Heatwaves and PM2.5 from wildfire smoke, non-wildfire sources, and all sources in relation to preterm birth: a nationwide cohort study in Canada

With climate change, the frequency and intensity of heatwaves and wildfires are increasing. We examined the independent associations of acute heatwaves and wildfire smoke-impacted particulate matter ≤2.5 µm diameter (PM 2.5 ) with preterm birth outcomes, and their combined effects. We conducted a nationwide population-based cohort study of singleton births in Canada (2010–2021). Temperature and PM 2.5 exposure during each gestational week were assigned from satellite data and a machine learning-based prediction model, respectively. Heatwaves were defined as 95th or 98th percentile regional weekly temperatures lasting 2–4 days (denoted as 95th-D2/D3/D4, 98th-D2/D3). PM 2.5 was categorized by regional percentile bins (≤50 th [ref.], 50–75th, 75–90th, >90th) and absolute bins (<6 µg/m 3 , ≥6–8 µg/m 3 , ≥8–10 µg/m 3 , ≥10–12 µg/m 3 , ≥12–14 µg/m 3 , ≥14–16 µg/m 3 , and ≥ 16 µg/m 3 ). Cox models assessed independent effects, and the relative excess risk due to interaction (RERI) assessed combined effects. Among 2,569,810 singleton births, 80,757 (3.1%) were preterm. Both PM 2.5 and heatwaves were independently associated with an increased risk of preterm birth. We observed a synergistic interaction (RERI > 0) between heatwaves and non-wildfire PM2.5 that increased in magnitude with higher concentration percentiles . The magnitude of this interaction increased as the heatwave threshold rose from 95th to 98th percentile, and as the duration increase from 2 to 4 days. Notably, the synergistic risk associated with 95th-D3 heatwaves and moderate (50th–75th) relative concentrations of wildfire smoke-impacted PM 2.5 (RERI = 0.35; 95% CI: 0.05, 0.65) was more than double the synergistic risk observed for corresponding moderate non-wildfire PM 2.5 (RERI = 0.16; 95% CI: 0.07, 0.25). Co-exposure to PM 2.5 and heatwaves exerts a synergistic effect on preterm birth risk that is significantly amplified when the particulate matter originates from wildfires. These findings highlight the need for early warning systems and public health messaging targeting moderate wildfire smoke events.