They may not breathe air or even have lungs yet, but the ash, car exhaust and dust from air pollution may affect the development of fetuses, according to a new study of pregnant mothers in Boston.

Those effects might contribute to certain health consequences later in life, said Dr. Rosalind Wright, a professor of pediatric medicine and public health at the Icahn School of Medicine at Mount Sinai and one of the study's authors.

Specifically, Wright and her research team measured something called "heart rate variability." Your heart rate naturally speeds up and slows down as you inhale and exhale, a kind of synchronization that scientists believe helps optimize oxygen distribution. Heart rate variability reflects the difference between those rates.

“Lower heart rate variability is considered maladaptive,” said Whitney Cowell, one of Wright’s post-doctoral researchers at Mount Sinai and the lead author on the study. “Children with that pattern are at a higher risk for a number of health problems like depression and cardiovascular disease.”

Wright and Cowell studied 237 women from Boston and their children. While the women were pregnant, the researchers measured the amount of air pollution in their neighborhoods daily, using a mix of air monitors and satellite data.

Six months after the infants were born, the researchers measured the babies’ heart rate variability. Cowell said the more air pollution they found in the mother's neighborhood during her pregnancy, the lower her child’s heart rate variability was likely to be.

“For every increase in air pollution, the outcome seems to get worse,” Cowell said.

Then, the researchers stressed the babies. In a healthy body, heart rate variability should vanish in stressful situations, Wright said.

“If you’re stressed, you need to increase your heart rate in a kind of fight or flight response,” she explained.

But that stress response wasn’t as clear in babies that had experienced more air pollution in utero.

“They look very stressed to an observer who sees them cry. Despite that, we are not seeing the stress physiologically in that system,” Wright said. “It’s blunted. Ideally, you want to see that system more capable of responding to stressors.”

The team, which reported the findings Wednesday in the journal Environmental Health Perspectives, cautioned that the effects they observed still need to be replicated by other researchers. But the stress response and heart rate variability observed by the team might set these kids up for certain negative health outcomes later in life, such as worse heart health and mental illness.

“The concern is we see this atypical pattern already in a 6 month old," Wright said. "If nothing changes and their system continues to operate in this way, when they face challenges as they go forward, they could be at risk for long-term complications and health outcomes if they are also genetically predisposed and so on."

It’s unclear how air pollution might be causing this outcome in infants, said Noel Mueller, an assistant professor of epidemiology at Johns Hopkins University who was not involved with the study.

“The leading hypothesis is that air pollution is leading to stress and inflammation of the placenta,” he said, and that increased inflammation might harm the baby's heart rate system.

Or, it’s possible that specks of air pollution are crossing the placenta and affecting the fetus directly, Wright and Cowell said. That might explain why fetuses are sensitive to such minute levels of pollution, the researchers suggested.

The air pollution in the pregnant women's neighborhoods ranged from 6 to 10.3 micrograms of particulate matter per meter cubed – well within the Environmental Protection Agency’s standard for air quality.

But Wright said while that level of pollution may not affect adults or children much, fetuses may be more vulnerable.

“Their organ systems and related processes are developing," she said. "Also, their ability to protect themselves from chemicals and pollutants has not had time to mature, so they cannot detoxify these chemical exposures like you and I can."

Whether fetal air pollution exposure truly leads to worse health outcomes later in life is difficult to know, since it hasn’t been studied before, Wright said. But infants still have an opportunity to correct problems like heart rate variability.

“These systems are still malleable and still going to be shaped as the kid continues to grow," she said. "We still have time to get it on a more optimal developmental trajectory.”

Pregnant women can also try to limit their exposure to air pollution, Wright added.

“Avoiding exercise near major roadways where traffic is dense," she said, "and [avoiding] exercise outside when the air quality index is high is a good idea."