WASHINGTON, May 14 (Xinhua) -- A new study published on Monday in the American Heart Association's journal Hypertension showed that those who had been exposed to higher levels of air pollution during the third trimester of the mother's pregnancy had a higher risk of elevated blood pressure in childhood.

Children exposed to higher levels of fine-particulate pollution in the womb during the third trimester were 61 percent more likely to have elevated systolic blood pressure in childhood compared to those exposed to the lowest level, according to the research.

The concentrations of PM2.5 in the highest category in this study (11.8 micrograms per cubic meter or higher) were slightly lower than America's National Air Quality Standard (12 micrograms per cubic meter).

PM2.5 or fine particulate matter of 2.5 microns or less 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.

Previous studies found that the direct exposure to fine air pollution was associated with high blood pressure in both children and adults and is a major contributor to illness and premature death worldwide.

"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, the senior author of the study and an assistant professor of epidemiology at the Johns Hopkins University.

The 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 three to nine years old. A systolic blood pressure was considered elevated if it was in the highest 10 percent for children the same age on national data.

Researchers also adjusted for other factors known to influence childhood blood pressure, such as birthweight and maternal smoking.

The study found that the association with elevated blood pressure regardless of whether a child was of low, normal or high birthweight.

This study established an association but it did not prove a direct cause-and-effect relationship.