, 1.01; 95% CI [0.91–1.11]; I2 = 82%) was less clear, although a significant association could not be ruled out, and there was high heterogeneity across studies.
Discussion
Existing evidence suggests a significant association between exposure to PM2·5 and incidence of dementia and nonsignificant association between dementia and NOX, NO2, and O3 exposure. However, results should be interpreted in light of the small number of studies and high heterogeneity of effects across studies.
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© 2022 American Academy of Neurology.
Publication History
Received: October 5, 2021
Accepted: August 31, 2022
Published online: October 26, 2022
Published in print: January 10, 2023
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The authors report no relevant disclosures. Go to Neurology.org/N for full disclosures.
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Abolhasani et al. conducted a meta-analysis to evaluate the association between air pollution and incidence of dementia.1 Pooled hazard ratio (HR) (95% confidence interval [CI]) of particulate matter with an aerodynamic diameter less than 2·5 μm (PM2·5) with 10 μg/m3 increment for the risk of dementia was 1.03 (1.02-1.05). In contrast, there were no significant associations of dementia incidence with nitrogen oxides, nitrogen dioxide, or ozone. The authors recommended further studies to keep enough number of studies for stable estimates. There are additional studies related to types of dementia and indicators of air pollution.
Cheng et al. also conducted a meta-analysis by specifying two subtypes including Alzheimer disease (AD) and vascular dementia (VaD).2 The age of the subjects was ≥40 years, which was the same age range studied by Abolhasani et al. Pooled HRs (95% CIs) of long-term exposure to PM2·5 with 10 μg/m3 increment for the risk of all-cause dementia, AD, and VaD were 1.40 (1.23-1.60), 1.47 (1.22-1.78), and 2.00 (1.30-3.08), respectively. Period and level of exposure to environmental pollutants may be difficult to determine because there are fluctuations in indoor and outdoor generation sources in each individual. In addition, time trends of exposure may also be difficult to determine by long-term monitoring. Taken together, accumulation of air pollution into the body may be difficult to estimate. It is possible that environmental monitoring data could be used as a substitute for individual exposure to air pollution.
Dhiman et al. conducted a meta-analysis by additional use of particulate matter with an aerodynamic diameter less than 10 μm, carbon monoxide, and sulfur dioxide.3 The increased risk of all-cause dementia, AD, and VaD were recognized only in PM2·5 exposure, presenting pooled HRs (95% CIs) of 1.03 (1.01-1.06), 1.08 (1.01-1.15), 1.03 (1.01-1.06), respectively. Other indicators did not present significant associations with dementia incidence. They also observed that there was a wide variation for evaluating the association between air pollution and cognitive decline. Risk assessment of dementia may be difficult, because many factors would affect cognitive performance such as hypertension, drinking, obesity, smoking, depression, social isolation, lack of exercise, air pollution and diabetes.4
There are many individual risk factors of dementia incidence, and intrinsic and extrinsic factors may interact with each other during long-term periods. Deposition of amyloid beta was closely associated with exposure to air pollution5, and APOE alleles were significant contributors for the progression of cognitive impairment.6 Air pollution may be an important risk factor for AD and other types of dementia, and appropriate indicators, such as PM2·5, should be included in the risk assessment.
References
1. Abolhasani E, Hachinski V, Ghazaleh N, et al. Air Pollution and Incidence of Dementia: A Systematic Review and Meta-analysis. Neurology 2023;100(2):e242-e254.
2. Cheng S, Jin Y, Dou Y, et al. Long-term particulate matter 2.5 exposure and dementia: a systematic review and meta-analysis. Public Health 2022;212:33-41.
3. Dhiman V, Trushna T, Raj D, et al. Is Air Pollution Associated with Increased Risk of Dementia? A Meta-Analysis of Epidemiological Research. Neurol India 2022;70(3):1004-1019.
4. Livingston G, Huntley J, Sommerlad A, et al. Dementia prevention, intervention, and care: 2020 report of the Lancet Commission. Lancet 2020;396(10248):413-446.
Author disclosures are available upon request([email protected]).