Each year around the world, approximately 15 million people suffer from a stroke.

Of these, 6 million dies and 5 million are left permanently disabled, making stroke the second leading cause of disability.

What you may not know is that New studies link air pollution as a leading risk factor for stroke worldwide this includes environmental and household air pollution.

The researchers discovered that around 30 percent of the disability associated with stroke is linked to air pollution.

This article discusses the adverse effects of air pollution leading to a stroke.

What is Air pollution?

Air pollution is an umbrella term consisting of a complex mixture of thousands of components from a wide range of different sources. Many of the world’s large cities today have bad air quality owing to urbanization.

The main pollutants currently recognized to pose risk to human health include airborne particulate matter (PM) and gaseous pollutants such as ozone (O3), sulphur dioxide (SO2), carbon monoxide (CO), and nitrogen oxides including nitrogen dioxide (NO2) and nitrogen oxide (NOx).

A study published in the year 2016 reveals, the presence of toxic nanoparticles from air pollution in the human brains in “abundant” quantities.

Thus it is evident that particulate matters (PM) can evade the natural Blood-brain barrier and lodge itself in the brain tissue or blood vessels therein.

What is a Stroke?

A stroke is a “brain attack” it occurs when the brain’s blood supply is blocked or when a blood vessel bursts, resulting in a loss of oxygen to the brain that in turn, injures or kills brain cells.

Having a stroke can result in death or permanent disability, including loss of vision, speech, paralysis, and confusion. The severity of the stroke depends on where it occurs in the brain and how much damage it has caused.

For example, someone who had a small stroke may only have minor problems such as temporary weakness of an arm or leg. People who have larger strokes may be permanently paralyzed on one side of their body or lose their ability to speak.

There are three types of stroke depending on the severity;

  • Ischemic Stroke: Caused by a blockage or clot in a blood vessel in your brain. The blockage can be caused when a substance called plaque builds up on the inside wall of an artery.
  • Haemorrhagic stroke: Caused when an artery in the brain breaks open. The interrupted blood flow causes damage to your brain. High blood pressure weakens arteries over time and is a major cause of haemorrhagic stroke.
  • Transient ischemic attack (TIA): Caused by a small clot that briefly blocks an artery. It is sometimes called a mini-stroke or warning stroke.

The 10 leading stroke risk factors around the world were high blood pressure, low fruit intake, environmental air pollution, high body mass index (BMI), high sodium intake, smoking, low vegetable intake, household pollution, low whole grain intake, and high blood sugar.

How Air Pollution is Leading Risk Factor of Stroke

There are well-documented studies that link air pollution and stroke. Air pollution is known to cause a chronic inflammatory response, oxidative stress, blood vessels endothelial cell injury, and a pro-thrombotic (clotting) state, severe arterial vasoconstriction (narrowing of arteries), and atherosclerosis (blockage of blood vessels or arteries).  All these processes are known to increase the risk of either Ischemic stroke or transient ischemic attack.

The mechanism of these processes are further discussed in detail in following sections –

air pollution and stroke - FB

1. Air pollution can stimulate an inflammatory response leading to stroke

The inflammation theory is that particles inhaled into the lungs are swallowed by white blood cells of the immune system, activating a local inflammatory response within the lung.

Inflammatory cells then spread into the blood circulation leading to an increased blood coagulation that effects the cardiovascular system.

Exposure to urban PM has been shown to cause inflammation in the lung tissue and elevated levels of white blood cells and pro-inflammatory cytokines such as tumour necrosis factor alpha (TNFα), interleukin-1, interleukin-6, C-reactive protein, and fibrinogen.

An overproduction of all these inflammatory mediators can cause blockage and thickening of blood vessels.

Also, many pro-inflammatory mediators are capable of encouraging the process of platelet activation or aggregation. The sticking together of platelets in the blood leading to the formation of a thrombus (clot) is termed as platelet activation or aggregation.

All these inflammatory responses can lead to an event of ischemic injury that is lack of oxygen to the brain tissue, the neighbouring healthy brain cells initiate a severe and prolonged inflammatory process.

This process further fuels in the increased production of pro-inflammatory proteins, and various types of inflammatory cells including neutrophils, different subtypes of T cells, monocyte, macrophages, and other cells into the ischemic brain tissue.

A population-based study conducted in Germany indicates that long-term residential exposure to high levels of PM2.5 is associated with systemic inflammatory markers in humans. This might provide a link between air pollution and coronary atherosclerosis.

