Babies are at higher risk of being born prematurely – that is, before 37 weeks of the normal 40 weeks of pregnancy – if their mothers are exposed to fine particle air pollution when they are expecting.

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High exposure to fine particle pollution in the third trimester was linked to the highest risk of premature birth.

This was the conclusion of a study by researchers at Cincinnati Children’s Hospital Medical Center and the University of Cincinnati, OH, published in the journal Environmental Health.

The team identified a 19% higher risk of women giving birth prematurely if they are exposed to fine particle air pollution during pregnancy, with the risk being highest if the exposure happens in the third trimester.

First author Emily DeFranco, an associate professor of maternal-fetal medicine at the University of Cincinnati, says:

“Although the risk increase is modest, the potential impact is robust, as all pregnant women are potentially at risk.”

Harmful particle pollution is one of the most common air pollutants in the US. The particles are of various sizes and shapes, come from numerous sources and comprise hundreds of different chemicals. In urban areas, a high proportion of particle pollution comes from diesel exhaust.

The ability to harm health depends on the size of the particles. Smaller particles are the most injurious because they can be inhaled and cause serious problems, such as heart and lung diseases.

Fine particles – of diameter up to 2.5 micrometers, also referred to as PM 2.5 – have been the focus of much health research and regulation. They are more stable than ultrafine particles and travel farther than coarse particles. To get an idea of its size, consider that a fine particle is about 30 times smaller than the thickness of human hair.

For their study, the researchers examined records of births in Ohio between 2007-2010, during which there were nearly 225,000 singleton – as opposed to multiple – live births, including more than 19,000 premature ones (8.5% of the total).

The team correlated the birth record data with average daily levels of PM 2.5 fine particles obtained from measures of air quality recorded at 57 Environmental Protection Agency (EPA) stations across the state.

They found that high PM 2.5 exposure occurred frequently during the study period, with 11% of the pregnant women having high exposure in all three trimesters.

After adjusting for other potential risk factors, the researchers found women with high PM 2.5 exposure through pregnancy had a 19% higher risk of premature birth.

Assessed per trimester, the analysis showed the third trimester had the highest premature birth risk, at 28%, when the pregnancy was exposed to high PM 2.5 levels.

The researchers classed exposure to PM 2.5 as high if the average over the specified time period went above 15 μg/m³, which was the EPA annual standard at the time of the study.

The Clean Air Act requires that the EPA set and periodically review national air quality standards for particulate matter and other pollutants considered harmful to health.

Prof. DeFranco, who is also a physician-researcher at the Center for Prevention of Preterm Birth at Cincinnati Children’s, notes what effect reducing fine particle pollution might have:

We estimate that decreasing the amount of particulate matter in the air below the EPA’s standard threshold could decrease preterm birth in women exposed to high levels of small particulates by about 17%, which corresponds to a 2.22% decrease in the preterm birth rate in the population as a whole.”

Meanwhile, Medical News Today recently learned of a study that found depression in fathers increases risk of premature birth for offspring, providing insight into a much neglected area of research: how the father may affect the mother-fetus pair and birth outcomes.