Could ADHD be triggered by mothers being exposed to air pollution while pregnant? 

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Scientists at Columbia University found children exposed to high levels of air pollution during pregnancy are five times more likely to have ADHD

Children exposed to high levels of pollution in the womb are at greater risk of suffering attention deficit hyperactivity disorder, a new study has found.

Scientists at Columbia University studied 233 non-smoking pregnant women living in New York.

They found children exposed to high levels of air pollution during pregnancy were five times more likely to have ADHD by the time they were nine years old. 

The nine-year study looked at levels of common pollutants polycyclic aromatic hydrocarbons (PAH).

Researchers measured the levels of PAH in maternal and umbilical cord blood shortly after delivery.

And they repeated tests when each of the children were three and five, measuring levels of PAH in their urine.

Thirty-three children who had high levels of exposure to PAHs, as measured at birth.

Of those, 13 were diagnosed with ADHD hyperactive-impulsive subtype, seven the inattentive subtype, and 13 had both. 

Professor Frederica Perera, first author of the study, said: 'Those children born to moms who were exposed to high levels of PAH during pregnancy had five times the odds of having an increased number of symptoms.'

PAHs are created when products like coal, oil, gas and rubbish are burned but not completely.

They don't burn easily, and as a result remain in the environment for long periods of time.

Most are used to conduct research though some are used to make dyes, plastics and pesticides.

One of the most common ways they enter the body is through breathing in contaminated air.

To establish children's exposure to PAHs in the womb, the scientists measured levels of fragments of the mothers' DNA bonded to PAH molecules, also known as DNA adducts, in umbilical cord blood.  

Previous studies carried out by Professor Perera and her team identified links between higher levels of prenatal PAH exposure and developmental delays in children by the age of three.

They also noted lower IQ scores at five, and increased risk a child will suffer anxiety, depression and attention problems at six and seven. 

The new study, published in the journal PLoS One, looked at the children's ADHD symptoms using the Child Behavior Checklist and the Conners' Parent Rating Scale - two screening tests used to diagnose the condition.

They studied 233 non-smoking pregnant women living in New York, measuring levels of a common pollutant in umbilical cord blood and then levels in the children's urine at ages three and five

Professor Perera said this is the first time a link has been established between prenatal PAH exposure and ADHD symptoms.

She told LiveScience: 'If replicated, then these findings could lead to new ways or stronger ways, better ways, to prevent ADHD.

'By nature, environmental exposures are preventable, this we consider one possible contributor to ADHD and one that's preventable, and the findings should be followed up so that necessary preventive strategies could be taken.' 

She said pregnant women concerned about the effect of pollution levels on their unborn babies, can eat plenty of fresh produce which helps offset the effects of pollutants.