Pollution isn’t good for pregnant women, and a 2016 research has found that even small amounts of air pollution seem to raise the risk of a condition in pregnant women linked to premature births and lifelong neurological and respiratory disorders in their children.

To conduct the study, the researchers analysed data from 5,059 mother-child pairs in the Boston Birth Cohort, a predominantly low-income minority population. They assessed the presence of intrauterine inflammation based on whether the mother had a fever during labour and by looking under a microscope at the placenta, which was collected and preserved after birth. They assessed maternal exposure to fine particulate matter (PM2.5) air pollution using data from EPA air quality stations located near the mothers’ homes. Boston, where the women lived, is known as a relatively clean city when it comes to air pollution. The majority of the women in the study were exposed to air pollution below the level that EPA deems acceptable, fewer than 12 micrograms per cubic meter. A subset of 1,588 women (or 31 percent) were exposed to air pollution at or above the EPA standard.

The researchers found that pregnant women who were exposed to the highest levels of air pollution were nearly twice as likely as those exposed to the lowest levels to have intrauterine inflammation and it appeared that the first trimester might be a time of highest risk. These results held up even when researchers accounted for factors including smoking, age, obesity and education levels.

Intrauterine inflammation is one of the leading causes of premature birth, the researchers say. Babies born prematurely can have lifelong developmental problems.

According to the study’s lead author Rebecca Massa Nachman, PhD, a postdoctoral fellow in Department of Environmental Health Sciences at the Bloomberg School: “This study raises the concern that even current standards for air pollution may not be strict enough to protect the fetus, which may be particularly sensitive to environmental factors. We found biological effects in women exposed to air pollution levels below the EPA standard.”

The study’s senior author, Xiaobin Wang said: “Twenty years ago, we showed that high levels of air pollution led to poor pregnancy outcomes, including premature births. Now we are showing that even small amounts of air pollution appear to have biological effects at the cellular level in pregnant women.”