279 primary schoolchildren aged 8-12 years from the BREATHE Study underwent brain neuroimaging between 2012 and 2013 in Barcelona, Spain. Several aspects of their cognition and their behavior were evaluated: ADHD symptoms using the ADHD-DSM-IV scale, general behavior using the Strengths and Difficulties Questionnaire, attention using the Attention Network Test, working and superior working memory using the n-back test. Air pollution levels were measured at the children´s school at the time of the study, and estimated at the residential address of each participant during the prenatal period.
We performed statistical analyses to examine the relationships between 1) brain structures volumes and air pollution levels, 2) brain structures volumes and cognition and behavior, 3) air pollution levels and cognition.
Firstly, we found that an increase in polycyclic aromatic hydrocarbons (PAHs) levels , and in particular an increase of 70 pg/m3 in outdoor benzo[a]pyrene (BAP) levels in the school environment, was associated with a reduction corresponding to almost 2% of the mean caudate nucleus volume (when the mean caudate nucleus volume is only 7 to 8% approximately smaller in children with ADHD). It is important to highlight that the levels of BAP in the different schools of the study ranged between 20 and 304 pg/m3 (median of 86 pg/m3), levels far below the legislated annual target levels of 1000 pg/m3 for BAP established in the European Union (European Council Directive 2004/107/EC,
http://eurlex.europa.eu/eli/dir/2004/107/oj(odnośnik otworzy się w nowym oknie)).
Secondly, we observed that an increase of approximately 7µg/m3 in the concentration of fine particles (particles matter with aerodynamic diameter <2.5µm, PM2.5) during the third trimester of pregnancy was significantly associated with a reduction corresponding to almost 4% of the mean volume of the corpus callosum (when the mean corpus callosum volume is only 16% approximately smaller in children with autism), and with an increase corresponding to almost 13% of the mean lateral ventricles volume. These prenatal PM2.5 levels ranged between 12.7 and 45.0 µg/m3 (median of 23.6 µg/m3), when the limit target value established by the European Union for 2015 was 25 µg/m3 (European Council directive 2008/50/EC,
http://eur-lex.europa.eu/eli/dir/2008/50/oj(odnośnik otworzy się w nowym oknie)).
These relationships between air pollution exposure and brain structural changes were observed independently of children´s age, sex, intracranial volume, maternal education and residential neighborhood socioeconomic status.
The BAP-related caudate nucleus change appeared to be subclinical given that we only observed a trend for an association between a reduction in the caudate nucleus volume and an increase in ADHD symptoms, but the relationship was not statistically significant.
In contrast, the reduction in the corpus callosum and the increase in the lateral ventricles volumes were significantly associated with an increase in behavioral problems (higher hyperactivity subscore on the ADHD scale and higher total score on the Strengths and Difficulties Questionnaire).
