Exposure to wildfires during pregnancy has negative effects on infant health, according to a study recently published by the Centre for Demographic Studies (CED) at the UAB in the journal Perspectives Demogràfiques. Based on 3.4 million births in Spain between 2008 and 2021, the research identifies two mechanisms: stress and air pollution caused by fine particulate matter (PM2.5) carried by smoke. One effect is a 10% increase in the likelihood of low birth weight. Another effect is an increase in preterm births.
The reduction in birth weight is roughly equivalent to the impact of one week of smoking during pregnancy.
The study Wildfires before birth: How stress and smoke are affecting newborn health in Spain uses data on approximately 3.4 million births registered in Spain between 2008 and 2021, provided by the Spanish National Statistics Institute (INE). The data include all births registered in municipalities with more than 10,000 inhabitants, representing around 85% of total births.
To measure exposure to wildfires, they consider the occurrence of at least one large wildfire—defined as one with a burned area exceeding 500 hectares—in the mother's municipality of residence during pregnancy, based on data from the European Forest Fire Information System (EFFIS). This proximity measure, based on satellite observations of the burned area, aims to capture not only exposure to air pollution but also the stress, displacement, and disruptions to daily life that characterise these events
To analyse the impact of wildfires on childbirth, the study's authors—Maria Rubio-Cabañez, researcher at the CED and the UAB, and Risto Conte Keivabu, researcher at the Max Planck Institute for Demographic Research—compare mothers residing in municipalities affected by a wildfire during pregnancy with those living in unaffected municipalities, controlling for individual, geographic, and temporal characteristics.
The results show that wildfires harm pregnancy through two distinct mechanisms: on the one hand, the stress, fear, and displacement caused by proximity to the fires, and on the other, air pollution generated by the smoke. "A comprehensive response must address both, especially since the consequences of low birth weight or premature birth can persist throughout life," explains Maria Rubio-Cabañez.
Effects on birth weight
One of the effects is a 10% increase in the likelihood of low birth weight. Thus, newborns of pregnant women living near a large fire—covering more than 500 hectares—have a 0.4 percentage point higher probability of being born with low birth weight or prematurely (Figure 1). The reduction in birth weight is roughly equivalent to the impact of one week of smoking during pregnancy.
"Although it may seem like a modest increase, during a severe wildfire season, thousands of pregnant women can be exposed simultaneously, and even small variations translate into a significant number of affected babies," Rubio-Cabañez points out. "Furthermore, newborns who are premature or have low birth weight face a higher risk of respiratory problems, developmental difficulties, and poorer health throughout their lives," she adds.
However, the effects of fires extend far beyond the area where the flames are burning. Smoke can carry harmful particles hundreds of kilometers from the source of the fire, affecting pregnant women who have never even seen the flames. Wildfires release large quantities of fine particulate matter (PM2.5) that penetrate deep into the respiratory system and enter the bloodstream, triggering inflammatory responses and oxidative stress that can hinder the transfer of oxygen and nutrients to the fetus.
Northwestern part of Spain is more exposed
Pollution from wildfires was distributed very unevenly across Spain throughout the study period, with northwestern areas being the most exposed. Much of central and eastern Spain recorded fewer than 25 days with high levels of wildfire smoke pollution, whereas some municipalities in the northwest—located downwind of the country's highest-risk forest areas—accumulated more than 100 such days, as shown in Figure 2.
Breathing this smoke during pregnancy also harms babies. The more days a mother is exposed to high levels of smoke pollution during pregnancy, the greater the likelihood that her child will be born prematurely or with low birth weight. While this effect—like proximity to the fire itself—may seem minor on an individual level, it becomes a significant public health issue when viewed in the context of a fire season where thousands of pregnant women may be exposed simultaneously.
The two mechanisms operate at different stages of pregnancy: proximity to the fire has a greater impact during the first and third trimesters, whereas smoke is particularly harmful during the second. This indicates that a fire is not merely a situation causing pollution: the fear and displacement it causes affect fetal development independently of the smoke.
The study concludes that pregnant women should be treated as a particularly vulnerable group during wildfire emergencies, and proposes integrating reproductive health into emergency plans, ensuring the continuity of prenatal care for displaced mothers, and issuing early warnings so that pregnant women can reduce their exposure to smoke. "Given that climate change intensifies conditions conducive to wildfires in the Mediterranean, the consequences for future generations begin even before birth," concludes researcher Maria Rubio-Cabañez.
