Latest Research: “Associations Between Wildfire Smoke Exposure During Pregnancy and Risk of Preterm Birth in California”

A new study from Stanford University suggests that exposure to wildfire smoke during pregnancy increases the risk of preterm birth. The study, published August 14 in Environmental Research, finds there may have been as many as 7,000 excess preterm births in California attributable to wildfire smoke exposure between 2007 and 2012. These births occurred before 37 weeks of pregnancy, when incomplete development heightens the risk of various health complications and even death.

The research comes just a year after a historic wildfire season burned more than 4 million acres in California, producing some of the worst daily air pollution ever recorded in the state. Wildfire smoke contains high levels of PM 2.5, a harmful type of pollution from particles so fine they can embed deeply into the lungs and pass into the bloodstream. While the association between PM 2.5 pollution and adverse birth outcomes is already established, the new study is among the first to isolate the effect of wildfire smoke on early births.

The Published Study (click to expand)
Sam Heft-Neal, Anne Driscoll, Wei Yang, Gary Shaw, Marshall Burke, Associations between wildfire smoke exposure during pregnancy and risk of preterm birth in California, Environmental Research, Volume 203, 2022, 111872, ISSN 0013-9351,


Research Methods (click to expand)
The researchers paired data on smoke days for over 2,600 ZIP codes with estimates of ground-level PM 2.5 pollution and birth outcomes from California medical records.

Smoke days were assembled from satellite data of smoke plumes from the National Oceanic and Atmospheric Administration. Ground-level pollution estimates were developed using a machine learning algorithm that incorporates data from air quality sensors, satellite observations, and computer models of how chemicals move through Earth’s atmosphere. Birth records included 3 million California births and covered a wide range of socioeconomic groups. 

After accounting for other factors known to influence preterm birth risk, the researchers looked at how patterns of preterm birth within each zip code changed when the number and intensity of smoke days rose above normal for that location.


Researchers found that every additional day of smoke exposure during pregnancy increased the risk of preterm birth - regardless of race, ethnicity or income. On average, every day of wildfire smoke upped the risk of preterm birth by half a percentage point. A full week of exposure meant a 3.5% greater risk compared to a pregnant woman who had no exposure to wildfire smoke.

Mothers exposed during the second trimester experienced the highest risk of preterm birth. (click to expand)
An additional day of smoke exposure during the second trimester of pregnancy (weeks 14-26) was associated with a 0.83% increase in the risk of a preterm birth, as compared to a 0.68% increase for exposure in the third trimester. The effect of exposure during the first trimester was not found to be significant.


Smoke exposure had the strongest impact on less-frequently-exposed groups. (click to expand)
Mothers residing in zip codes with infrequent smoke exposure were found to be more affected by an additional day of wildfire smoke compared to mothers in regions where smoke is more common.


Observed associations were driven by higher intensity smoke days. (click to expand)
Exposure to low intensity smoke-days was not found to be significant, while additional medium or high intensity smoke days were both associated with an increase in preterm risk.


What does this mean for our children? (click to expand)
While the study only incorporated data from California, wildfires are becoming more frequent, intense, and widespread both nationally and globally. A 2017 report by the US Global Change Research Program recorded a “profound increase in forest fire activity” in recent decades. As a result, the health burden from smoke exposure - including preterm births - is likely to increase.

Climate change is a major driver for the increased risk and extent of wildfires. Rising temperatures evaporate more moisture from the ground, drying out the soil, and making vegetation more flammable. Stronger policies to limit the global temperature rise, such as stricter regulations for greenhouse gas emissions, are urgently needed.  

These findings also highlight the need for enhanced resilience and preparedness for climate-related disasters. Tools such as the CDC’s Social Vulnerability Index help emergency response planners and public health officials identify, map, and plan support for vulnerable communities. CDC also helps state, tribal, local, and territorial health departments prepare for potential health effects from climate change through its Climate-Ready States & Cities initiative. Currently the program funds 16 states and 2 cities. Expansion of this program, and increased support to health agencies across the country, will better address maternal and child health needs in our changing climate.