Mothers who live in polluted areas may give birth to less intelligent children as they get exposed to air pollution particles while in the womb, a study suggests.

Researchers from the University of California, San Francisco said that PM10 particles, which are easily inhaled, can pass through the placenta and affect brain development, thus reducing the child's IQ.

Children living in highly polluted areas were found to have an average IQ of 2.5 points lower – and up to 6.8 points down – by the time they were four than those living in the less polluted regions.

“We found that children... exposed to higher ambient PM10 in utero had lower IQ in early childhood,” researchers explained as the study’s focus area was Shelby County which has high emissions inventories.

“We observed no evidence that IQ is associated with... prenatal exposure to traffic-related air pollution,” the paper added, drawing attention to industrial works as the main cause.

Folic acid, recommended for pregnant women, may protect against brain damage, the study noted.

“It is possible that higher folate levels increase the antioxidant capability of the diet, buffering oxidative stress associated with PM10 exposure. It may also be that folate itself is protective, as folate plays an important role in healthy neurodevelopment regardless of air pollution exposure,” one of the researchers, Kaja LeWinn, told The Times.