Outdoor air pollution causes millions of deaths each year — 4.2 million premature deaths globally, according to the World Health Organisation. Over time, fine particles of inhaled smog cause cardiovascular and lung issues, like lung cancer and stroke.
These air pollutants may have equally insidious effects on the brain. Over the past decade or so, in both animals and humans, in the lab and in the real world, scientists have documented associations between air pollution and brain-related issues, like anxiety, poor attention, and memory deficits. Children appear to be especially susceptible.
Fine particulate matter, meaning air pollutants measuring 2.5 microns across or less (30 times smaller than the width of a human hair), are the likely culprits.
“Once they are inhaled,” said Devon Payne-Sturges, an environmental health researcher at the University of Maryland, “they can reach to the really deep areas of the lung and they can get into the bloodstream and get carried into the brain.” Fine particulate matter is small enough to freely cross the blood-brain barrier that keeps out larger toxins. Alternatively, if inhaled through the nose, the fine particles can get to the brain via the olfactory nerve in the nasal cavity.
Once the particles arrive, in rodent brains at least, neural immune cells called microglia move in to engulf and destroy the particles. A similar process may occur in humans. However, the microglia may not remove all particles, leaving them to accumulate in the brain. These particles can trigger inflammation which may lead to more serious brain disorders and cognitive effects.
Scientists have grown increasingly concerned about these effects in children, who have more permeable blood-brain barriers. “Some researchers are beginning to use MRI scans to look at these neurodevelopmental impacts of pollutants, and showing actual changes in the brain,” said Payne-Sturges.
In a recent study, Kelly Brunst and her team from the University of Cincinnati looked at how traffic-related air pollution exposure might affect the developing brain and children’s mental health. “Our study was one of the first to link traffic pollution with functional changes in the brain that are also associated with anxiety,” said Brunst. Scientists had documented associations between anxiety and air pollution before, but this work, published in August in Environmental Research, showed how the brain was involved.
Brunst and her colleagues analysed MRI scans of 145 12-year-olds and used their address information to estimate their recent exposure to pollution, including fine particles. The researchers also factored demographic information into their analysis to ensure that things like socioeconomic status and race weren’t responsible for any effects.
After comparing the preteens’ recent traffic-related air pollution exposure levels, reported anxiety symptoms, and brain imaging data, the team found that children with higher levels of pollution exposure also reported more generalised anxiety symptoms. Children with greater symptoms also had higher levels of a chemical called myo-inositol in an area of the brain that processes emotions, the anterior cingulate cortex. Myo-inositol is normally found in the brain, but abnormal levels are associated with brain disorders.
Anxiety is a complex disorder, and in this study, the disruption of myo-inositol accounted for just 12 per cent of the association between traffic pollution and anxiety symptoms. The symptoms were primarily caused by other factors.
“But,” Brunst said, “on a wider population level, what could be happening is that air pollution [exposure] can result in increased risk for anxiety on the whole.”
“It’s really quite an interesting paper,” said Frederica Perera, an environmental health researcher at Columbia University who was not involved in the research.
Perera herself has conducted several studies examining the effects of pollution exposure, focusing on American minorities in particular. In one study of 40 New York City minority children and their mothers, Perera and her team sought to determine if a component of city smog might affect cognition and externalising behaviour, like aggression or hostility.
This study, published in 2015 in JAMA Psychiatry, looked at children’s prenatal exposure to polycyclic aromatic hydrocarbons (PAH). PAH is a component of particulate matter released into the air by burning fossil fuels, garbage, tobacco, and wood, and is common in minority and low-income communities. PAH also easily crosses into the placenta.
Researchers had the women wear monitors for two days during their third trimester to estimate their chronic PAH exposure. When the children reached 7-9 years of age, they came to Columbia University for academic tests, questionnaires, and MRI scans.
Children with greater prenatal PAH exposure had lower white matter volume in the left side of the brain. White matter is a pale, fatty material that insulates neurons, helping them communicate with each other via electrochemical signals efficiently.
The lower white matter volumes, in turn, were correlated with more externalising behaviours, ADHD symptoms, and slower processing speed. But these findings may not be generalisable to all children, since researchers only studied 40 children and focused on pollutant exposure during a slim prenatal window.
In 2018, Perera took a closer look at these findings. Examining a demographically similar sample of 351 children, she saw that those with high PAH exposure who also dealt with lack of nutritious food, housing, utilities, and clothing during their lives showed more ADHD symptoms than those who didn’t experience these hardships. The researchers write that these results support the idea that pollution acts with social stressors to affect neurodevelopment.
“The whole purpose of this [research] is to get a better handle on the risks to the young, and to use that information to help guide public health and environmental policies,” said Perera.
Although scientists still aren’t sure how fine particulate matter exerts effects on the brain, the evidence for a causal association is convincing. In its draft of the Integrated Science Assessment for Particulate Matter published last October, Environmental Protection Agency scientists concluded for the first time that the association between long-term exposure to air pollution and nervous system issues is “likely to be causal.”
“But,” said Payne-Sturges, while the assessment might be a step forward in recognising the science, “it doesn’t mean anything until the administration acts on it.”
In the meantime, people most affected by air pollution, such as children from low-income families and children of colour, will continue to bear most of the burden, as polluting sources and heavily trafficked roadways often coincide with these communities. “We are all exposed and might be affected, but certain groups are likely to suffer more because of the greater exposure,” said Perera.
Scientists now have good reason to believe that airborne pollution directly affects the brain. Additional research in the coming years is likely to reveal a lot more about what effects these tiny particles have on our mental and physical well-being.