Plos: Human brain development is a complex and lengthy process. During
pregnancy, the basic structures of the brain are formed, and the neural
circuits that will eventually control movement, speech, memory, and
other cognitive (thinking) functions, as well as the function of many
organs, begin to be established. By the time of birth, the brain is
about a quarter of its adult size, and the neural circuits that control
vital bodily functions such as breathing are well developed.
By
contrast, the cerebral cortex—the brain region that is involved in
thought and action—is poorly developed. Much of the development of the
cerebral cortex happens during the first two years of life. For example,
babies usually learn to crawl at about nine months. Other aspects of
brain function take longer to develop. Thus, the cognitive functions
that are essential for learning undergo considerable development between
the ages of 6 and 10 years, and further brain changes occur during
adolescence.
Why Was This Study Done?
Exposure
to the air pollutants produced by the combustion of fossil fuels by
vehicles during pregnancy or infancy has been associated with delays in
cognitive development. Moreover, experiments in animals suggest that
traffic-related air pollution is a developmental neurotoxicant—a factor
that disrupts brain development. However, although many schools are
located next to busy roads and although traffic-related air pollution
levels peak during school hours, it is not known whether exposure of
school-age children to traffic-related air pollutants impairs their
cognitive development and thus their ability to learn. Here, in a
prospective cohort study (the BREATHE study), the researchers assess
whether exposure of children aged 7–10 years to traffic-related air
pollutants in schools in Barcelona, Spain, is associated with impaired
cognitive development. A prospective cohort study is an observational
investigation that studies groups (cohorts) of individuals who differ
with respect to a specific factor to determine how exposure to this
factor affects specific outcomes.
What Did the Researchers Do and Find?
The
researchers used computerized tests to measure the development of
working memory (the system that holds multiple pieces of transitory
information in the mind where they can be manipulated), superior working
memory (working memory that involves continuous updating of the working
memory buffer), and attentiveness every three months over a 12-month
period in 2,715 primary school children attending 39 schools exposed to
high or low levels of traffic-related air pollution and paired by
socioeconomic index. That is, the researchers compared three cognitive
development outcomes in the children attending each school where
exposure to air pollution was high with the same outcomes in children
attending a school with a similar socioeconomic index where exposure to
pollution was low; school pairing was undertaken to avoid “confounding”
by social class, a factor that is known to affect cognitive development.
Statistical analyses of these data indicated that the increase in
cognitive development over time among children attending highly polluted
schools was less than that among children attending paired lowly
polluted schools, even after adjusting for additional factors that
affect cognitive development. Thus, for example, there was an 11.5%
12-month increase in working memory at the lowly polluted schools but
only a 7.4% 12-month increase in working memory at the highly polluted
schools. Other analyses indicated that children attending schools with
higher levels of traffic-related air pollutants in either the courtyard
or in the classroom experienced a substantially smaller increase over
the 12-month study in all three cognitive measurements than those
attending schools with lower levels of pollutants.
What Do These Findings Mean?
These
findings suggest that, compared with attendance at schools exposed to
low levels of traffic-related air pollution, attendance at schools
exposed to high levels of traffic-related air pollution is associated
with a smaller increase in cognitive development over a 12-month period
among 7- to 10-year-old children in Barcelona. The accuracy of these
findings may be limited by residual confounding. That is, the children
attending schools where traffic-related pollution is high might have
shared other unknown characteristics that affected their cognitive
development. Importantly, these findings do not prove that
traffic-related air pollution causes impairment of cognitive
development. Rather, they suggest that the developing brain may be
vulnerable to traffic-related air pollution well into middle childhood, a
conclusion that has implications for the design of air pollution
regulations and for the location of new schools.