Summary

Air pollution induced inflammation potentially magnifies the biological and physical actions of inflammatory cells and its associated proteins leading to the development and progression of the stroke symptoms.

2. Particulate matters induce oxidative stress

Oxidative stress is caused due to an imbalance between pro-oxidants and antioxidants and resulting in excessive production of reactive oxygen species (ROSs) and Reactive Nitrogen Species (RNSs).

Both ROSs and RNSs has two major roles, playing a role in normal biological processes and are also involved in a number of disease processes.

There are several studies linking the effects of PM inhalation and an increased oxidative stress response in the brain as well as other circulatory organs.

It has been shown that exposure to different size fractions of PM may induce biological as well as physical changes in the central nervous system (CNS), where inflammation, oxidative stress, and impaired protein structures are effectively activated.

ROSs cause damage to cell structures, including lipids, membranes, proteins, and DNA leading to cell-death via autophagy, apoptosis, and necrosis.

The “cerebral vasculature” meaning the network of blood vessels in the brain, is a major target of oxidative stress playing a critical role in the development of ischaemic brain injury.

All these damages trigger an inflammatory response in the brain cells leading to an inflammation-related process as mentioned in the above section responsible in the development and progression of a stroke.

Summary

Oxidative stress and its products ROSs or RNSs cause both physical and functional damages to major cell organelles leading to an inflammatory process that can cause a stroke.

3. Air pollution exposure can cause impaired vascular functions

Several studies have linked exposure to combustion-derived air pollution with an increase in cardiovascular diseases and mortality (death).

Living in proximity to major roadways is associated with cardiovascular events including increased atherosclerosis and thickening of the walls of the heart’s main pumping chamber in adult populations.

ROSs and its derivatives have been shown to cause the dilation of the blood vessels (vasodilation) via the opening of potassium channels and altered vascular reactivity, the breakdown of the blood-brain barrier and focal destructive lesions in animal models of ischaemic stroke.

Vasodilation increases the blood flow which in turn increases the level of oxygen supplied to the tissue and stimulates the second burst of ROS generation, which leads to an ischemia-reperfusion injury.

Ischemia-reperfusion injury is the tissue damage caused when blood supply returns to tissue (re- + perfusion) after a period of ischemia or lack of oxygen (anoxia or hypoxia)

Exposure to diesel exhaust leads to the generation of oxidative stress in the blood vessels that decrease the availability of Nitrogen Oxide (NO), a key regulator of the diameter of blood vessels (vascular tone) and blood pressure.

A decrease in NO bio-concentration leads to the disruption of two main functions of blood vessels in humans, the regulation of vascular tone and the breakdown of clots in blood vessels and arteries (endogenous fibrinolysis).

Impairments in these biological process lead to reversible cerebral vasoconstriction, and platelet activation or aggregation.

The reversible cerebral vasoconstriction syndrome (RCVS) is a rare condition that occurs as the result of a sudden constriction (tightening) of the vessels that supply blood to the brain.

Summary

Oxidative stress in the brain’s blood vessels caused due to the air pollution exposure, is responsible for a number of biological and physical damages, resulting indirect effects on atherosclerotic plaque stability, the vascular endothelium, platelet function, and thrombosis that collectively leads to the build-up of ischemic-stroke.

4. Air Pollution is associated with Ischemic Stroke via Cardiogenic Embolism

Studies that compared ischemic and haemorrhagic stroke suggest that the association between air pollutants and stroke is mainly confined to ischemic stroke.

A cardioembolic stroke a subtype of an ischemic stroke occurs when the heart pumps unwanted materials into the blood circulation leading to brain, resulting in the blocking of a blood vessel and damage to the brain tissue termed as “cardiogenic embolism”.

Cardioembolic stroke accounts for approximately 20% of ischemic strokes each year.

A study showed that the short-term exposure to particulate matter <10 μm (PM10), NO, black carbon, and SO concentrations were independently associated with an increased risk of cardioembolic stroke.

The study also suggested that the pre-existing risk factors for stroke, such as the geographic factors, lifestyle-related risk factors (diet, obesity, etc.) and pre-existing medical conditions like diabetes, existing heart diseases can increase the vulnerability to air pollution related to ischemic stroke.

Summary

It is a known fact that PM2.5 can translocate itself in the lungs, heart, and brain through the air-blood circulatory system and can cause physical and biological damages that can lead to an ischemic stroke.

