Review shows association between pollutants, ozone, or heat and adverse outcomes

Worsening air quality and rising global temperatures brought on by climate change were significantly linked with serious adverse pregnancy outcomes, researchers reported.

In a systematic review, a total of 57 studies (84% on air pollutants; 90% on heat) showed a significant association between air pollutant and heat exposure and birth outcomes in the U.S., according to Bruce Bekkar, MD, who is retired from the Southern California Permanente Medical Group in Del Mar, and co-authors.

Exposure to fine particle matter (PM2.5) or ozone was associated with increased risk of preterm birth in 79% of the studies, and low birth weight in 86% of the studies, while exposure to heat was tied to those same outcomes in 90% of the studies, they stated in JAMA Network Open.

“This review suggests that increasingly common environmental exposures exacerbated by climate change are significantly associated with serious adverse pregnancy outcomes across the U.S.,” the authors stated. “It appears that the medical community at large and women’s health clinicians in particular should take note of the emerging data and become facile in both communicating these risks with patients and integrating them into plans for care.”

On a related note, the COVID-19 pandemic has led to better air quality in general because of quarantine and stay-at-home orders. The ongoing BiSC project in Spain looks at the impact of pollution during the pandemic on pregnant women.

“Pregnant women are particularly at risk for health compromise by air pollution because of their altered cardiopulmonary physiology and by high temperatures since their ability to thermoregulate is compromised at all gestational ages,” explained Linda C. Giudice, MD, PhD, of the University of California San Francisco, in an editorial accompanying the study.

She pointed out that the current review demonstrated a “disproportional impact on vulnerable populations,” specifically minority women, African American women in the U.S., Hispanic women in California, and women with asthma.

Bekkar’s group reported that 10 studies reported the association of racial/ethnic disparities with increased risk of preterm birth among minority mothers, and eight outlined a consistently higher risk for black mothers (77% of 13 studies). One study showed a higher risk for preterm birth among patients with asthma.

Less than half a dozen studies founds ties between preterm birth and lower educational level, government insurance, and/or lack of early prenatal care.

“Additionally, increased risks for poor pregnancy outcomes were noted in some studies among women with low socioeconomic status and residential proximity to power plants and freeways,” Guidice stated.

For instance, Bekkar and co-authors reported that a study of traffic-generated PM2.5 in Los Angeles and Orange County, California found that preterm birth overall increased 3% (95% CI 1% to 6%), deliveries <35 weeks increased 7% (95% CI 3% to 12%), and deliveries <30 weeks increased 18% (95% CI 10% to 26%) per interquartile range of 1.35 µg/m3.

Guidice also urged healthcare providers to be aware of the increased risks these women face from environmental causes and more, “with the goal to minimize these exposures if possible.”

She acknowledged that “direct control over climate is often beyond the control of an individual,” but providers can keep abreast with information from professional groups, such as the International Federation of Gynecologists and Obstetricians, which has issued a statement on Climate Crisis and Health “to raise awareness among its members about these important issues.”

The authors included English-language studies — predominantly comparative observational cohort studies and cross-sectional studies with comparators — on air pollutants or heat and obstetrical outcomes that were done between January 2007 and April 2019 with “no requirement for minimum sample size for inclusion.” Out of more than 1,000 studies, 68 articles met the study criteria, and about 34 million births were analyzed.

For PM2.5 exposure and ozone, there were 24 (41%) studies on preterm birth, 29 (50%) on low birth weight, and five (9%) on stillbirth. Out of 58 studies addressing air pollution, 56 (96%) included PM2.5, 23 (40%) included ozone, and 21 (36%) analyzed both.

For heat exposure, there were 10 studies looking at the association between heat exposure and obstetrical outcomes, specifically five (50%) on preterm birth, three (30%) on low birth weight, and two (20%) on stillbirth.

The authors discussed some possible mechanisms of action behind these findings. For preterm birth, there may be “maternal hematologic transport of inhaled noxious chemicals, the triggering of systemic inflammation, or alterations in function of the autonomic nervous system.”

Low birth weight may be linked with air pollutants by “direct toxic effects from fetal exposure, altered maternal cardiac or pulmonary function, systemic inflammation from oxidative stress, placental inflammation, altered placental gene expression, or changes in blood viscosity,” and these effects may occur simultaneously.

Finally, “heat exposure may contribute to prematurity through labor instigation from dehydration… from altered blood viscosity, and/or by leading to inefficient thermoregulation… heat exposure may impair fetal growth by reducing uterine blood flow and altering placental-fetal exchange.”

Review limitations included the fact that it was done in observational studies with heterogeneous sources of air pollution and heat exposure. “For both air pollution and heat exposure, different study designs may complicate direct comparison of the data even within a single study,” the authors added.

Bekkar’s group suggested that future research should focus on “at-risk populations, high-exposure geographic areas, and effects of seasonality,” and that “improved geographic information systems,” could be mapped onto public health databases, such as the national Kids’ Inpatient Database.

  1. There was a statistically significant association between heat, ozone, or fine particulate matter and adverse pregnancy outcomes in women in the U.S., according to a systematic review.

  2. Exacerbation of air pollution and heat exposure related to climate change may be significantly tied to a risk of adverse pregnancy outcomes in the U.S.

Shalmali Pal, Contributing Writer, BreakingMED™

Bekkar and co-authors reported no relationships relevant to the contents of this paper to disclose.

Giudice reported serving as chair of the International Federation of Gynecologists and Obstetricians Committee on Reproductive and Developmental Environmental Health (2019-2022), and being the founder University of California San Francisco Program on Reproductive Health and the Environment.

Cat ID: 41

Topic ID: 83,41,41,138,192,149,925