According to the European Union 25 ug/m3 is the safe level of fine particle.
Previous studies have linked acceptable air pollution levels with impact on lungs, heart, and other organs including cognitive decline and foetal growth development.
However, the new study showed that air pollution levels related to brain alterations in the foetal brain, which is in developing stages and has no mechanisms to protect against or remove environmental toxins, were below those considered to be safe.
"We observed brain development effects in relationship to fine particles levels below the current EU limit," said lead author Monica Guxens, MD, of Barcelona Institute for Global Health (ISGlobal) in Spain.
The findings showed that exposure to fine particles during foetal life was associated with a thinner outer layer of the brain, called the cortex, in several regions.
These brain abnormalities contribute in part to difficulty with inhibitory control -- the ability to regulate self-control over temptations and impulsive behaviour -- which is related to mental health problems such as addictive behaviour and attention-deficit/hyperactivity disorder.
"The observed cognitive delays at early ages could have significant long-term consequences such as increased risk of mental health disorders and low academic achievement, in particular due to the ubiquity of the exposure," Guxens said.
"Therefore, we cannot warrant the safety of the current levels of air pollution in our cities," he added.
For the study, published in Biological Psychiatry, the team assessed air pollution levels, including levels of nitrogen dioxide -- a prominent air pollutant caused by traffic and cigarette smoking --, coarse particles, and fine particles, at home during the foetal life of 783 children.
Brain imaging performed when the children were between six and 10 years old revealed abnormalities in the thickness of the brain cortex of the precuneus and rostral middle frontal region.
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