Research shows that exposure to air pollution during pregnancy increases the risk of postpartum depression for at least three years

Women exposed to higher levels of nitrogen dioxide (NO₂) or inhalable particulate matter (PM₁₀) are nearly four times more likely to develop postpartum depression during the second trimester of pregnancy, compared to women exposed to lower levels of those air pollutants. That higher risk persists for at least three years, according to a study just published in the journal Science of the total environment.

“What’s really new about this work is that we were able to extend the study of depression beyond the first year after delivery, and we showed that air pollution during pregnancy has a lasting effect on depressive symptoms for up to three years after delivery,” said Tracy Bastain, Ph.D., associate professor of clinical population and public health sciences at the Keck School of Medicine of USC and lead author of the study.

NO₂ comes from the combustion of fossil fuels in vehicles and power plants, while PM₁₀which refers to particles smaller than 10 micrometers in diameter, can include everything from dust and pollen to pollutants from factories and forest fires. Exposure to these chemicals in high concentrations or over long periods of time is known to increase the risk of a range of health problems, including asthma, heart attacks and strokes.

A new line of research, supported by the Keck School of Medicine team, is now examining the long-term effects of these and other chemical exposures on maternal and infant health.

The longitudinal study followed 361 mothers from the beginning of pregnancy to three years after giving birth. Researchers collected data on mothers’ depressive symptoms one, two, and three years after giving birth, then compared that data to weekly measurements of air pollution near their homes during pregnancy. They found a robust association between pollution levels and later depressive symptoms.

Women with higher exposure to NO₂ during weeks 13 to 29 of pregnancy had a 3.86 times higher risk of postpartum depression for up to 3 years. Women with higher levels of PM₁₀ exposure during weeks 12 to 28 had a 3.88-fold increased risk. Overall, 17.8% of women had depressive symptoms at one year, 17.5% at two years, and 13.4% at three years.

“Our study actually found a higher rate of clinically significant depression compared to recent CDC data“, Bastain said. “That’s quite significant — that there’s probably more postpartum depression than even our national prevalence data shows.”

The findings point to the need for continued mental health screening, including postpartum, for 12 months after delivery, the researchers said. They also suggest that minimizing exposure to air pollution during the second trimester may help reduce the risk of depression.

The risk of high exposure

The study included data from 361 mothers recruited through the Maternal and Developmental Risks from Environmental and Social Stressors (MADRES) Center for Environmental Health Disparities at the Keck School of Medicine. MADRES follows primarily Hispanic/Latino families in Los Angeles from pregnancy through childhood to measure the effects of various environmental exposures on long-term health.

Bastain and her colleagues used the Center for Epidemiologic Studies-Depression (CES-D) scale to measure clinically significant depressive symptoms at 12, 24, and 36 months postpartum. They also assessed air pollution exposure for each participant by comparing their home address (including any address changes) to air quality data from outdoor air pollution monitors in Southern California. For each participant, they calculated weekly average exposure levels during pregnancy for four separate pollutants: NO₂prime minister₁₀prime minister_2.5 and ozone (O₃).

The researchers calculated the effect of air pollution exposure on the risk of postpartum depression and then interpreted the data by scaling the effect to an interquartile range of exposure to each pollutant. They compared a relatively low exposure level (the 25th percentile of the cohort) with a relatively high exposure level (the 75th percentile of the cohort) to quantify the difference in depression risk for each pollutant.

A higher level of NO₂ exposure during weeks 13 to 29 of pregnancy increased the risk of depression 1 to 3 years after delivery by a factor of 3.86. Higher levels of PM exposure₁₀ increased risk of postnatal depression by 3.88. Exposure to PM_2.5 and O₃ did not increase the risk of postnatal depression.

Reduce the risk of depression

The findings suggest that reducing exposure to air pollution during the second trimester of pregnancy may reduce the risk of postpartum depression. Exercise during pregnancy remains important, Bastain said, but women should try to exercise outdoors during high-pollution periods, such as morning and evening rush hours or during wildfires. In the summer, it may also help to stay indoors (preferably in air conditioning) and away from busy roads during the hottest hours of the day.

“Another important implication of our work is that depression can persist well beyond the first 12 months postpartum. Mothers should talk to their health care provider if they continue to experience depressive symptoms,” Bastain said.

Next, Bastain and her colleagues at the MADRES Centre will continue their research into the effects of air pollution and chemical exposure on maternal and child health over time, as well as the biological processes underlying various health risks.

“We will continue this research so that we can protect the long-term health of mothers and children,” she said.