ADHD and air pollution in pregnancy

Here at Research the Headlines, we have frequently written about environmental factors that may be linked with developmental disorders such as Attention Deficit Hyperactivity Disorder (ADHD) or Autism Spectrum Disorder (ASD). We have done so because the media frequently reports on such studies. So is there really evidence for a link between ADHD and mothers being exposed to air pollution during their pregnancies as current media headlines would suggest?

What did the study involve?

The NHS Behind the Headlines team have written an excellent summary of the methodological design and key results of this study (see here).  In brief, the study examined ADHD symptoms when children were aged 9 and their mother’s levels of polycyclic aromatic hydrocarbons (PAHs) taken from the mother’s blood and cord samples at birth. PAHs are organic chemicals found in crude oil and also generated by incomplete combustion of carbon-based fuels such as wood, coal and oil, and are persistent pollutants in both the soil and the air.  The assessment of ADHD symptoms was not part of a diagnostic interview but was provided by a general measure of psychiatric symptoms and symptoms on an ADHD specific questionnaire.

What were the basic findings?

The researchers reported a correlation (i.e. an association between two things rather than a cause-and-effect relationship) between subscales of the ADHD questionnaire and maternal PAH blood levels. Mothers with high PAH blood levels had increased odds of being categorised as having “moderately to markedly atypical” scores on the “inattentive” and “total” subscales of the ADHD questionnaire, but not the hyperactive-impulsive subscale, in comparison to mothers with low PAH levels. There were no significant relationships between responses on the general measure of psychiatric symptoms (which included ADHD symptoms) and PAH levels. No relationships were reported between PAH cord levels and ADHD symptoms.

How should we interpret these findings?

As the NHS Behind the Headlines team conclude, this study merely shows an association between two variables rather than an causal relationship.  We have previously discussed the need to be cautious in interpreting relationships between variables that are associated only by correlation in our ‘How to Research the Headlines’ series. The association between PAH levels and ADHD symptoms may have been influenced by a whole range of confounding variables. It is also very important to point out that the researchers were not relating a link between a diagnosis of ADHD (for which both parental and teacher report is essential and as evidenced from a full diagnostic assessment by a clinician) and PAH levels. So in summary, the study does not shown a causal link between pollution and ADHD as a diagnostic disorder.

Should we believe the headline?

This is a definite case of not stopping at the headline and seeking out alternative opinions of the story presented.  Both the NHS Behind the Headlines team and ourselves as part of the multi-discipline Research the Headlines team regularly write about stories like this. If you see a headline you are unsure of that we have not written about please let us know!

Perera, F. P. et al. (2014). Early-Life Exposure to Polycyclic Aromatic Hydrocarbons and ADHD Behavior Problems. PLOS ONE. DOI:10.1371/journal.pone.0111670