September 13, 2021

Evidence that Wildfire Smoke Exposure is Associated with an Increase in Preterm Labor by Phyllis Leppert, MD, PhD

Recently a research article published by Stanford University scientist made headline in the national press. The researchers looked at data from three million births in California between 2007 and 2012. They correlated the birth information with data regarding wildfire smoke and reported a 3.4% increase in preterm births in California due to wildfire smoke exposure.(1) They adjusted for other factors, such as reported racial identity, and income, known to be associated with preterm birth.  They further reported that in 2008 when 1.6 million acres burned in California due to 6,200 wildfires, there was an increase of 6.3% in preterm births in the state. Considering that as of September 12, 2021, the day I wrote this blog, a total of 2,243,663 acres burned in California from  a total of 7377 reported wildfires, (2) it  could be assumed that an increase of preterm births could be happening now. This is a serious crisis for the health of mothers and babies.

Wildfire smoke contains particulate matter (PM10, PM2.5 and even smaller particulate matter). Wildfire smoke also contains carbon monoxide, nitrogen oxide, nitrogen dioxide and volatile organic compounds. Wildfire smoke particulate matter can be smaller than the size usually found in air pollution is urban industrialized environments. These smaller particles have been found to contain more oxidative compounds such as oxygenated polycyclic aromatic hydrocarbons and quinones.  They also contain more pro-inflammatory components such as aldehydes and nitrogen oxide.(3) These are very toxic substances.  The small particles are able to penetrate the small vessels of placentas and there they trigger damage that will trigger preterm labor. The Stanford study also suggests that  wildfire exposure in the second trimester is linked to a higher increase in preterm birth compared to exposure at other times in pregnancy.

The findings of this paper supports the findings from previous studies.  It is true that confounding variables in population-based studies make it difficult for epidemiologists to demonstrate the causative factor contributing to adverse  birth outcomes. In the case of wildfire smoke exposure, the confounding variables are many.  In addition to those mentioned above, they include pre-existing maternal disease and conditions and the background pollution in the air of the environment prior to the wildfire. This is why scientists state that further studies are necessary. However as more and more studies are conducted and reported, it adds to knowledge and lends increased credence to the fact that wildfire smoke does indeed increase the risk of preterm birth.

There are many reasons as to why wildfires are increasing across the globe. Summers are hotter and the hotter temperatures are lasting longer, summer rainfall is decreasing leading to drier land and forests. Due to changing ocean-atmospheric oscillations like El Nino, global surface wind is increasing. Finally, because of changes in the mass of ice particles in clouds, lightning strikes are increasing. (3)  We are learned early in life that fire needs fuel, oxygen and an ignition source. Because of climate change, all three of these factors are increasing across the world.  Soil and forests are dry, increased surface wind increases oxygen and increased lightning strikes increases the ignition.  It is interesting to note that a large percentage of the wildfires in California this year are reported to be due to lightning strikes.

Wildfires will harm human health directly through burns, injury and serious smoke inhalation and can cause death. They directly cause serious mental health conditions. They also cause indirect harms thought wildfire smoke exposure. It is time for all of us to understand the impact of increased wildfires on reproductive health.


(1.)  Heft-Neal S, Driscoll A, Yang W,  Shaw, Burke M. Associations between wildfire smoke exposure during pregnancy and risk of preterm birth in California, Environmental Research, Volume 203,2022,111872,ISSN00139351

(2.) Wikipedia.  Accessed September 21, 2021.

(3,) Xu R,  Yu P, Abramson MJ et al., Wildfires, Global Climate Change, and Human Health. N Engl J Med 2020; 383:2173-2181 DOI: 10.1056/NEJMsr2028985