Monday November 10 2014
This study didn't prove a link between pollution and ADHD
“Could ADHD be triggered by mothers being exposed to air pollution while pregnant?,” asks the Mail Online.
Pregnant women have enough to worry about, without going round in a gas mask or moving to the country. Fortunately, the study that this news relates to doesn’t find a connection between exposure to pollution while pregnant and attention deficit hyperactivity disorder (ADHD).
In fact, the study looked at just 250 African-American and Dominican children in three suburbs of New York. It looked at whether symptoms of ADHD (rather than diagnoses) at the age of nine were associated with their pregnant mother’s exposure to environmental pollution, derived from traffic fumes and domestic heaters. The pollution – polycyclic aromatic hydrocarbons (PAHs) – was measured by levels of PAH DNA in maternal and cord blood samples taken at birth.
The researchers found there to be an association between PAH levels in maternal blood and ADHD symptoms. The mothers with high PAH levels had increased odds of being categorised as having “moderately to markedly atypical” scores on “inattentive” and “total symptom” scales.
However, there is no evidence that the association between symptoms and PAHs in the mothers’ blood was caused by environmental pollution. The researchers found no association between maternal blood PAH levels and air-measured PAH levels, nor estimations of dietary PAH intake.
This relatively small study of a specific population sample demonstrates an association, but does not provide conclusive evidence of a link between exposure to pollution during pregnancy and a child’s chances of developing ADHD.
Where did the story come from?
The study was carried out by researchers from Columbia University in New York, and was funded by the National Institute for Environmental Health Sciences and the US Environmental Protection Agency. The study was published in the open access, peer-reviewed medical journal PLOS One.
The media appears to have taken these results at face value, but not considered the various limitations of this small study, which make the results far from conclusive.
What kind of research was this?
This was a US cohort study that investigated whether there is an association between childhood symptoms of ADHD and maternal exposure to PAHs during pregnancy
PAHs are toxic air pollutants released during incomplete combustion of fossil fuels. They are produced by traffic and residential heating, among other sources. As the researchers say, urban minority populations often have much higher exposure to air pollution than other populations.
This is a health concern because foetuses and developing children are potentially susceptible to PAHs and other pollutants. Previous laboratory studies have suggested a range of neurodevelopmental and behavioural effects from PAH exposure. Results from this cohort of mothers has already found that exposure to PAH before birth is associated with developmental delay at three years old, reduced IQ at five, and symptoms of anxiety or depression and attention problems at six to seven years old.
As ADHD is the most common behavioural disorder in childhood, the researchers also wanted to see if it was associated with ADHD at nine years of age.
However, a cohort study such as this can only demonstrate an association – it can’t prove cause and effect, as the relationship may be influenced by other factors.
What did the research involve?
This cohort study recruited a sample of African-American and Dominican women from antenatal clinics in three suburbs of New York City between 1998 and 2006. The women were all aged 18 to 35, non-smokers and did not use any other drug substances.
The researchers measured PAH exposure by levels of PAH-modified DNA in maternal and umbilical blood samples taken after delivery. They also measured air PAH levels during pregnancy, and questioned the women about their exposure to passive smoke and dietary PAH consumption (through grilled, fried or smoked meat).
Child ADHD behaviour problems were assessed when the children were nine years old using two validated parent-reported rating scales:
- the CBCL: a screening instrument assessing various child functioning problems
- the CPRS-Revised: a focused assessment of ADHD
The CBCL and CPRS-revised scales also assessed child anxiety and depression symptoms.
The researchers analysed the association between PAH metabolites and ADHD symptoms, adjusting for other measured health and environmental factors, such as child age, sex, mother’s educational level and her own ADHD symptoms. The researchers also measured levels of PAH breakdown products detected in the child’s urine samples when aged three and five years, so that they could adjust for PAH exposure after birth.
The final sample included 250 children with complete data.
What were the basic results?
The researchers found that all CPRS subscales scores were significantly associated with levels of PAH-modified DNA in maternal blood.
The researchers then analysed the information to see whether there was an association with “moderately to markedly atypical” scores. Compared to those whose maternal blood was categorised as having low PAH levels, those with high levels had increased odds of being categorised as having “moderately to markedly atypical” scores on the “inattentive” and “total” DSM-IV subscales of the CPRS, but not the hyperactive-impulsive subscale.
There was some association between maternal blood PAH and the ADHD problems on the CBCL checklist scores, but this did not reach statistical significance.
Levels of PAH DNA in umbilical cord blood were available for fewer participants. There were no significant associations between PAH cord blood levels and CPRS or CBCL scores.
How did the researchers interpret the results?
The researchers conclude that their results, “suggest that exposure to PAHs encountered in New York City air may play a role in childhood ADHD behaviour problems”.
Overall, this relatively small cohort study demonstrates an association, but does not provide conclusive evidence, that exposure to pollution (in the form of PAHs) before birth is associated with the development of ADHD.
There are a number of limitations to consider. These include the fact that the study includes a relatively small sample of 250 children, with all of them from two specific ethnic groups (African-American and Dominican), and from three suburbs of New York City. The findings may not be generalisable to other populations.
While the researchers used valid assessment scales, it has not focused on examining actual diagnoses of ADHD.
Importantly, the only association identified by the researchers was between ADHD symptoms and levels of PAH DNA in maternal blood at the time of birth. There was no association between maternal blood PAH levels and environmentally-measured PAH levels or dietary PAH intake. Therefore, the source of this exposure is not known, and it cannot be reliably assumed to be due to environmental causes. Levels of PAH-modified DNA not only reflect exposure, but also an individual’s uptake, detoxification and DNA repair rates.
Finally, there remains the possibility that if there is an association between maternal levels of PAH and child ADHD symptoms, it could be influenced by a variety of unmeasured health, lifestyle and socioeconomic factors.
While the findings are undoubtedly worthy of further research, there does not appear to be firm evidence from this study to support the media conclusion that exposure to environmental pollutants during pregnancy could lead to the development of ADHD.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.