Air Pollution: Brain Health And The Toxic Threat

Air pollution has long been known to harm the lungs and cardiovascular system. However, recent studies have found that air pollution may also be a risk factor for neurological disorders such as autism, dementia, and stroke.

Air pollution is a complex mixture of gases, organic compounds, and inorganic compounds. One of the most common components is ozone, a ground-level gas or smog that can blanket a city. Air pollution also consists of small particles of varying types and sizes, which can be breathed in or ingested. These particles can be made up of heavy metals, such as lead, and other neurotoxic metals.

The exact mechanisms by which air pollution affects the brain are not yet fully understood. However, researchers suspect that air pollution affects the brain through direct and indirect pathways. In the first, particles travel up the nose into the olfactory nerves that lead to the brain, where they can directly irritate the tissue. In the second, particles enter the bloodstream through the lungs and make their way to the brain and elsewhere in the body. Other scientists suggest that pollutants indirectly influence the brain by setting off an inflammatory cascade in lung tissue, which in turn dumps irritating substances into the bloodstream.

In addition, air pollution has been found to affect the developing brains of young children. Research from Mexico City, which has high levels of air pollution, suggests that exposure to dirty air may be associated with cognitive deficits and brain abnormalities in children.

Air pollution can cause inflammation in the brain, which is linked to several neurological conditions

Air pollution is known to have harmful effects on the respiratory and cardiovascular systems. However, emerging research suggests that air pollution can also have significant impacts on the brain and mental and cognitive health. According to the World Health Organization (WHO), 99% of the global population is exposed to unsafe levels of air pollution.

The primary way air pollution affects mental health is through inflammation. Air pollution creates inflammatory responses within the body, and chronic inflammation in the brain can damage neurons and the nervous system's regulatory responses. Animal research shows that air pollution can enter the brain and cause inflammation through various pathways:

• By crossing the blood-brain barrier, which works to protect the brain from pathogens and toxins in the blood.
• By entering the olfactory neurons connecting the nose to the brain, where polluted air can damage neurons.
• By entering the stomach and digestive system, where the enteric nervous system, often referred to as the "second brain," can impact mood and well-being.

Chronic inflammation in animals can mimic symptoms like depression and bipolar disorder in humans. While more research is needed to fully understand the impacts of air pollution on the brain, there is sufficient evidence to suggest that it can affect cognitive and mental health throughout an individual's lifetime.

Air Pollution's Impact on the Brain Before Birth

Air pollution has been found to impact brain development even before birth. Toxic air pollution particles, such as black carbon, a component of particulate matter pollution, can be breathed in by pregnant mothers and passed through the bloodstream to the fetus. Exposure to high levels of certain air pollutants during pregnancy has been linked to an increased risk of behavioral problems, attention issues, and anxiety and depression symptoms in children.

Air Pollution's Impact on Children and Adolescents

Air pollution exposure is a significant predictor of academic performance and cognitive and mental health in children. Poor air quality has been associated with lower school attendance rates and lower performance on educational tests. Exposure to high levels of air pollution during childhood has been linked to poorer behavioral functioning, cognitive performance, and increased anxiety and depression.

Air Pollution's Impact on Adults

Air pollution has been linked to various mental and physical health problems in adults, including anxiety, depression, and neurodegenerative disorders. Exposure to elevated levels of particulate matter has been associated with short-term cognitive decline and negative emotions, which can lead to a decrease in social trust.

Air Pollution's Impact on Older Adults

Older adults are also vulnerable to the negative effects of air pollution. Airborne pollutants have been associated with an increased risk of dementia, Alzheimer's disease, and general cognitive decline. Research suggests that air pollution may drive increased dementia risk by causing inflammation in the brain and nervous system and negatively impacting the lungs and heart.

While the exact mechanisms by which air pollution affects the brain are still being studied, the evidence suggests that air pollution can cause inflammation in the brain, which is linked to various neurological conditions and cognitive impairments throughout an individual's lifetime.

Particulate matter in air pollution can enter the brain through the olfactory nerves and the bloodstream

Air pollution is a major environmental health problem that has been associated with cardiovascular morbidity and mortality. An emerging body of evidence has raised concern regarding the potentially harmful effects of inhaled pollutants on the central nervous system. Traffic-related air pollution (TRAP) exposure has been associated with adverse effects on cognitive, behavioural, and psychomotor development in children, and with cognitive decline and a higher risk of dementia in the elderly.

Particulate matter (PM) is a major public health concern. It includes organic and elemental carbon, metals, and polycyclic aromatic hydrocarbons (PAHs). PM can enter the brain through the olfactory nerves and the bloodstream.

Inhaled particles can travel through a nerve running from the nasal cavity into the brain, bypassing the lungs. Ultrafine particulate matter (UFPM) can also pass through the blood-brain barrier, which normally protects the brain. UFPM can also enter the bloodstream through the lungs and make its way to the brain and elsewhere in the body.

Animal studies have shown that inflammation and oxidative stress, identified as common and basic mechanisms through which air pollution causes damage, may also affect the central nervous system by inducing neuronal death or synaptic toxicity. Increasing levels of circulating cytokines due to systemic inflammation may have a peripheral impact on the brain, and air pollutants may themselves be proinflammatory.

The effects of air pollution on the brain are still being studied, but the evidence suggests that particulate matter in air pollution can enter the brain through the olfactory nerves and the bloodstream.

