High blood pressure typically occurs in adulthood, so when children develop the condition, it often means something is very wrong. A child might have kidney disease, hyperthyroidism or a heart problem. Obesity can also be a factor.
In a paper published Monday in the American Heart Association's journal Hypertension, researchers reported that children of mothers who were exposed in their third trimester to higher levels of fine particulate pollution — the tiny airborne matter that causes haze in many cities around the world — were at a 61 percent higher risk of elevated blood pressure.
The study of families in the Boston area involved 1,293 mothers and their children, ages 3 to 9. About 160 of the children had elevated blood pressure. Pollution levels were gauged by looking at home addresses and nearby Environmental Protection Agency monitors to measure air quality. Those readings are taken as often as every three days.
Noel Mueller, an assistant professor of epidemiology at the Johns Hopkins University Bloomberg School of Public Health, and his colleagues zeroed in on the microscopic solids and droplets that are 2.5 microns or less in width and are particularly insidious because they can get into the lungs, bloodstream and heart.
There is a large body of research — the EPA cites thousands of studies — that shows the severe impact of fine particulate matter on human health, particularly the respiratory system. But this is among the first to show that a pregnant mother's exposure may harm her offspring.
Mueller said that it is possible the particles may have caused intrauterine inflammation in the mothers, thus altering fetal growth patterns and “programming a baby to have a higher risk for blood pressure during childhood.” But he emphasized in an interview that this is only a theory and that much more work must be done to confirm the association and understand what might be happening.
While high blood pressure in children does not usually produce any symptoms, it can lead to early heart attack, stroke or other serious health issues. In 2017, the American Academy of Pediatrics updated its guidelines for screening and managing high blood pressure in youth. Diagnosing hypertension in adults is simple, with the threshold now set at 130/80. But the bodies of children change so quickly that the definition of “normal” has to take into account age, sex and height.
In the study, the researchers adjusted for race and ethnicity, birth weight, maternal smoking and alcohol intake, as well as other factors known to influence blood pressure.
The highest level of pollution exposure the mothers experienced was 11.8 micrograms per cubic meter or greater, which is just below the EPA’s air-quality standard of 12 micrograms per cubic meter. Exposures at higher levels are considered to be risky, but Mueller said the new study provides more support for maintaining or even lowering the standard to ensure that Americans are breathing healthful air.
It also suggests some practical advice for pregnant women in the third trimester: “You might consider not going outside in highly polluted areas during that time because of the risk,” he said.
The families involved are part of the long-term Boston Birth Cohort study, which is partially funded by the National Institute of Child Health and Human Development, so researchers hope to continue tracking them and publish follow-up reports. A previous study from California found a similar relationship between maternal exposure to a different pollutant — nitrogen dioxide, which comes from cigarette smoke or the burning of fossil fuels — and high blood pressure in children.
In an opinion piece published with the study, Diane Gold and Antonella Zanobetti of the Harvard T.H. Chan School of Public Health noted that the research did not look at the season of a child's birth, which might affect pollution levels in a temperate zone such as Boston's. Nor did it look at differences in the city's pollution levels over time. The young subjects were enrolled from 1998 to 2012, a period when much of the country woke up to the harmful effects of pollution and undertook greater efforts to reduce pollution.