Highlights
– In a prospective nationwide cohort (ECHO, N=20,034 births), wildfire-specific PM2.5 exposure was associated with higher odds of preterm birth in the US West but not in the pooled national sample.
– In the US West (N=5,807), each 1 µg/m3 increase in mean daily wildfire PM2.5 across pregnancy was associated with 13.9% higher odds of preterm birth (OR 1.139, 95% CI 1.001–1.296).
– Exposure-response relationships were observed for smoke-day intensity and duration (smoke days at ≥5 and ≥10 µg/m3 and ≥4-day smoke waves) and for mid-to-late pregnancy smoke exposure at the national level.
– Findings support public health efforts to reduce pregnant people’s exposure to wildfire smoke and motivate targeted interventions during wildfire seasons.
Background and Clinical Context
Preterm birth (delivery before 37 completed weeks of gestation) is a leading cause of neonatal morbidity and mortality and contributes substantially to long-term neurodevelopmental and cardiometabolic risk. Environmental exposures, including ambient fine particulate matter (PM2.5), have been associated with elevated risk of preterm birth in multiple epidemiological studies. Wildfire smoke, an increasingly important source of episodic PM2.5 in many regions, has distinct chemical composition and temporal patterns compared with urban PM2.5. As wildfire frequency, duration, and intensity have risen across large parts of North America, determining whether maternal exposure to wildfire-specific PM2.5 increases the risk of preterm birth — and identifying the critical exposure windows — is essential for clinical counseling and public health policy.
Study Design and Methods
This report summarizes a prespecified cohort analysis from the US Environmental Influences on Child Health Outcomes (ECHO) Consortium (Sherris et al., Lancet Planet Health, 2026). The investigators included live singleton births with known gestational age and birthweight conceived from Jan 1, 2006 to March 20, 2020. Residential address history was linked to census-tract level daily estimates of wildfire-specific PM2.5 derived from a previously described machine-learning model covering 2006–2020.
Exposure metrics included:
- Mean daily wildfire PM2.5 across pregnancy (µg/m3).
- Counts of smoke days above prespecified thresholds (>0, ≥2.5, ≥5.0, and ≥10.0 µg/m3).
- Counts of consecutive smoke days (2, 3, or ≥4 days) — termed smoke waves — at those thresholds.
Primary outcome was preterm birth (<37 weeks). Nationwide associations were estimated using adjusted pooled logistic regression; stratified analyses were performed for the US West census region. Gestational-week–specific associations (weeks 0–35) between smoke days and preterm birth were assessed with logistic regression in the national sample. Models adjusted for relevant covariates (demographics, maternal characteristics, and temporal factors); the paper provides full model lists and sensitivity analyses.
Key Findings
The analysis included 20,034 births from 30 ECHO sites, with maternal residences during pregnancy spanning the 48 contiguous US states and DC. Overall preterm birth prevalence was 8.4% (1,687/20,034). Mean daily wildfire PM2.5 across pregnancy averaged 0.36 µg/m3 (SD 0.46), and the mean number of smoke days >0 µg/m3 was 22.2 days (SD 16.6).
Nationwide pooled analyses returned estimates that included the null across the primary exposure metrics. However, in the US West (N=5,807), exposures were associated with increased odds of preterm birth:
- Mean daily wildfire PM2.5: OR 1.139 per 1 µg/m3 increase (95% CI 1.001–1.296).
- Each additional smoke day with wildfire PM2.5 ≥5.0 µg/m3: OR 1.018 (95% CI 1.003–1.032).
- Each additional smoke day with wildfire PM2.5 ≥10.0 µg/m3: OR 1.030 (95% CI 1.006–1.054).
- Each additional ≥4-day smoke wave at ≥5.0 µg/m3: OR 1.185 (95% CI 1.044–1.347).
- Each additional ≥4-day smoke wave at ≥10.0 µg/m3: OR 1.232 (95% CI 1.029–1.475).
Gestational timing analyses at the national level identified associations between smoke days and preterm birth during mid-pregnancy for thresholds >0, ≥2.5, and ≥5.0 µg/m3, and during late pregnancy for the highest threshold (≥10.0 µg/m3). This pattern suggests that both cumulative exposure and late-pregnancy acute high-intensity events may be relevant to triggering preterm delivery.
Interpretation of Effect Sizes
Although absolute increases in PM2.5 attributable to wildfire smoke (in µg/m3) may appear small on average, the episodic nature of wildfire smoke and the observed exposure–response with intensity and duration (smoke days and smoke waves) indicate clinically relevant increases in risk for populations in heavily affected regions. For example, multiple additional high-intensity smoke days or smoke waves during wildfire seasons could meaningfully shift population preterm birth risk among pregnant people living in wildfire-prone areas.
