Hot off the press: Heat and air pollution linked to poor pregnancy outcomes

EBM Focus - Volume 15, Issue 32

Reference: JAMA Netw Open. 2020 Jun 1

At the time of writing, millions of acres are burning across California, Oregon, and Washington – fires that have been impacted by record-setting heat as a result of climate change. Rising temperatures and poor air quality have long been linked to cardiorespiratory disease and cancer in nonpregnant adults. Newer data suggest that heat and pollution may also be associated with adverse pregnancy outcomes according to a recently published systematic review of observational studies.

This systematic review evaluated 68 cohort, cross-sectional, time-series, and case control studies with over 32 million births in the US in 2007-2019, and examined the effects of air pollution and heat on pregnancy outcomes, including preterm birth, low birth weight, and stillbirth. Exposure to air pollution was defined by the level of fine particulate matter (PM2.5) and/or the level of ozone (O3) and other pollutants. Heat was assessed using geographic temperature for the duration of pregnancy. Heterogeneity among studies precluded pooling of data and meta-analysis.

Exposure to PM2.5 was associated with increased risk of preterm birth (median 11.5% increase) in 19 of 24 studies, low birth weight (< 2500 g) (median 10.8% increase) in 25 of 29 studies, and stillbirth (median 14.5% increase) in 4 of 5 studies. Heat exposure was likewise associated with increased risk of preterm birth (median 15.5%) in 4 of 5 studies, low birth weight (median 31%) in 3 of 3 studies, and stillbirth in 2 of 2 studies. Preterm births increased with every 5.6°C rise in temperature in 3 studies in California. The associations between heat exposure and preterm birth and stillbirth were particularly high in Black and Asian women and in Black and Hispanic women respectively.

This study raises a number of important concerns about the impact of the environment on pregnancy outcomes. Any conclusions are naturally limited by the observational nature of the study designs, which is an issue with any analysis where randomization is not possible. Nonetheless, this review underscores the relationship between heat and air pollution exposure and adverse pregnancy outcomes, and highlights the impact on women of color, especially Black women. Although the absolute rates of preterm birth, low birth weight, and stillbirth may seem low, don’t forget that these outcomes are additive, are not happening in isolation, and likely have implications that we may not see for many years. Like the COVID-19 pandemic, the climate crisis will continue to exacerbate existing health inequities.

Only recently has climate change been widely perceived as a crisis with downstream health consequences such as increased exposure to heat and pollution, altered vector-borne disease transmission, threatened water quality and food security, increased risk of mental illness, and mass migration of people. This review supports one hypothesis attributing the observed drop in the number of premature births around the world during COVID-19 lockdowns to a decline in air pollution and adds to a growing body of literature indicating that air quality and temperature can have adverse effects on obstetric outcomes. It also demonstrates how climate change can disproportionately affect people of color. With more evidence that the health consequences of climate change go beyond flooding and storm damage, perhaps we need to focus public messaging regarding climate change on the associated health risks to highlight the urgency of action needed to curb its effects.

For more information, see the topic Risk Factors for Preterm Labor and Premature Birth in DynaMed.

DynaMed EBM Focus Editorial Team

This EBM Focus was written by Nicole Jensen, MD, Faculty Development Fellow and Clinical Instructor of Family Medicine at the University of Virginia. Edited by Alan Ehrlich, MD, Executive Editor at DynaMed and Associate Professor of Family Medicine at the University of Massachusetts Medical School; Dan Randall, MD, Deputy Editor for Internal Medicine at DynaMed; and Katharine DeGeorge, MD, MS, Associate Professor of Family Medicine at the University of Virginia.