Researchers examined data on more than 1.3mn healthy singleton pregnancies from 30 provinces in mainland China in 2013 and 2014. Almost 105,000 of the babies, or 8%, were born before 37 weeks’ gestation, making them premature arrivals.
To see how air quality may have influenced the risk of a preterm delivery, researchers used mothers’ home addresses to estimate their exposure to an extremely small type of fine particulate matter known as PM 1, a mixture of solid particles and liquid droplets smaller than 1 micrometer in diameter that’s found in traffic exhaust and can include dust, dirt, soot, and smoke.
Over their entire pregnancies, half of the women in the study were exposed to average PM 1 levels of 46 micrograms per cubic meter of air (ug/m3). For each 10 ug/m3 increase in PM 1 exposure, women had a 9% higher risk of a preterm delivery.
“We’ve long known that air pollution contributes to preterm birth,” said Dr Leonardo Trasande, an environmental medicine researcher at New York University School of Medicine in New York City, who wasn’t involved in the study.
“What’s novel in this study is the careful examination of smaller particles which are more readily inhaled,” Trasande said by e-mail.
Much of the previous research linking air pollution to preterm births has focused on what’s known as PM 2.5, or fine particulate matter that’s smaller than 2.5 micrometers in diameter, Xu Ma of the National Research Institute for Family Planning in Beijing and colleagues note in JAMA Paediatrics.
The current study offers fresh evidence of the harmful effects of PM 1, which makes up about 80 percent of PM 2.5. Compared to larger particles, PM 1 has a higher surface area to mass ratio and can reach alveoli, the tiny air sacs in the lungs, Ma and colleagues write. Ma didn’t respond to requests for comment.
Women living in the Beijing, Tianjin, and Hebei regions, the Yangtze River delta, the Sichuan Basin, and the Pearl River delta were exposed to PM 1 levels greater than 52.7 ug/m3 over their entire pregnancy, the study found.
For every 10 ug/m3 increase in PM 1 exposure levels throughout their pregnancy, women were 20% more likely to have very preterm births, with babies arriving between 28 and 31 weeks gestation, and 29% more likely to have extremely premature babies, with infants delivered between 20 and 27 weeks.
In the weeks immediately after birth, preemies often have difficulty breathing and digesting food. They can also encounter longer-term challenges such as impaired vision, hearing and cognitive skills, as well as social and behavioural problems.
One limitation of the study is its reliance on satellite data to estimate mothers’ exposure to air pollution, a method that might not always reflect women’s actual exposure because it can’t account for the quality of air indoors, at work, or during commutes.
Still, pregnant women should take precautions to limit their exposure to air pollution, whether by reducing their time outdoors during high-traffic hours or wearing an air filtration mask, advised Dr Shruthi Mahalingaiah, an environmental health researcher at Boston University Medical Campus who wasn’t involved in the study.
Women considering pregnancy might also ask their doctor whether taking baby aspirin or progesterone might help reduce their risk of a preterm delivery, Mahalingaiah said by e-mail.
