April 6, 2022

Wildfire Smoke Affects Pregnancy Outcomes

Results from an observational, fixed-cohort study
Risk of preterm birth increases with the number and intensity of smoke days.


Heft-Neal S, Driscoll A, Yang W, Shaw G, Burke M. Associations between wildfire smoke exposure during pregnancy and risk of preterm birth in California. Environ Res. 2022;203:111872.

Study Objective

To explore the effects of nearby wildfire smoke exposure during pregnancy on preterm birth in California


Observational, prospective, fixed-cohort study


Researchers first limited the study to infants born at the gestational ages of 23 to 41 weeks in California between 2006 and 2011 (of which there were 3,493,242 births); however, based on the estimated dates of conception, they included only 3,063,672 births in the present study to avoid fixed-cohort bias. (Births that occurred 23 weeks before October 2006 and after 41 weeks before December 31, 2012, were excluded from the study.)

Demographics included mother’s age, ethnicity, education, income, location of birth, parity, and history of cigarette usage, and the child’s sex and gestational age. Multiple births were excluded.

Study Parameters Assessed

Researchers drew data on births from birth certificates at the Department of Health in California for all 2610 zip codes. They characterized wildfire smoke exposure by the number of days the Hazard Mapping System (HMS) data correlated to the zip code of the mother’s residence during the gestational period, matching accumulation of weekly exposures to weeks of pregnancy. Researchers assessed exposure for each trimester with periods distinguished weekly relative to conception date. Trimester 1 was characterized as weeks 1 to 13, trimester 2 as weeks 14 to 26, and trimester 3 as the last 4 weeks of pregnancy. Exposure for the entire third trimester was not assessed to avoid having overlap of exposure prior to week 31.

Researchers measured the amount of particulate matter with diameters less than 2.5 µm (PM2.5) and other pollutants from the wildfire smoke using an intensity metric with data on the location based on zip codes allotted to grid cell level bins 0–50th, 50–75th, and >75th percentiles and the timing of wildfire smoke. They used National Oceanic and Atmospheric Administration (NOAA) imaging to identify smoke days and calculated estimates of ground-level pollution using air-quality sensors, satellite images, and computer modeling of atmospheric movements.

An HMS determined smoke and fire activity. Researchers calculated accumulated exposure to varying smoke intensities by classifying smoke days into different intensity bins. They took baseline air particulates into account so that the difference between baseline and wildfire-induced particulate exposure was tracked (=anomalies). These were divided into 3 categories per day:

  1. Low Intensity: Days with PM2.5 anomalies <5 μg/m3
  2. Medium Intensity: Days with PM2.5 anomalies 5–10 μg/m3
  3. High Intensity: Days with PM2.5 anomalies ≥10 μg/m3

Primary Outcome Measures

The researchers used a multivariable analysis using covariates based on demographics (ethnicity, race, education, income, residence, and parity) influencing risk of preterm birth (PTB) upon exposure to wildfire smoke during pregnancy. This was done per trimester and PM2.5 concentrations (low, medium, or high intensity) in the residing zip code of the mothers, using generalized regression models.

Key Findings

Within each zip code, there was an increase in preterm birth when the number and intensity of smoke days increased from baseline. Unlike in prior studies, these findings were independent of race, income, or ethnicity.


Exposure to PM2.5 medium-intensity smoke resulted in 0.95% (95% Cl: 0.47%–1.42%) increase in risk of PTB, while PM2.5 high-intensity smoke exposure had 0.82% (95% Cl: 0.41%–1.24%) increased risk. Smoke days for medium and high-intensity PM2.5 were higher for both the 2nd and 3rd trimester, with increased associated risks. PM2.5 low-intensity smoke had a value of zero for the entire pregnancy.


In total, 187,913 PTBs were attributable to exposure to wildfire smoke during the years 2007 to 2012. Additional wildfire smoke exposure during the pregnancy period was associated with a 0.49% (95% Cl: 0.41%–0.59%) increase in risk of PTB (<37 weeks). Smoke days during the 2nd trimester had the largest effects as additional days of smoke resulted in a 0.83% (95% Cl:0.71%–0.96%) increase in the risk of PTB in comparison to a 0.68% (95% Cl:0.49%–0.87%) increase for 3rd trimester exposure. The 1st trimester had a value of zero. During pregnancy, an additional smoke day had a 0.88% (95% Cl: 0.52%–1.24%) increase in risk of PTB <32 weeks and a 0.55% (95% Cl: 0.051.15%) increase in risk of PTB <28 weeks, with the 2nd trimester being more affected upon exposure than 3rd trimester.