5. Particulate matters inhalation is responsible for a stroke

PM2.5 air pollution promotes the development of cerebrovascular atherosclerosis by inducing macrophage (type of large immune cells) production, and the consequent cytokine release stimulates systemic superoxide radicals production by neutrophils.

These superoxide radicals rapidly increase the concentration of ROS in the bloodstream.

Many blood-borne molecules are subsequently oxidized, but the oxidation of low-density lipoproteins (LDLs) also known as bad cholesterol contributes to atherosclerotic pathogenesis.

Oxidized LDLs have a high affinity for sub-endothelial macrophages (white blood cells).

These macrophages engulf large quantities of oxidized LDLs and become foam cells, which contribute to the size and instability of sub-endothelial plaques.

Thus, people having high LDLs or cholesterol concentration are more susceptible to the increased risk of stroke and myocardial infarction, which is observed in populations in high PM areas.

Summary

Particulate matters (PM2.5) plays a key role in development of atherosclerosis plaque in the blood vessels leading to the brain.

Air pollution can increase the rate of mortality in stroke patients

Air pollution ranks within the top five risk factors for mortality in emerging economies such as India and China.

It was estimated that 99% of deaths attributed to household air pollution and 89% of deaths attributed to outdoor air pollution occurred in low and medium income countries.

The Global Burden of Diseases study estimated that in 2015, air pollution accounted for 19% of all cardiovascular death, 21% of deaths due to stroke and 24% deaths due to ischaemic heart disease.

The study accounted that emissions from residential energy use such as heating and cooking, prevalent in India and China, have the largest impact on stroke mortality.

Patients with existing cardiovascular disease share many risk factors that can increase stroke mortality such as obesity, hypertension, smoking, poor diet, alcohol consumption, and air pollution.

Air pollution differs from other risk factors because exposure to air pollution, for the large majority of the population, is unavoidable.

In several studies conducted in Korea and China, it was concluded that ischemic and haemorrhagic stroke, was found to be significantly associated with short-term exposure to air pollutants.

In another study conducted in the Netherlands, it was shown deaths due to cerebrovascular diseases, and atherosclerosis was more strongly associated with air pollution than cardiovascular deaths alone.

 This study shows that people with existing cerebrovascular diseases are more susceptible to stroke mortality when exposed to air pollution.  

However, lowering the level of air pollution exposure could considerably reduce the risk of stroke mortality along with other lifestyle-related modifications.

Are You Safe Indoors? Indoor Pollution & Stroke

Worldwide, one in eight deaths is attributed to the indoor pollution often from fuel burned indoors for cooking. Seven million people die each year because of exposure to air pollution.

In Asia, cooking and heating with solid fuels are the main sources of indoor air pollution, whereas in Europe, several sources contribute to indoor pollution, such as volatile organic compounds from organic solvents, household products and PM from cooking and wood burning.

In a population-based study in China showed that 24 of the subjects who had no previous history of heart disease and stroke died from stroke.

Fumes from the biomass fuel and coal used for domestic purposes were found to be an independent risk factor for stroke in addition to diastolic blood pressure, age, and cigarette smoking.

Daily cigarette smoking increases the risk of fatal stroke by three and a half times. Combined cigarette and pipe or cigar smoking had a higher risk than smoking cigarettes only.

Passive-smoking is equally as harmful and considered a major risk factor leading to stroke.

Nevertheless, a recent European study demonstrated that 60% of the global burden of indoor air pollution-related disease comes from outdoor PM2.5 penetrating indoors via air exchange.

Using ventilation to dilute contaminants, filtration, and source control are the primary methods for improving indoor air quality in most buildings.

Conclusion

There is significant evidence linking air pollution and cardiovascular diseases including stroke.

Population-based studies have demonstrated that both short- and long-term exposure to air pollution increases the risk of stroke.

Exposure studies in humans and animals alike as well as experimental studies have provided insight into the physical and biological mechanisms leading to the initiation of the dysfunctional endothelial cells that line the interior surface of blood vessels, atherosclerosis, platelet activation, and susceptibility for coagulation.

Air pollution should be recognized more widely as one of the most important modifiable risk factors for the prevention and management of cardiovascular disease.

Healthcare professionals have an important role in promoting the awareness of this data, not just to improve the health of individual patients, but also to place pressure on policy makers for air pollution to be a public health priority.

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Team AirHealth.in
Author

This article is written by AirHealth's team which consists of writers with degrees in medicine, biotechnology, pharmacy, nutrition, engineering, etc. The work is peer-reviewed by the core team or by our Advisors before it is published. Check out more about us in "Our Team" section.

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