Air Pollution: Brain Health And The Toxic Threat

Air pollution is a major environmental health problem that has been associated with cardiovascular morbidity and mortality. Common sources of outdoor air pollution include the combustion of fossil fuels and industrial and agricultural processes. In the last decade, an emerging body of evidence has raised concerns 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 mental development and behavioural functions such as attention, reduced global IQ, a decrease in memory and academic performance, and higher prevalence of attention-deficit hyperactivity disorder and autism spectrum disorder.

Findings from epidemiological studies also provide support for a relation of TRAP exposure to cognitive decline and dementia in the elderly. 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.

Recent studies have interfaced environmental epidemiology with magnetic resonance imaging to investigate the effects of TRAP on the human brain. The findings revealed that cerebral white matter, cortical grey matter, and basal ganglia might be the targets of TRAP. The detected brain damage could be involved in cognition changes.

Evidence suggests that air pollution can enter the brain directly and indirectly. Inhaled particles can travel through a nerve running from the nasal cavity into the brain, bypassing the lungs. Ultrafine particulate matter (UFPM) carried in the bloodstream can pass through the barrier that normally protects the brain. Animal studies have also shown that chemical markers of brain inflammation can increase due to air pollution.

In humans, emerging evidence suggests links between air pollution and harmful brain effects in the elderly. Two recent studies revealed associations between air pollution and dementia. A study in Ontario, Canada found that the risk of dementia increased the closer people lived to major roadways. A study of US women showed a higher risk of cognitive decline and dementia for those exposed to levels of fine particulate matter above the national standard.

While research is still relatively early, scientists believe that the primary way air pollution affects mental health is through the avenue of 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, or the protective barrier between the brain’s blood vessels and the cells that make up brain tissue that 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.

Air pollution can enter the brain through the olfactory nerve, bypassing the lungs

The olfactory nerve is a pathway that allows air pollutants to enter the brain and potentially cause harm. Studies have shown that exposure to air pollution, especially particulate matter, can lead to an increase in chemical markers of brain inflammation. This indicates that air pollution can have direct effects on the brain and is associated with potentially harmful consequences.

The impact of air pollution on the brain is an emerging area of research, and more studies are needed to fully understand the mechanisms by which pollutants enter the brain and their specific effects. However, the existing evidence suggests that air pollution can have negative effects on brain health and cognition. For example, studies have found that exposure to air pollution is associated with poorer performance on standardised psychological tests and an increased risk of dementia.

In conclusion, air pollution can enter the brain through the olfactory nerve, bypassing the lungs, and this pathway may contribute to the harmful effects of air pollution on brain health and cognition.

Air pollution can cross the blood-brain barrier

Air pollution can enter the brain through various pathways, including via the olfactory nerve and by crossing the blood-brain barrier.

The blood-brain barrier is a protective barrier between the brain's blood vessels and the cells that make up brain tissue. It works to create a defence against pathogens and toxins present in the blood. However, studies have shown that air pollutants can cross this barrier and enter the brain.

Animal studies have shown that air pollutants can enter the brain through the olfactory nerve, which connects the nose to the brain. Inhaled pollutants can travel up the nerve and enter the brain, causing damage to neurons at the site.

Air pollutants can also enter the bloodstream and be distributed around the body. Small particles can slip through the plasma membrane of alveoli (the tiny air sacs in the lungs) and get picked up by capillaries. These particles can then cross the blood-brain barrier and enter the brain.

Once in the brain, air pollutants can cause inflammation and oxidative stress, which can lead to neuronal death and synaptic toxicity. This can have harmful effects on brain function and cognition.

Studies have found that exposure to air pollution is associated with poorer performance on cognitive tests and an increased risk of dementia and other neurological problems.

More research is needed to fully understand the impacts of air pollution on the brain and to determine which pollutants are the most harmful. However, the evidence suggests that air pollution can cross the blood-brain barrier and have negative effects on brain health.

Air pollution can cause inflammation in the brain

Air pollution can enter the brain through various pathways. Animal studies have shown that inhaled pollutants can travel through the olfactory nerve, which connects the nose to the brain, bypassing the lungs. Pollutants can also enter the bloodstream and pass through the blood-brain barrier, which normally protects the brain from harmful substances.

Air pollution has been linked to increased inflammation in the brain, which can damage neurons and disrupt the brain's regulatory responses. This can lead to a range of cognitive and mental health issues, including anxiety, depression, and neurodegenerative disorders.

Who is most at risk?

While air pollution affects everyone, certain groups are more vulnerable to its effects on the brain. Fetuses, children, and older adults are particularly susceptible to the harmful impacts of air pollution on brain health and cognitive function. Additionally, people living in disadvantaged communities, such as low-income areas and communities of color, often face higher levels of air pollution due to historical discrimination and redlining.

Reducing air pollution exposure is crucial to protecting brain health. This can be achieved through individual actions, such as wearing masks or installing air filtration systems, and societal efforts, such as implementing stricter air quality regulations and transitioning to cleaner energy sources.

Air pollution has been linked to poorer performance in mazes and other learning and memory problems

In humans, children exposed to higher levels of air pollution have been found to have lower scores on memory, cognition, and intelligence tests. A study of 200 children in Boston found that those exposed to the highest levels of black carbon had the lowest scores on memory and verbal and non-verbal IQ tests. Another study of 200 children in New York City found that children exposed to higher levels of urban air pollutants while in utero were more likely to experience attention problems and symptoms of anxiety and depression.

In adults, air pollution has been linked to a variety of mental and physical health problems, including anxiety, depression, and neurodegenerative disorders. A study of almost 20,000 women between the ages of 70 and 81 found that those exposed to the highest levels of fine particulate matter air pollution experienced the greatest cognitive decline. Another study of older women in the US showed that exposure to levels of fine particulate matter above the national standard was associated with a higher risk of cognitive decline and dementia.

The mechanisms by which air pollution impacts mental and cognitive health are still being studied. However, it is believed that air pollution can enter the brain and cause inflammation through various pathways, including crossing the blood-brain barrier and entering through the olfactory neurons connecting the nose to the brain. Air pollution is also known to create inflammatory responses within the body, and chronic inflammation in the brain can damage neurons and the nervous system's regulatory responses.

Air pollution has been associated with an increased risk of dementia

Further studies have shown that air pollution can affect the brain directly and indirectly, with inhaled particles travelling through a nerve running from the nasal cavity into the brain, bypassing the lungs, and entering the bloodstream and passing through the barrier that normally protects the brain. Animal studies have shown that air pollution can cause inflammation in the brain, with increased levels of circulating cytokines due to systemic inflammation possibly having a peripheral impact on the brain.

In humans, emerging evidence suggests links between air pollution and harmful brain effects in the elderly, with studies revealing associations between air pollution and dementia. A study in Ontario, Canada found that the risk of dementia increased the closer people lived to major roadways, and a study of US women showed a higher risk of cognitive decline and dementia for those exposed to levels of fine particulate matter above the national standard.

With 99% of the global population exposed to unsafe levels of air pollution, it is important to recognise the impact of air pollution on brain health and to take action to reduce air pollution exposures and protect public health.

Frequently asked questions