Air pollution linked to increased risk of stroke and dementia transition, study reveals

By Neha MathurDec 17 2023Reviewed by Sophia Coveney

In a recent study published in the journal eClinicalMedicine, researchers assess the detrimental effects of long-term air pollution exposure on the dynamic transitions of stroke and dementia, especially the influence of air pollution during different time intervals, using the United Kingdom (UK) Biobank data.

Specifically, they evaluated the health effects of multiple air pollutants in ambient air. This comprised a mixture of both particulate matter (PM) and gaseous pollutants.

Background

The outcome of stroke can be fatal, especially among aged people. Stroke accounts for 11.6% of all global deaths and is a leading cause of neurological death and disability worldwide.

In 2019, more than 50 million people had dementia. This is projected to increase to 152 million by 2050.

Given the non-availability of interventions to prevent dementia onset and fatal stroke outcomes, investigation of the alterable risk factors of both conditions remains a priority.

Notably, both stroke and dementia create reciprocal risks, which necessitates investigation of the risk factors involved in the “transition” from stroke to comorbid dementia and dementia to comorbid stroke.

Furthermore, emerging evidence suggests that the risk from stroke or dementia to comorbidity varies with the duration of the disease. Thus, studying its impacts (that changes over time) could be critical to optimizing prevention and management strategies for stroke or dementia.

Study methodology

In the present study, researchers first calculated the air pollution score using levels of pollutants: PM_2.5, PM_2.5-10, PM_coarse, and nitrogen dioxide (NO₂), measured in the study area using passive samplers.

Next, they combined these measurements with residential addresses (of participants) to develop LUR models to elucidate any significant spatial variance in air pollutants.

Finally, they created an air pollution score using principal components analysis (PCA) based on the measured pollutants. The team followed up with all participants until lost to follow-up, death, or February 2020.

The researchers then identified stroke and dementia cases using the UK Biobank data from death registries and hospitals.

They used Cox proportional hazard models to assess the associations of air pollution with stroke, dementia, comorbidity, and all-cause mortality. Multi-state models were used to determine the association of air pollution with the dynamic transitions of stroke and dementia. 

The results, i.e., each interquartile range (IQR) increase in air pollution score and individual air pollutants, were presented as hazard ratios (HRs) with 95% confidence intervals (CIs).

The study models further stratified the observed associations between air pollution and transitions according to age (<65 vs. ≥65 years), gender (female vs. male), Townsend deprivation index (<median vs. ≥ median), and education (university/college degree vs. others), and assessment center.

Finally, they performed several sensitivity analyses to assess the robustness of the results.

Results

Of 413,372 participants (54.2% females) included in the primary analysis, the number of individuals diagnosed with stroke or dementia during a 10.9-year follow-up period was proportionally low: 6,484 (1.6%) with stroke and 3,813 (0.9%) with dementia.

Some participants were diagnosed with both conditions; 238 participants with stroke developed dementia, and 138 participants with dementia suffered a stroke.

Cox proportional hazard and competing risk models revealed positive associations between air pollution, specifically increased PM_2.5 and NO₂, dementia, comorbidity of both conditions and all-cause mortality. Likewise, multi-state models showed a link between air pollutants and dynamic transitions of stroke and dementia.

Potential confounders did not markedly modify the observed association between air pollution and transitions from stroke or dementia to comorbidity, and the study results also remained robust in sensitivity analyses.

The current US Environmental Protection Agency (EPA) annual standards for air pollution are PM_2.5: 12 μg/m³, NO₂: 53 ppb, and the limits of the UK and the European Union (EU) are PM_2.5: 20 μg/m³, NO₂: 40 μg/m³ and PM_2.5: 25 μg/m³, NO₂: 40 μg/m³, respectively.

The study found that air pollutants below these standards were also associated with the transition from stroke to comorbid dementia, with increased risks within one year and over five years after stroke, with the HR of 1.61 and 1.82 (95% CI) for each IQR increase in the air pollution score, respectively. 

Although air pollution had no association with the overall transition from dementia to comorbid stroke, it still adversely impacted this transition during 2–3 years, with an HR of 1.75 (95% CI) for each IQR increase in air pollution score.

Conclusions

The study findings provide novel evidence for reducing the risk of neurological disorders related to air pollution during critical time intervals. 

Considering that many countries’ air quality standards are still above the World Health Organization (WHO) Global Air Quality standards, updated in 2021, implementing stricter controls for air pollution could help alleviate the burden of these diseases.

Furthermore, research on pathophysiological mechanisms governing these associations is needed.