Children exposed to higher levels of air pollution during the third trimester of their mother's pregnancy face a higher risk of elevated blood pressure in childhood, according to a study. Fine particulate matter of 2.5 microns or less (PM2.5) is a form of air pollution produced by motor vehicles and the burning of oil, coal and biomass, and has been shown to enter the circulatory system and negatively affect human health. "Ours is one of the first studies to show breathing polluted air during pregnancy may have a direct negative influence on the cardiovascular health of the offspring during childhood," said Noel T Mueller, an assistant professor at the Johns Hopkins University in the US. "High blood pressure during childhood often leads to high blood pressure in adulthood and hypertension is the leading cause of cardiovascular disease," said Mueller, senior author of the study published in the journal Hypertension. Researchers examined 1,293 mothers and their children who were part of the large, ongoing Boston Birth Cohort study. Blood pressure was measured at each childhood physical examination at 3- to 9- years old. The researchers found that children exposed to higher levels of ambient fine-particulate pollution in the womb during the third trimester were 61 per cent more likely to have elevated systolic blood pressure in childhood compared to those exposed to the lowest level. Higher exposure to air pollution in the third trimester, when foetal weight gain is the most rapid, was already known to influence (lower) birthweight, researchers said. However, the study found the association with elevated blood pressure regardless of whether a child was of low-, normal- or high birthweight. A woman's fine-particulate matter exposure before pregnancy was not associated with blood pressure in her offspring, thus providing evidence of the significant impact of in-utero exposure. "These results reinforce the importance of reducing emissions of PM2.5 in the environment," Mueller said. "Not only does exposure increase the risk of illness and death in those directly exposed, but it may also cross the placental barrier in pregnancy and effect fetal growth and increase future risks for high blood pressure," he said.