Mechanistic Plausibility
Biological mechanisms plausibly linking wildfire PM2.5 exposure to preterm birth include systemic and placental inflammation, oxidative stress, endothelial dysfunction, and alterations in maternal immune responses. PM2.5 can translocate particles or particle-associated constituents to the circulation and placenta, potentially affecting placental perfusion and fetal growth signals. Short-term high-intensity exposures (smoke waves) may precipitate inflammatory cascades that increase the risk of labor onset or compromise gestation, consistent with the study’s findings of mid-to-late pregnancy vulnerability.
Strengths
- Large, prospective, multisite cohort with geographic coverage across the contiguous US and detailed residential history linkage.
- Use of a wildfire-specific PM2.5 model allowed attribution of fine particles to wildfire sources rather than general ambient PM2.5.
- Granular exposure metrics (intensity, duration, consecutive smoke days) and timing-by-week analyses provided nuanced insight into exposure–response and critical windows.
Limitations and Considerations
- Exposure misclassification is possible: census-tract exposure assignments may not capture indoor exposures, individual mobility, occupational exposures, or household air filtration use. Differential misclassification could bias associations toward the null in the national analysis.
- Residual confounding remains a concern, including unmeasured socioeconomic factors, access to healthcare, co-exposures (e.g., heat during wildfire events), and maternal behaviors during smoke events.
- Generalizability: Stronger associations in the US West may reflect regional differences in wildfire characteristics (fuel types, fire intensity, distance), population susceptibility, or exposure contrast; results may not directly translate to other regions worldwide.
- Although adjusted for many covariates, the observational design cannot prove causality; however, consistency with mechanistic data and exposure–response patterns strengthens causal inference.
Clinical and Public Health Implications
For clinicians caring for pregnant patients, these findings underscore the importance of counseling about wildfire smoke exposure during wildfire seasons, particularly for patients in high-risk regions such as the US West. Practical advice can include strategies to reduce exposure during smoke events: staying indoors with windows closed, using high-efficiency particulate air (HEPA) filtration or portable air cleaners, limiting strenuous outdoor activity, and following public health notices regarding air quality and evacuation orders. In some situations, appropriately fitted respirators (e.g., N95) may reduce exposure for brief necessary outdoor activities, although their use in pregnancy should be individualized and balanced against tolerance and cardiopulmonary load.
At the population level, these data support the prioritization of pregnant people as a vulnerable group in wildfire preparedness planning: targeted public health messaging, distribution of air filtration resources, establishing clean air shelters, and integrating wildfire smoke risk into prenatal care guidelines during wildfire seasons.
Research and Policy Gaps
- Interventional studies are needed to test whether exposure-reduction strategies (e.g., HEPA filters, community clean-air centers) during pregnancy reduce preterm birth risk in wildfire-affected areas.
- Further mechanistic research is required to delineate pathways linking episodic high-intensity exposures to parturition triggers and to identify biomarkers of effect and susceptibility.
- Standardizing exposure metrics and improving individual-level exposure assessment (personal monitors, indoor measurements) would reduce measurement error and strengthen causal inference.
- Policy analyses should consider cost-effectiveness of targeted mitigation for pregnant populations and integrate wildfire smoke into air quality regulations and public health emergency planning.
Conclusion
The ECHO cohort analysis provides robust, prospective evidence that wildfire-specific PM2.5 exposure is associated with increased odds of preterm birth in the US West, with consistent exposure–response relationships for intensity and duration and temporal vulnerability in mid-to-late pregnancy. These findings reinforce the classification of pregnant people as an at-risk population during wildfire events and support both clinical counseling and public health interventions to reduce exposure. Given the projected increases in wildfire activity with climate change, integrating wildfire smoke risk mitigation into prenatal care and community resilience planning is an urgent priority.
Funding
The ECHO Program analysis was supported by the US National Institutes of Health Office of the Director. The primary report: Sherris AR et al., Wildfire-specific fine particulate matter and preterm birth: a US ECHO Cohort analysis. Lancet Planet Health. 2026 Sep 9:101324. doi: 10.1016/j.lanplh.2025.101324. Epub ahead of print. PMID: 41197644.
References
1. Sherris AR, Dearborn LC, Goin DE, et al.; ECHO Cohort Consortium. Wildfire-specific fine particulate matter and preterm birth: a US ECHO Cohort analysis. Lancet Planet Health. 2026 Sep 9:101324. doi:10.1016/j.lanplh.2025.101324. PMID: 41197644.
2. World Health Organization. WHO global air quality guidelines 2021. Geneva: WHO; 2021. (Guidance on PM2.5 and health effects.)
3. Centers for Disease Control and Prevention. Wildfire smoke — Information for clinicians and public health professionals. CDC; 2024. Available at: https://www.cdc.gov/disasters/wildfires/health.html (accessed Nov 2025).
Thumbnail prompt (AI-friendly)
A pregnant woman looking out a living room window at a distant landscape covered in dense orange-gray wildfire smoke; inside, a portable HEPA air purifier runs and a clinician’s pamphlet titled “Pregnancy & Air Quality” sits on the table — photorealistic, muted tones, empathetic and informative mood.
