Is Dirty Air Bad for Your Brain? – September 2024

Researchers have long known of the impact of air pollution on human health. There are numerous studies linking air pollution exposure and increased risk of cardiovascular and respiratory disease. A less studied topic, but one for which there is mounting evidence, is the effect of air pollution exposure on cognitive abilities. Recent studies have shown links between air pollution exposure and short-term cognitive impairment as well as long-term cognitive decline.

Air pollution is often categorized by the amount and size of particulate matter, or PM, that is present. PM refers to the mixture of solid particles and liquid droplets suspended in air from sources such as industrial emissions, vehicle exhaust, fires, construction sites, and unpaved roads, to name a few. The US EPA regulates two categories of particle pollution based on size: PM₁₀ is coarse inhalable particles with diameters of 10 micrometers or less, and PM_2.5 is fine inhalable particles with diameters of 2.5 micrometers (about 30 times smaller than the width of a human hair) or less.⁽¹⁾ Fine particles can more easily penetrate the body’s defenses and be inhaled deeply into the lungs, potentially leading to serious health issues. The US EPA recently set the National Ambient Air Quality Standard (NAAQS) for PM_2.5 at 9.0 µg/m³ (annualized average), down from
15.0 µg/m³ in 2012.⁽²⁾

Like the way inhaled PM has been shown to cause irritation and trigger nerves that regulate heart rate and breathing, or incite immune responses to inflammation that accelerate plaque formation in arteries and increase the risk of heart attack and stroke⁽³⁾, exposure to PM is believed to affect the brain and central nervous system (CNS) via analogous pathways. Studies have shown that inhaled particles can travel from the nasal cavity to the brain via the olfactory nerve and bypass the lungs altogether. Once in the bloodstream, ultrafine particles can pass though the blood-brain barrier and cause neuroinflammation and oxidative stress.^(4,5) Once in the brain, particles may accumulate and contribute to the formation of amyloid plaques, a hallmark of Alzheimer’s disease.

A study led by Dr. Jennifer Weuve of Rush Medical College examined the impact of long-term exposure to high levels of PM pollution on the rate of cognitive decline in older women. Data were gathered from a population of more than 19,000 women across the United States, between the ages 70 and 81, using address history to estimate PM exposure over a period of 7 to 14 years. The research showed increased rates of cognitive decline, as measured by cognitive skill tests, in women with long-term exposure to high levels of pollution compared to other women of similar age.⁽⁶⁾

A similar study led by Melinda Power of the Harvard School of Public Health examined the correlation between exposure to black carbon, a type of fine PM associated with diesel exhaust, and cognitive performance in a sample of 680 older men in Boston. Each subject’s exposure was determined by cross-referencing their address with established models that provide daily estimates of black carbon concentrations around the Boston area. The research showed reduced cognitive performance, equivalent to nearly 2 years of aging, in men exposed to high levels of black carbon compared to those with less exposure.⁽⁷⁾

Research has also shown that the impact of air pollution exposure on cognitive abilities is not just a concern for the older population but can affect young minds as well. Studies have found that children exposed to higher levels of urban pollution and fossil fuel combustion byproducts often score worse on tests of memory and IQ and are more likely to experience symptoms of anxiety and depression.^(8,9)

In a first of its kind randomized double-blind crossover study, a reduction in short-term exposure to indoor particulate air pollution resulted in improved test scores among a sample of college students in China. There were 162 participants randomly assigned to two similar classrooms and completed a standardized test on consecutive weekends. Air purifiers were placed in each classroom, one with active filtration and one with the filter removed, and were switched between the two test days. Average particulate pollution reductions of 56% for PM₁₀ and 68% for PM_2.5 in the classrooms with active filtration corresponded with higher test scores, suggesting improved cognitive function.⁽¹⁰⁾

Though air quality has generally improved over time due to emission regulations, mounting evidence linking air pollution and cognitive impairment warrants additional scrutiny. Much is still unknown about the specific mechanisms by which inhaled PM affects the brain and continued research could offer new ways to think about treating and preventing disease.