A study of children from Tarragona, Spain, found that higher prenatal exposure to air pollution (PM10 and PMcoarse particles, NO2, and NOx gases) was associated with modestly higher teacher-reported ADHD symptom scores in school-age participants. However, the study did not find an association between air pollution exposure and a clinical ADHD diagnosis. The paper was published in Research on Child and Adolescent Psychopathology.
Air pollution is the presence of harmful substances in the air, such as gases, particles, smoke, and chemical pollutants. It can come from traffic, factories, power plants, heating systems, agriculture, fires, and natural sources such as dust storms. Air pollution is typically described in terms of the substances that comprise it 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 especially concerning because they can enter deep into the lungs and may even pass into the bloodstream. PM10 refers to particles with a diameter of 10 micrometers or less, including dust, pollen, soot, and other larger particles. PMcoarse usually refers to the larger part of PM10, often particles between 2.5 and 10 micrometers in diameter.
NO2, or nitrogen dioxide, is a harmful gas produced mainly by combustion, especially from vehicles and power generation. NOx refers to nitrogen oxides as a group, mainly nitric oxide and nitrogen dioxide, both of 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 diseases, and contribute to premature death.
Study author Sharanpreet Kaur and his colleagues investigated the association between prenatal exposure to air pollutants (while a child is still in the womb) and the likelihood of ADHD symptoms in an area of Spain with high petrochemical activity. They considered specific symptoms of ADHD and the levels of exposure to air pollutants during different trimesters of pregnancy. These researchers hypothesized that higher exposure to air pollutants would be associated with more severe symptoms of ADHD, and that the association would be stronger in boys than in girls.
They analyzed data from the Neurodevelopmental Disorders Epidemiological Project (EPINED), a study carried out in the region of Tarragona, a province in the north-eastern part of Spain. The data were collected between 2014 and 2019.
The study consisted of two phases. In the first phase, a total of 6,894 children were screened for symptoms of ADHD. Of these, 54% of families consented to participate in the study, resulting in 3,727 children becoming study participants (1,929 were girls). The participating children belonged to two age groups: a preschool group, aged 4-5 years, and a school-age group, aged 10-11 years. Parents and teachers completed questionnaires about the presence of ADHD symptoms in these children. Ultimately, 334 children exceeded the ADHD classification threshold, indicating a high risk that they suffer from ADHD.
In the second phase, 781 of the participating children (a mix of high-risk and low-risk kids) were individually evaluated by two qualified psychiatrists and psychologists for ADHD. Children diagnosed with autism were excluded from the analysis. Therefore, a total of 723 children were included in the diagnostic analyses this study is based on. Of these children, 174 suffered from clinical ADHD, and 549 did not.
The study authors combined parent-reported data on where they lived when their children were born with data from the European Study of Cohorts for Air Pollution Effects (ESCAPE) project. This allowed them to estimate the parents’ exposure to air pollution—in the form of different types of particles and gases—during their pregnancies.
The results showed that higher prenatal exposure to PM10 and PMcoarse particles, as well as NO2 and NOx gases, was associated with more severe teacher-reported ADHD symptoms in the group of school-age children (from the first phase of the study). In contrast, in the group of preschool children, only prenatal exposure to higher levels of O3 (ozone, considered an air pollutant when found near the ground) was found to be associated with teacher-reported emotional lability symptoms.
Further analyses dividing the pregnancies by trimester revealed a critical window of vulnerability. Exposure to PM2.5, PM10, PMcoarse, NO2, and NOx during the first and second trimesters of pregnancy was associated with increased inattention. Furthermore, as the researchers hypothesized, these associations were significantly stronger in males than in females.
However, the study did not find any association between exposure to air pollutants and a formal ADHD diagnosis or a specific pattern of ADHD symptoms.
“Our findings suggest that even modest increases in ADHD symptoms may reflect subtle neurodevelopmental effects of prenatal air pollution exposure. These results highlight early gestation as a vulnerable period and the need for further research on long-term impacts,” the study authors concluded.
The study contributes to the scientific understanding of the links between environmental factors and ADHD symptoms. However, it should be noted that the study design does not allow any definitive causal inferences to be derived from the results. Also, the strength of the observed associations was modest and found only for teacher-reported symptoms, not parent-reported symptoms or clinical diagnoses.
Finally, the air pollutants associated with ADHD symptom scores differed between the school-aged and preschool children. The researchers suggest this is likely due to developmental shifts; preschoolers are rapidly developing emotional regulation (which appears sensitive to ozone), whereas older children face greater cognitive demands in school, making executive function deficits (driven by particulate matter and nitrogen gases) more apparent.
The paper, “Prenatal Exposure to Air Pollution and Risk for Attention-Deficit/ hyperactivity Disorder in Children,” was authored by Sharanpreet Kaur, Josefa Canals-Sans, Paula Morales-Hidalgo, Mònica Guxens, Sami Petricola, and Victoria Arija.