Air Pollution: Silent Killer, Stroke Risk

Air pollution is a major environmental problem and a leading cause of mortality and morbidity, contributing to millions of deaths and disability-adjusted life years (DALYs) globally. Evidence suggests that air pollution is an emerging risk factor for stroke, with increasing air pollution levels estimated to be responsible for a growing number of stroke-associated deaths. The biological mechanisms underlying the association between air pollution and stroke are still under investigation, but studies have found a positive association between air pollution exposure and stroke hospital admissions, incidence, and mortality. While the relative risk at an individual level may be small, the widespread exposure to air pollution poses a significant public health concern, especially in low and middle-income countries.

Air pollution is a leading cause of strokes and deaths worldwide

Air pollution is a major environmental problem and a significant risk factor for various cardiovascular diseases, including strokes. It is now recognised as a leading cause of morbidity and mortality, contributing to millions of deaths and disability-adjusted life years (DALYs) worldwide.

Several studies have found a positive association between exposure to air pollution and an increased risk of stroke hospital admissions, incidence, and mortality. The Global Burden of Disease Study (GBD) reported approximately 80.1 million stroke patients and 5.5 million deaths attributed to stroke globally in 2016. Air pollution is estimated to be responsible for 14% of all stroke-associated deaths, making it the sixth leading cause of stroke death globally from 1990 to 2017.

The adverse health effects of air pollution are well-known, but the underlying biological mechanisms through which pollutants affect the vasculature are still under investigation. Evidence suggests that smaller nanoparticles, such as PM2.5, can penetrate the alveolar spaces of the lungs and potentially enter the bloodstream, leading to systemic inflammation, oxidative stress, thrombosis, and vascular endothelial dysfunction. These pathways are believed to contribute to the development of strokes.

Additionally, air pollution is of particular concern in low and middle-income countries, where rapid industrialisation is expected to increase pollution levels. Studies indicate that 99% of deaths attributed to household air pollution and 89% of deaths linked to ambient air pollution occur in these regions. The impact of air pollution on stroke risk is influenced by individual factors such as age, health status, and traditional cardiovascular risk factors like diabetes, hypertension, and smoking.

Overall, the growing body of evidence highlights the significant role of air pollution in causing strokes and contributing to mortality worldwide. Addressing this issue and reducing exposure to air pollutants, especially in vulnerable populations, is crucial for public health and the reduction of stroke-related morbidity and mortality.

Outdoor air pollution is a major public health issue

The continuous increase in air pollution levels over the past decades has resulted in a corresponding rise in its contribution to stroke-associated deaths. It is now estimated that air pollution is responsible for around 14% of all deaths related to strokes. This figure is even higher in low and middle-income countries, where air pollution is projected to rise due to rapid industrialisation. The Global Burden of Disease Study (GBD) reported approximately 5.5 million deaths from stroke in 2016 globally, with stroke being the second most common cause of death worldwide.

While the exact biological mechanisms by which air pollutants affect the vasculature are still being debated, several pathways have been proposed. These include systemic inflammation, oxidative stress, thrombosis, and vascular endothelial dysfunction. Smaller nanoparticles, such as PM2.5, can penetrate the alveolar spaces of the lungs and potentially enter the bloodstream, reaching systemic organs. These particles, often derived from combustion, contain a range of harmful chemicals, including reactive transition metals and organic hydrocarbons, which are believed to contribute to inflammation and oxidative stress.

The association between air pollution and stroke is complex, and the interpretation of previous literature is challenging due to the lack of distinction between ischaemic and haemorrhagic strokes and their respective causes. However, recent studies have employed various approaches, including case-crossover and time series analyses, to better understand the relationship. These studies have found significant associations between air pollution and stroke hospital admissions, incidence, and mortality, particularly in relation to PM10 and O3 pollutant levels.

Long-term exposure to PM 2.5 increases the risk of strokes

Air pollution is a major public health issue, contributing to millions of deaths and disability-adjusted life years worldwide. It is now recognised as one of the leading causes of mortality and morbidity, with stroke being a significant contributor.

While the specific pathophysiological mechanisms of air pollutants leading to stroke remain uncertain, increasing evidence suggests that long-term exposure to PM 2.5 is an important risk factor for stroke. PM 2.5 refers to particulate matter with an aerodynamic diameter of ≤2.5 μm, which can be inhaled and penetrate the alveolar spaces of the lung. These particles may even infiltrate the bloodstream and reach systemic organs. The smaller the nanoparticles, the greater the risk of adverse health effects.

