How does wildfire smoke affect pregnancy outcomes?

As wildfires blaze in the U.S., Canada, and other places around the globe this summer, millions of people are gearing up yet again to protect themselves and their families from the health risks of wildfire smoke. And as wildfires have become more intense over the years—quadrupling in size over the last four decades—researchers have begun to focus on one group’s vulnerability to smoke exposure in particular: pregnant people.

So how exactly do wildfires impact pregnancy? The answer lies in tiny air pollutant particles released in wildfire smoke, called PM2.5 (so named because this “particulate matter” measures less than 2.5 microns in size). Exposure to these fine particles can enter the nose, lungs, and eventually into your blood. Being exposed to small amounts of PM2.5 causes symptoms like runny nose, coughing, sneezing, and eye irritation. Prolonged exposure can lead to more serious conditions, like chronic bronchitis, asthma, and cardiovascular disease.

During pregnancy, the growing fetus depends on maternal blood to thrive. Oxygen reaches the fetus through the placenta, which allows for organ development and growth. Up until recently, it was unclear whether pollutants inhaled during pregnancy could actually reach the fetus, but researchers have now established that fine pollutants released from wildfire smoke, like PM2.5, can also cross the placental barrier. Previous research studies have found wildfire smoke exposure linked to low birth weight; this trend is especially apparent when exposure occurs in the first trimester. Smoke exposure during pregnancy may also increase the risk of preterm birth, especially if exposed during the second trimester.

Interested in learning more about how wildfire smoke affects pregnancy outcomes, our team at UCSF conducted a study on the impact of smoke exposure on newborn birth weight. We used data from San Francisco-based air pollution monitors during the 2018 wildfires and found the peak of outdoor PM2.5 to have occurred over a two-week period in November 2018. We then looked at birth data from UCSF and sorted pregnancies by whether or not they were exposed to the November 2018 peak. Although the results are preliminary, we saw an association between PM2.5 exposure in the second trimester and babies born large for their gestational age, a term used to describe neonates born in the top 90th percentile for weight. We also found that this association was especially strong in pregnancies with gestational diabetes.

Babies with macrosomia, the condition termed for neonates born large for their gestational age, are at risk of trauma secondary to complicated vaginal deliveries and conditions like hypoglycemia (low blood sugar) independent of whether gestational diabetes was present during pregnancy. Being born with macrosomia therefore has important clinical significance and should be further investigated to understand how to mitigate risks. Few research projects have analyzed links between wildfire smoke and macrosomia, so more studies are needed to better understand this relationship.

Our team is especially interested in understanding how varying degrees of structural privilege determine risk of adverse birth outcomes during wildfires. One of our most pressing questions is how pregnancies differ depending on socio-economic status, ability to work from home when air quality is hazardous, and how housing infrastructure affects people’s protection indoors. To learn more about the disparities in wildfire exposure and pregnancy outcomes, more information needs to be collected about pregnant people’s occupational and socio-economic realities, as well as any racial and language barriers that limit access to public health resources and medical services during wildfire season. We are planning a new study to enroll and follow pregnant people at UCSF to determine if factors such as location, resources and behaviors may impact the growth and birth of the babies.

To learn more about the health risks of wildfire smoke, why children are more vulnerable, and how to protect children and families from smoke, please check out this interview with Dr. Stephanie Holm, a pediatrician and co-director of the Western States Pediatric Environmental Health Specialty Unit at UCSF. Video courtesy of SciLine and the American Association for the Advancement of Science.

About the author

AC Fernández is a second year in the UC Berkeley-UCSF Joint Medical Program and an EaRTH Center Environmental Scholars Program recipient. A graduate of Cornell University and a former case manager and Spanish language interpreter at Memorial Sloan Kettering’s Immigrant Health & Cancer Disparities Service, she worked this summer with Dr. Amy Padula to study the impact of wildfire smoke on pregnancy outcomes and how marginalized communities are more vulnerable. She hopes to explore the community health benefits of federal and local renewable energy transitions in future research.