Air pollution may impair children's memory

Air pollution kills more people than any other environmental cause. The deadly effects include stroke, heart disease and lung cancer. But the tiny toxic particles in dirty air could also be dimming our wits, according to an emerging field of research. Now a new study links early life exposure to haze with impaired memory and poor control of attention by boys and girls (Environ. Health Perspect. 2019, DOI: 10.1289/EHP3169).

When people burn wood and fossil fuels like gasoline and coal, the fuels generate a smoky haze full of particles made of hundreds of chemicals. The smallest are known as PM_2.5, which includes airborne particles less than 2.5 µm in diameter. When people breathe in a lungful of dirty air, these particles can enter the bloodstream and wreak havoc in organs and cells by promoting inflammation and damaging DNA.

Earlier studies in lab animals exposed to PM_2.5 have uncovered inflammation in the brain, but it hasn’t been clear how these effects play out in people. “We want to understand whether this is affecting children in real life,” says Ioar Rivas, an environmental health researcher at King’s College London. In previous work, she and her colleagues found a direct correlation between daily spikes in dirty air and slumps in children’s short-term memory and attention. They wondered if chronic exposures to PM_2.5 had lasting effects. “We hypothesized that exposure to high levels of air pollution in the prenatal period and the very first years of life could lead to significant effects since these years are periods of high brain development,” she says.

To test this idea, Rivas and her team recruited 2,221 children ages 7 to 10 years in Barcelona, Spain. The researchers tested the children four times over the course of one year on the power of their working memory and ability to focus. As a group, the annual PM_2.5 exposures of these children exceeds the World Health Organization’s guidelines. The recommended limit is an annual average of 10 µg per m³ of air—but few cities actually meet this target, and Barcelona is no exception. Based on national air quality data, the team calculated that during their first 7 years, the children were exposed to an annual average of 16.8 µg of PM_2.5 per m³ of air.

To correlate pollution exposure with cognitive development, the scientists created a history of PM_2.5 levels at home for each child using a model of air pollution for the city. They developed statistical models that computed the change in the children’s average test scores associated with cumulative increases in PM_2.5 concentrations from conception to age 7. They measured differences in attention and memory correlated with every additional 10 µg/m³ of PM_2.5 the children were exposed to over their lifetimes.

To measure the childrens’ attention control, the researchers showed them a row of five fish with the central fish pointing in the same or opposite direction from the others. The scientists measured how quickly the children pressed a button to indicate whether the central fish pointed to the left or right. For every additional 10 µg/m³ of PM_2.5 the children were exposed to over their lifetimes, the speed of their responses slowed by about 18%.

The memory test presented the children with a sequence of numbers on a computer screen and asked them to click a button when the number on the screen matched the one they saw three steps before. The scientists scored the children based on the proportion of correct and incorrect clicks. A roughly 16% drop in working memory scores was associated with lifetime exposures of 10 µg/m³ of PM_2.5. The boys’ scores declined more sharply than those of the girls.

“The findings in this paper are very similar to our study of 20,000 adults in China finding that PM_2.5 worsened people’s verbal test performance and that men were more affected than women,” says Xi Chen, a health economist at the Yale School of Public Health. He says that the effects of air pollution could be due to inflammation in the brain.

Rivas adds that during pregnancy, air pollution may depress the transport of oxygen and nutrients to the developing fetus. She and her colleagues have a new project to assess exposure of pregnant women to air pollutants and image the brains of fetuses and newborns.