A meta-analysis of 16 cohort studies with over 2.2 million participants found a significant association between long-term PM 2.5 exposure and the incidence of stroke. The pooled hazard ratio for each 5 μg/m3 increment in PM 2.5 concentration was 1.11 for both the incidence and mortality of stroke. This means that for every 5 μg/m3 increase in PM 2.5 levels, the risk of stroke increases by approximately 11%. Region-specific analyses showed a significant association in North America and Europe, while the pooled result for Asia was not significant.

Another review and meta-analysis of 42 studies published by December 31, 2019, found that a 10-µg/m3 increase in long-term PM 2.5 exposure was associated with an increased risk of 24% for cerebrovascular mortality and incident stroke. This suggests that long-term exposure to PM 2.5 increases the likelihood of stroke and stroke-related death, with the risk rising in line with PM 2.5 concentration levels.

Inconsistent results in studies on the association between air pollution and strokes

Several factors contribute to inconsistent results in studies investigating the association between air pollution and strokes. Firstly, most studies use the term "stroke" as an umbrella term, encompassing both ischaemic and haemorrhagic strokes without differentiating between these distinct subtypes and their unique causes and associations with cardiovascular risk factors. This lack of differentiation can lead to challenges in interpreting the literature and understanding the specific relationships between air pollution and different types of strokes.

Secondly, the methods for measuring air pollutants and estimating individual exposure can result in exposure misclassification. Many studies extrapolate data from ambient air pollutant concentrations reported at a regional level or recorded by the nearest fixed air quality monitor. This approach assumes uniform exposure across an area without adequately accounting for within-region variability or differences in personal exposure. For example, individuals may spend significant time outside their residential areas for work or vacation, leading to potential underestimation or overestimation of exposure levels. Additionally, these methods often fail to differentiate between time spent indoors and outdoors, which can vary across age groups and between men and women.

Thirdly, the majority of studies rely on administrative data, which typically lacks detailed clinical information. This limitation can result in misclassification of stroke occurrence, making it challenging to establish precise associations between air pollution and stroke events. Furthermore, many studies do not adequately adjust for traditional risk factors for cardiovascular disease, such as diabetes, hypertension, smoking, and socioeconomic deprivation. These factors can act as confounders in the relationship between air pollution and stroke, potentially influencing the results and contributing to inconsistencies.

While epidemiological studies have reported significant associations between air pollution and stroke, establishing a direct causal relationship between specific air pollutants and stroke incidence remains challenging. This complexity arises from the various biological mechanisms through which air pollutants may exert their effects on the vasculature, a topic that is still under active investigation. Nonetheless, an increasing number of human epidemiological studies continue to explore the potential links between air pollution exposure and stroke hospital admissions, incidence, and mortality, with some studies reporting positive associations while others do not.

Air pollution is a significant environmental risk factor for all-cause mortality

It is a particular concern in low- and middle-income countries, where air pollution is projected to rise due to rapid industrialisation. In these countries, 99% of deaths attributed to household air pollution and 89% of deaths attributed to ambient air pollution occurred. Overall, air pollution is the sixth-leading cause of stroke death globally, and 28.1% of DALYs of stroke are attributed to environmental factors exposure.

The adverse health effects of air pollution are well-known, but the impact on the cardiovascular system is less widely understood. Evidence shows a strong association between air pollution and cardiovascular diseases, including stroke. While the relative risk for an individual may be small, the widespread exposure to air pollution means the absolute risk at a population level is significant. The biological mechanisms through which air pollutants affect the vasculature are still under discussion, but nanoparticles penetrating the alveolar spaces of the lungs and entering the bloodstream are thought to be a crucial factor.

Several studies have found a positive association between air pollution and stroke hospital admissions, incidence, and mortality. For example, a meta-analysis of over 23 million participants found an association between exposure to air pollution and an increased risk of stroke hospital admissions, incidence, and mortality for various pollutants, including PM2.5, PM10, SO2, and NO2. Another study by NYU Langone researchers found that long-term exposure to PM2.5, or fine particulate matter, was associated with a greater chance of stroke. These findings suggest that reducing fine particulate matter could lower the risk of stroke.

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