How Air Pollution Affects Preterm Birth Rates
We all know air pollution is bad for our health, but a recent study suggests air pollution is a likely culprit for millions of preterm births around the world.
The meta-analysis, published in PLOS Medicine, was the first of its kind to calculate the global impact of outdoor and indoor pollution combined. Researchers analyzed data from 124 existing studies investigating the link between air pollution and negative birth outcomes, such as low birth weight (LBW) and preterm birth (PTB), from the United States, Europe, Australia, India, China, South America and sub-Saharan Africa.
After controlling for other risk factors, including smoking and alcohol use, poor nutrition and pregnancy weight, the study found air pollution was a leading cause of low birth weight and premature birth globally. About 16 percent of all babies born with low birth weight (about 5.5 pounds) and approximately 36 percent of preterm births (defined as before 37 weeks of pregnancy) were attributable to air pollution, according to the report.
Air pollution is measured by exposure to fine particulate matter (PM2.5) with a diameter of 2.5 micrometers or smaller, formed as a result of chemical reactions between pollutants. PM2.5 can be absorbed into the bloodstream when inhaled and lead to health complications. The study authors suggested that about 2.8 million births of babies with low birth weight and 5.9 million preterm births in 2019 could have been averted if PM2.5 exposure during the entire pregnancy was reduced to the minimum risk levels.
More than 92 percent of people worldwide live in areas where the outdoor air quality is below the World Health Organization's recommended limits, while around 49 percent of the global population is exposed to high levels of indoor air pollution. The main source of indoor pollution is burning solid fuels, such as coal and wood, for cooking and heating, which is especially common in developing areas of Southeast Asia and sub-Saharan Africa.
"The effect is small, but the burden is not," said Rakesh Ghosh, Ph.D., an epidemiologist at the University of California, San Francisco, and the study's lead researcher. "PTB and LBW occur in every population across the world, which means millions of neonates are affected every year due to maternal exposure to air pollution during pregnancy. If exposure is reduced, millions of babies will be born on time and will not be low birth weight. Thus, millions of babies will have better chances of living through the early period—reduce neonatal mortality—and living a healthy life thereafter—reduce neonatal morbidity."
About 16 percent of all babies born with low birth weight and approximately 36 percent of preterm births were attributable to air pollution.
Low birth weight and preterm birth can increase the risk of infant mortality in the first year of life and can have other long-term health implications, such as developmental disabilities, lung diseases including asthma, and digestive issues.
Exposure to air pollution during pregnancy can affect the health of a pregnant person and, therefore, a developing fetus, because contaminants can affect the development of the placenta and umbilical cord.
"In other words, the supply of food and gaseous exchange to the fetus is affected. Pollutants also cause inflammation of the protective membranes, causing early rupture and preterm birth. Some organic pollutants may also cause mutations, causing long-term damage," Ghosh said.
As somebody's belly is growing, they take shorter and more frequent breaths, because their lung capacity is getting smaller as the baby's pushing up, said Cindy M. Duke, M.D., Ph.D., a Las Vegas-based reproductive endocrinologist and America's only dual fertility expert and virologist.
"If a pregnant person is exposed to viral particles, they're going to inhale more in the end than a nonpregnant person. If someone has an underlying lung disease, like an asthmatic, air pollution can compromise their ability to breathe," Duke explained.
"If you're not breathing well, you're not getting enough oxygen, so your baby's not going to grow well, your risk of miscarriage goes up, but also your risk of pregnancy complications like preeclampsia and toxemia goes up, as well," Duke continued. "The complications they have can lead to preterm delivery, even pregnancy loss. We now believe that if mom is having problems with air pollution and struggling while pregnant, her child is also going to grow up to be at higher risk for that, too."
According to Ghosh, several factors determine the specific effects of air pollution on a pregnancy. The pollutant in question, the amount and length of exposure, and the point in the pregnancy when the exposure occurred all play a role.
"PM2.5 has the most evidence and is arguably the most harmful. Chemical composition also matters, for example, polycyclic aromatic compounds are more harmful because they could affect the genes," he said. "The level as well as the timing—first, second or third trimester during pregnancy—of exposure are important."
According to Ghosh's research, socioeconomic status and smoking likely increase the risk of air pollution exposure.
"We definitely see it in inner cities and urban centers, places that have a lot of cars," Duke confirmed. "African American women in the U.S. have higher rates of preterm delivery, and then when you really look at it, it has to do with those who are in urban centers, which, again, is where you have higher rates of air pollution."
Geographic and population differences should also be taken into account.
"South Asian countries have very high levels of pollutants, so do sub-Saharan and some South American countries, so the population differences are largely due to differences in exposure levels," Ghosh said.
His research showed that strategies including using clean fuels, such as liquefied petroleum gas, and high-ventilation cookstoves reduced exposure to indoor pollution. Where clean fuel wasn't an option, the use of PM2.5 masks also reduced chronic exposure to pollutants and might reduce the effects of indoor air pollution on pregnancy.
"We want the stakeholders to look beyond air pollution as an adult health or premature adult mortality problem," Ghosh said. "It affects babies and our future generations as severely as it affects adults."