While the effect of pollution on cardiovascular and respiratory health has been studied extensively, less is known about how breathing dirty air might affect the brain, Suglia and her team write in the American Journal of Epidemiology.

To investigate, she and her colleagues looked at 202 Boston children 8 to 11 years old who were participating in a study of maternal smoking. The researchers related several measures of cognitive function to the children's estimated exposure to black carbon, a component of the particulate matter emitted in automobile and truck exhaust, particularly by diesel vehicles.

The more heavily exposed children were to black carbon, the lower were their scores on several intelligence tests.

When the researchers adjusted for the effects of parents' education, language spoken at home, birth weight, and exposure to tobacco smoke, the association remained.

For example, heavy exposure to black carbon was linked to a 3.4-point drop in IQ, on average. Heavily exposed children also scored lower on tests of vocabulary, memory and learning.

"It's within the range for in utero tobacco exposure and lead exposure," Suglia said in an interview.

She pointed out that exposure to traffic pollution has been associated with a number of other harmful effects and that, short of moving away from heavy traffic areas, there's not much people can do to limit it.