Exposure of babies to high levels of air pollution in the womb may increase the risk of elevated blood pressure in childhood, according to a new 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. The findings suggest 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 in childhood compared to those exposed to the lowest level.

"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 co-author Noel T. Mueller, Assistant Professor at Johns Hopkins University Bloomberg School of Public Health in Maryland, US. For the study, published in the journal Hypertension, the researchers examined 1,293 mothers and their children. Blood pressure was measured at each childhood physical examination at three to nine years old.

A systolic blood pressure was considered elevated if it was in the highest 10 per cent for children of the same age on national data. Higher exposure to air pollution in the third trimester, when fetal weight gain is the most rapid, was already known to influence (lower) birth weight. But this study found the association with elevated blood pressure regardless of whether a child was of low, normal or high birth weight, the researcher said.

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, the researcher added. "These results reinforce the importance of reducing emissions of PM2.5 in the environment. 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 affect fetal growth and increase future risks for high blood pressure," Mueller noted.