Air pollution’s impact on birth weight

For years we have known that air pollution can be harmful, and in some cases that harm begins before birth. A growing body of research shows that pollution consisting of small particles suspended in the air, known as particulate matter (PM) pollution, is associated with poor birth outcomes like low birth weight (birth weight of less than 5.5 pounds). These negative outcomes can have life-long health effects, which is why it is crucial to find ways to reduce their likelihood.

In a new study published in the journal Environment International, researchers from the Texas A&M University School of Public Health and other institutions reviewed studies on PM and infant birth weight using a well-regarded systematic review methodology. Natalie Johnson, PhD, associate professor in the Department of Environmental and Occupational Health at the School of Public Health, and her colleagues evaluated existing research to determine the risk of bias and quality of evidence in each study and estimate the size of the effect for PM exposure.

PM air pollution is grouped into three categories based on the size of suspended particles. These include particles 10 microns in diameter and larger (PM10), particles 2.5 microns in diameter and smaller (PM2.5) and particles between these two sizes (PM2.5-10). Studies of prenatal PM exposure investigate links between one or more of these size groups and birth weight. The studies in this analysis looked into exposure during different periods of pregnancy, ranging from a single trimester to multiple trimesters or the entire pregnancy.

Johnson and colleagues used the Navigation Guide methodology to systematically review literature on PM pollution and birth weight. The Navigation Guide was developed in 2011 to improve how researchers evaluate data in environmental science and health fields. This methodology has proven its effectiveness in many environmental health studies since its creation.

The research team searched several literature databases and found 53 articles that met their criteria, 44 of which studied the relationship between PM pollution and birth weight in similar ways. They then investigated each study to determine their risk of bias and quality of evidence. Sources of bias include methods for assessing PM exposure, strategies for recruiting participants and incomplete data. Risk of bias, inconsistency and imprecision were a few qualities used to rate each study’s evidence quality. The analysis found that the relatively higher risks of bias in some of the studies were due to methods used to assess PM exposure. They also found variations in how much of an effect PM exposure had between studies.

The studies analyzed found that PM10 affected birth weight more than the other categories. However, most studies analyzed the effects of different PM categories separately. These categories most likely occur together, so models that investigate multiple sources of pollution may give a more accurate view of prenatal PM exposure. Additionally, most of the studies in the analysis did not consider other pollutants or heat, which can also affect birth weight.

The quality of evidence and risk of bias that Johnson and colleagues found mean that the current body of evidence is not strong enough to fully grasp the effects of PM pollution on birth weight. However, the evidence in the studies does support a relationship between increased PM pollution, particularly PM10, and low birth weight. The risk of bias and variation found in some studies point to the need for standardized and improved air pollution exposure assessment methods in future work. With more accurate and consistent evaluation methods, researchers will be able to more fully understand the relationships between PM pollution and poor birth outcomes like low birth weight. This would help inform interventions to improve birth outcomes, which could reduce health risks later in life.