Heatwaves and PM2.5: Sometimes-Surprising Associations with Preterm Birth

Pregnant Chinese woman walking outdoors and wearing a face mask on a sunny day.

2017. The investigators studied heatwave periods of at least two, three, and four consecutive days above various temperature thresholds ranging from 33 C (91:4 F) to 37 C (98:6 F).
Consistent with past studies, numbers of preterm births rose with heatwave exposure during the final week before birth. When the lowest temperature threshold was used to define a heatwave and the duration was less than 4 days, preterm birth was not increased. However, preterm birth did increase 1.1 times when a 33°C heatwave lasted 4 days. Associations increased with higher temperature thresholds and longer durations, with 1.9 times more preterm births after 3 days of 36:4 C or 2 days of 37 C.
Estimates of the combined effects of PM 2:5 and less intense heatwaves-which had weak or null associations with preterm birth when PM 2:5 was low-suggested that preterm birth increased more than expected when both were present, consistent with a synergistic effect. In contrast, when high PM 2:5 was combined with more intense heatwaves-which were positively associated with preterm birth even when PM 2:5 was low-the estimated combined effect was additive or slightly lower than expected. This suggests that higher PM 2:5 did not cause an additional increase in the risk of preterm birth beyond the risk associated with intense heatwaves alone.
The strong positive association between heatwaves and preterm birth increases with heatwave intensity. It is possible that, as the magnitude of the effect of heat increases, the relatively minor added effect of PM 2:5 doesn't matter-which may explain why a synergistic interaction between high PM 2:5 and extended hot temperatures seemed to be limited to less intense heatwaves. Image: © iStockphoto/torwai.

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The evidence that PM 2:5 acts synergistically only with less intense heatwaves to increase preterm birth-but not with more intense heatwaves-surprised the authors. Lead author Cunrui Huang, a professor of health policy and management at Sun Yatsen University, says behavioral adaptations may play some role. For example, he says, on days during less intense heatwaves, pregnant women may not feel the need to take protective actions. However, on extremely hot days, they may be more likely to drink more fluids or use air conditioning. "Such adjustments may offset some of the harmful effects of very intense heatwaves," Huang says. "More importantly, people tend to reduce outdoor activities during very hot days, thereby potentially reducing their exposure to both heatwaves and PM 2:5 ." "[This new research] strengthens the evidence that exposure to heat extremes and PM 2:5 can trigger preterm birth. The large sample size allowed exploration of different temperature thresholds and of the impact of consecutive numbers of hot days," says Kristie Ebi, a professor at the University of Washington's Center for Health and the Global Environment, who was not involved in the study. She notes that heatwaves of a week or longer are not uncommon. "With stronger associations on days three to four," she says, "it would have been interesting to understand whether the strength of the association continued to increase on days five and beyond." The authors treated gestational age as a time-to-event variable and performed a survival analysis to estimate short-term effects. To estimate the true effect of exposure at the week before birth, they controlled for the effects of longer-term gestational PM 2:5 and temperature exposures through an integrated model strategy. In addition, they took considerable care to test many different definitions of heatwaves. Bin Jalaludin, an epidemiology professor at the University of New South Wales, Australia, says the methodology is very sound. "This is important as there is no international agreement on what constitutes a heatwave," says Jalaludin, who also was not involved in the study. "With climate change, we anticipate more intense and more frequent heatwaves. Therefore, if these results are corroborated, we also anticipate more preterm births." It is never advisable to rely on just one study or a few studies to make policy or practice changes, adds Jalaludin. "It is important to obtain further evidence from regions with different air pollution levels, climatic conditions, and living conditions to corroborate the results from this study," he says.
As for Huang, he wants to continue and expand this line of research. "We hope to extend the target period to the whole gestation to see if earlier exposure to heatwaves could also affect preterm birth risk," he says. "In addition, we are conducting a prospective birth cohort study, where we have collected more detailed information, gestational complications, and also blood samples of pregnant women." Testing certain biomarkers may shed light on underlying mechanisms for the relationship between heatwaves, PM 2:5 , and preterm births.
Wendee Nicole is an award-winning freelance writer based in Houston. She has written for Nature, Scientific American, and other publications.