A study of children in Tarragona, Spain, found that higher prenatal exposure to air pollution (PM10 and PM coarse particles, NO2, and NOx gases) was associated with slightly higher teacher-reported ADHD symptom scores in school-age participants. However, this study found no association between exposure to air pollution and clinical diagnosis of ADHD. The paper is Research on psychopathology in children and adolescents.
Air pollution is the presence of harmful substances such as gases, particles, smoke, and chemical pollutants in the air. They can come from natural sources such as traffic, factories, power plants, heating systems, agriculture, fires, and sandstorms. Air pollution is usually described by the substances that make it up and the size of the particles in the air.
For example, PM2.5 refers to very small airborne particles with a diameter of 2.5 micrometers or less. These particles are of particular concern because they can penetrate deep into the lungs and even enter the bloodstream. PM10 refers to particles with a diameter of 10 micrometers or less, including dust, pollen, soot, and other larger particles. Coarse PM typically refers to the bulk of PM10, often particles between 2.5 and 10 micrometers in diameter.
NO2 (Nitrogen Dioxide) is a harmful gas mainly produced by combustion, especially vehicles and electricity generation. NOx is a general term for nitrogen oxides, mainly nitrogen monoxide and nitrogen dioxide, which are produced when fuel is burned at high temperatures. These pollutants can irritate the lungs, worsen asthma, increase the risk of respiratory and cardiovascular disease, and contribute to premature death.
Study author Sharanpreet Kaur and colleagues investigated the link between prenatal exposure to air pollutants (while the child was still in the womb) and the likelihood of ADHD symptoms in a region of Spain with a lot of petrochemical activity. They looked at specific symptoms of ADHD and levels of exposure to air pollutants during different periods of pregnancy. These researchers hypothesized that greater exposure to air pollutants would be associated with more severe symptoms of ADHD, and that this association would be stronger in boys than in girls.
They analyzed data from the Project on the Epidemiology of Neurodevelopmental Disorders (EPINED), a study conducted in the Tarragona region of northeastern Spain. Data was collected between 2014 and 2019.
This study consisted of two phases. In the first phase, a total of 6,894 children were tested for symptoms of ADHD. Of these, 54% of families agreed to participate in the study, resulting in 3,727 children (1,929 girls). The participating children belonged to two age groups: a 4- to 5-year-old preschool group and a 10- to 11-year-old school-age group. Parents and teachers completed a questionnaire regarding the presence or absence of ADHD symptoms in these children. Ultimately, 334 children exceeded the classification criteria for ADHD, indicating they were at high risk for developing ADHD.
In the second phase, 781 of the participating children (a mix of high-risk and low-risk children) were independently assessed for ADHD by two qualified psychiatrists and psychologists. Children diagnosed with autism were excluded from the analysis. Therefore, a total of 723 children were included in the diagnostic analysis underlying this study. Of these children, 174 had clinical ADHD and 549 did not.
The study authors combined data reported by parents about where their children lived at the time of their birth with data from the European Air Pollution Impact Cohort Study (ESCAPE) project. This allowed us to estimate the amount of air pollution that parents are exposed to during pregnancy in the form of different types of particles and gases.
The results showed that higher prenatal exposure to PM10, coarse PM particles, NO2 and NOx gases was associated with more severe teacher-reported ADHD symptoms in a group of school-aged children (from the first phase of the study). In contrast, in a group of preschool children, only prenatal exposure to high levels of O3 (ozone, considered an air pollutant if found near the ground) was found to be associated with teacher-reported symptoms of emotional instability.
Further analysis of pregnancies by gestational age revealed critical periods of vulnerability. Exposure to PM2.5, PM10, coarse PM, NO2, and NOx during the first and second trimesters was associated with increased inattention. Furthermore, as the researchers hypothesized, these associations were significantly stronger for men than for women.
However, this study found no association between exposure to air pollutants and formal ADHD diagnosis or specific patterns of ADHD symptoms.
“Our findings suggest that even small increases in ADHD symptoms may reflect subtle neurodevelopmental effects of prenatal air pollution exposure. These results highlight early pregnancy as a vulnerable period and the need for further research into long-term effects,” the study authors concluded.
This study contributes to the scientific understanding of the association between environmental factors and ADHD symptoms. However, note that the study design does not allow us to draw definitive causal inferences from the results. Additionally, the strength of the observed association was modest and only for teacher-reported symptoms, rather than parent-reported symptoms or clinical diagnoses.
Finally, air pollutants associated with ADHD symptom scores differed between school-age and preschool children. Researchers suggest this is likely due to developmental changes. Preschoolers are rapidly developing emotional regulation (which makes them more sensitive to ozone), but older children face greater cognitive demands at school and executive function deficits (caused by particulate matter and nitrogen gas) become more pronounced.
The paper, “Prenatal exposure to air pollution and the risk of attention-deficit/hyperactivity disorder in children,” was authored by Sharanpreet Kaur, Josefa Canals-Sans, Paula Morales-Hidalgo, Mònica Guxens, Sami Petricola, and Victoria Arija.