Air pollution has been linked to an increased risk of stroke, with short-term exposure to certain pollutants being associated with an increased risk of ischemic stroke

Air pollution and stroke

Air pollution is an expansive term consisting of a complex mixture of thousands of components from a wide range of different sources. The main pollutants currently recognised to pose a risk to 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).

Short-term exposure to air pollution and stroke risk

Short-term exposure to air pollution refers to the daily variation in air pollution concentration. Many studies have reported an association between short-term increased air pollutant levels and increased risk of ischemic stroke. A 2019 meta-analysis of 28 countries worldwide found a relative increase in hospital admissions or mortality due to stroke of 1.011 per 10 μg/m3 increment in PM2.5 concentration. A 2018 study in China found a 0.208% increase in ischemic stroke for every 12.0 μg/m3 increase in NO2 concentration.

Long-term exposure to air pollution and stroke risk

Long-term exposure to air pollution refers to exposure ranging from weeks to years. A 2019 meta-analysis reported a statistically significant association between long-term exposure to increased PM2.5 levels and all-cause stroke within a 1-4 year exposure window, but not for ischemic or haemorrhagic stroke separately. A 2019 study in China reported an increased risk of ischemic stroke of 1.20 per 10 μg/m3 increase in PM2.5 concentrations over a mean follow-up time of 7.5 years.

Exposure to air pollution during pregnancy can lead to permanent brain damage in the developing foetus

Air pollution and its impact on the brain

Air pollution has been linked to a variety of serious mental and physical health issues, including anxiety, depression, and neurodegenerative disorders. Emerging research shows that it also has significant impacts on our brain and mental and cognitive health. According to the World Health Organization (WHO), 99% of the global population is exposed to unsafe levels of air pollution.

Air pollution and pregnancy

Evidence has emerged showing that air pollution is impacting our brains before we are even born. Toxic air pollution particles have been found in the lungs and brains of foetuses. Black carbon, a component of particulate matter pollution, can be breathed in by the mother during pregnancy and passed through the bloodstream to the foetus, where it has been found in first and second-trimester placenta and in the organs of the foetus.

Air pollution and brain damage in the developing foetus

A study published in Environmental Health Perspectives found that mothers who had higher NO2 exposure during their pregnancy, especially in the first and second trimesters, had children who were more likely to have behavioural problems. Research also shows that those exposed to higher levels of certain urban air pollutants while in utero were more likely to experience attention problems and anxiety and depression symptoms as children.

Higher prenatal exposure to PM10 and PM2.5 was negatively associated with composite cognitive scores. In addition, higher average prenatal exposure to PM10 was negatively associated with composite motor, scaled motor, gross motor, fine motor, composite language, scaled language, and expressive communication scaled scores.

DLMs (distributed lag linear models) showed that higher prenatal air pollution exposure during mid and late pregnancy was inversely associated with motor, cognitive, and communication language scores.

Mechanisms of impact

While research is still relatively early, scientists believe that the primary way air pollution affects mental health is through inflammation. Air pollution is known to create inflammatory responses within the body, and chronic inflammation in the brain can damage neurons and the nervous system's regulatory responses. Animal research shows that air pollution can enter the brain and cause this inflammation through a variety of pathways:

• By crossing the blood-brain barrier, the protective barrier between the brain's blood vessels and the cells that make up brain tissue, which works to create a defence against pathogens and toxins present in our blood.
• By entering the olfactory neurons connecting the nose to the brain, where polluted air is inhaled into the nose and can damage neurons at the site.
• By entering the stomach and digestive system, where the enteric nervous system, often referred to as the "second brain", resides and impacts our mood and well-being.

These findings are especially concerning when considering the disproportionate impacts on low-income communities, communities of colour, and environmental justice communities. Exposure during mid to late pregnancy may be especially detrimental to neurodevelopment, which suggests the need for limiting air pollution exposure, especially during the latter half of pregnancy.

Air pollution has been linked to an increased risk of dementia, with certain pollutants being associated with higher levels of amyloid-beta, a biomarker for Alzheimer's disease

Air pollution has been found to have harmful effects on the brain and cognitive health. Research has shown that exposure to air pollution can lead to inflammation in the brain, which can damage neurons and the nervous system's regulatory responses. This can result in cognitive decline, behavioural issues, and an increased risk of dementia.

A study by Lilian Calderón-Garcidueñas found that children living in Mexico City, a highly polluted area, had higher levels of amyloid plaques and neurofibrillary tangles, which are markers for Alzheimer's disease. The children exposed to urban pollution also had lower scores on memory, cognition, and intelligence tests.

Another study by Melinda Power and colleagues found that exposure to black carbon, a type of particulate matter associated with diesel exhaust, was linked to reduced cognitive performance in older men in Boston. The men exposed to high levels of black carbon showed cognitive decline equivalent to ageing by about two years.

In addition, a study by Frederica Perera and colleagues found that children exposed to high levels of polycyclic aromatic hydrocarbons, a byproduct of fossil fuel combustion, while in utero were more likely to experience attention problems and symptoms of anxiety and depression.

While the exact mechanisms by which air pollution affects the brain are still being studied, the evidence suggests that air pollution can have harmful effects on brain health and increase the risk of dementia and Alzheimer's disease.

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