Practice Implications

Air pollution is associated with an array of adverse health risks.1 Fetal health can be affected as maternal exposure to PM2.5 in wildfire smoke, containing thousands of chemicals, can lead to elevated risk of PTB, low birth weight, and congenital anomalies.1 PM2.5 can elicit an extensive range of biological responses as its minute size allows it direct entry into the blood through the lung tissue.1 Both pathways of oxidative stress and inflammation can have a role in affecting pregnancy outcomes as exposure to air pollution allows particles to cross the blood-brain barrier and placental barrier.1 PTB has been found to occur in about 12% of pregnancies worldwide and is the predominant cause of neonatal morbidity and mortality.2

While the relative rates of PTB did not differ between sociodemographic cohorts in this study, the baseline estimate for PTB in Black mothers was at least 1/3 higher than the other groups (ie, absolute rate). The data show that Black mothers have substantially higher rates of PTB in comparison to other races due to various factors, including healthcare disparities.Those experiencing health inequities need adequate healthcare services to decrease environmental health risks as they are more prone to pre-existing health conditions. Other studies have found that exposure to a natural disaster before, during, or after pregnancy can cause maternal stress, which is associated with adverse pregnancy outcomes.This is in keeping with the Developmental Origins of Health and Disease theory, which suggests that much of the disease risk faced later in life is set at the time of fertilization, embryonic, fetal, and neonatal stages of life through the gene-environment interaction.5 Maternal health may also suffer from high stress levels. Stress can have programming effects on the maternal hypothalamic-pituitary-adrenal axis, influencing effects on embryonic development, and has been linked to the development of gestational diabetes.4

Various therapies have been evaluated for treating sarcopenia, though exercise appears to outperform all others.

Long-term exposure to air pollution can impair beta-cell function and decrease insulin-dependent glucose absorption, causing insulin resistance.6 In a systemic review of air pollution in adults, an 8% to 10% increase was associated with diabetes for a 10 μg/m3 increase in PM2.5. or nitrogen dioxide.6 Several studies found associations between exposure to PM2.5 during pregnancy and gestational diabetes.6 Pregnant women who smoke tobacco or live in a smoking household are at an even greater risk of adverse pregnancy and fetal outcomes due to compounding toxic impacts of both wildfire and tobacco smoke.7 Tobacco smoke and bushfire smoke have similar constituents, making both detrimental to maternal health.7

Several measures can be taken to help reduce PTB. Maintaining a healthy lifestyle during pregnancy is crucial to proper fetal development. Consuming a balanced diet with foods rich in critical nutrients is essential.8 Adequate consumption of micronutrients is vital to support pregnancy and breastfeeding.8 For example, folate is essential before conception and in early pregnancy to reduce risk of congenital birth defects.9 Nutrient requirements increase markedly during pregnancy, so dietary supplementation with iron, B vitamins, iodine, and vitamin A may be warranted in women with lower levels of micronutrients.9 Adequate vitamin D is also essential in pregnancy to help maintain maternal calcium levels needed for fetal bone development. Deficiency of vitamin D can result in low birth weight, neonatal hypocalcemia, and cardiac failure.9 Women living in areas with limited sun exposure or who have darker skin are at higher risk of vitamin D deficiency.9

Physical activity during pregnancy decreases risk of obesity and gestational diabetes.8 Intervention studies show that pregnant women who exercise for roughly 30 min/day have lasting benefits for maternal health and wellbeing.8 Weight management and healthy body mass index (BMI) is important during pregnancy as postpartum weight retention heightens risk of hypertension, diabetes, and stillbirth in subsequent pregnancies.9 Intake of fish and additional supplementation of omega-3 fatty acids lower risk of PTB prior to 34 weeks gestation.9 Other factors that improve maternal quality of life and improve outcomes include managing stress, maintaining healthy sleep habits, microbiome modulation, and avoidance of alcohol.8

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  3. Dominguez TP. Race, racism, and racial disparities in adverse birth outcomes. Clin Obstet Gynecol. 2008;51(2):360-370.
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