Estimating Intervention Impact: Racial Disparities in Phthalate Exposure and Preterm Birth

A dark-skinned person paints her own fingernails, shown in close-up, with blue polish

https://doi.org/10.1289/EHP14501Phthalates, a class of chemicals commonly found in hair care products and processed foods, are associated with preterm birth among pregnant women. 1 Higher levels of exposure among Black women may be contributing to high rates of preterm birth within this group, according to a study published in Environmental Health Perspectives. 2 Preterm birth is a leading cause of infant mortality and disability in the United States. 3Preterm infants who survive may face such health problems as cerebral palsy, developmental delays, and poor vision and hearing. 4In the United States, the proportion of live births that were preterm climbed to a peak of 10.49% in 2021, then dropped very slightly in 2022 to 10.38%, according to the US Centers for Disease Control and Prevention (CDC). 4,5he racial disparity in rates of preterm birth has persisted for years, with preterm birth among non-Hispanic Black women at 14.7% in 2021, according to the CDC's nationwide data. 6That year, the rate among Hispanic women was 10.2%, and among women in two other groups-Native Hawaiians and other Pacific Islanders, and American Indians and Alaskan Natives-the rate was over 12%.In comparison, preterm births in non-Hispanic White women that year were 9.5%.
For this study, the research team looked at biomarkers of exposures to phthalates-endocrine-disrupting chemicals also associated with increased risk for adverse impacts on childhood growth and development, and reproductive health. 7,8These synthetic plasticizers are found in many consumer products, including toiletries, food wrappers, and medications, especially capsules and coated tablets. 9,10The chemicals are thought to contribute to several outcomes that increase the risk of preterm birth, such as inadequate placenta development 11 and other effects of maternal endocrine disruption. 12"We also strongly believe inflammation and oxidative stress 13 are involved in these relationships," says Kelly Ferguson, a senior investigator at the National Institute of Environmental Health Sciences and senior author of the study.
Existing cohort studies of racial disparities in phthalate exposure during pregnancy are limited by their small size and lack of diversity, according to the authors.To strengthen the results, the coauthors pooled data from 16 prospective pregnancy cohorts involving 6,045 live births in the United States between 1983 and 2018.Their statistical analyses focused on Black, White, and Hispanic/Latina participants due to low representation of other racial/ethnic groups in the pooled studies.
"One of our primary motivations was to increase the sample size of preterm births," says Barrett Welch, an assistant professor at the University of Nevada, Reno, School of Public Health and first author of the new work."That was a limitation of prior studies of individual cohorts." In addition to their use in nail polish, shampoos, hair straighteners, and other personal care products, phthalates are used across the plastics industry in polyvinyl chloride production, food packaging material, and thousands of consumer products. 10Image: © iStock.com/BraunS.

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In each of the prospective cohorts they analyzed, participants' phthalate exposures had been based on levels of phthalate metabolites in urine samples taken at least once during their pregnancies.Welch and his colleagues analyzed data on nine phthalate metabolites.As they had expected based on other studies in the literature, the authors found large racial and ethnic disparities in exposure levels.Phthalate metabolite concentrations as a whole were up to 148% higher in Black participants and up to 94% higher in Hispanic/Latina participants compared with White participants after accounting for other factors that could cause differences in exposure across these groups.
Then the authors took their pooled study one step further."We asked, 'What if all the study participants, regardless of their race and ethnicity, had about the same level of phthalate exposure?'"Welch says.To answer that question, the researchers used g-computation, a statistical approach that can estimate the probability of the same outcome under different circumstances.In this case, they sought an estimate of the probability of preterm birth if phthalate metabolite levels were similar across racial and ethnic groups.They based their question on the idea that phthalate exposures among Black and Hispanic/Latina participants could drop to the level of White participants following hypothetical interventions, such as regulation, education, and manufacturers' phasing phthalates out of their products (none of which had actually occurred).
Ultimately, the team predicted that lowering exposures via successful interventions could potentially reduce preterm births by 13% for Black infants to 103 per 1,000 live births, which is nearer the rate of 93 per 1,000 for White infants.For Hispanic infants, the hypothetical interventions were predicted to reduce preterm births by 9%, down to 70 per 1,000.(These figures reflect the study populations and not the national averages mentioned earlier, in which Hispanic mothers have a higher incidence of preterm births than White mothers.) This study suggested that racial disparities in phthalate exposures were not driven by socioeconomic factors alone.For socioeconomic comparisons, the authors used education level as a proxy; they wrote that their analyses would have been improved if income data for all the individual cohorts had been available.Notably, racial and ethnic disparities in phthalate metabolite levels persisted across all levels of education.
The largest disparities in phthalate exposures were for metabolites linked to personal care products.The authors hypothesized that differences in phthalate levels among hair care products may contribute to higher exposure levels in Black participants.
Phthalate exposures from processed foods also may have contributed to the observed disparities, although the authors note that consumption habits were not studied.Food products may contain phthalates due to processing as well as contact with plastic packaging materials.Phthalates are soluble in oil and have an affinity for fat, making them especially prevalent in high-fat processed foods. 14The new study is a step in the right direction," says Lesliam Quirós-Alcalá, an assistant professor at Johns Hopkins Bloomberg School of Public Health who studies chemical exposures and who was not involved in the work.But, she adds, "The findings don't inform interventions."Before recommending interventions, Quirós-Alcalá says researchers need to understand what drives women's use of products made with phthalates: "It's critical to get a grasp of people's attitudes, knowledge, and behavior." Success with personal-level interventions would be difficult to achieve, the authors predicted.For example, in regard to personal care product use, often there is insufficient information about the chemicals in hair and skin care products for consumers to make educated choices."Suggesting that pregnant women use 'phthalate-free products' assumes they are equally able to identify and afford them," says Ferguson.Other reports have contended that advertisers take advantage of structural discrimination-rooted in private or public institutional policies-against minorities. 15ew laws in California 16 and Maryland 17 may help lessen exposures.Both states approved bans, set to take effect 1 January 2025, on the sale of toiletries made with several known and suspected toxicants, including dibutyl phthalate and diethylhexyl phthalate.Tamarra James-Todd, an associate professor of environmental reproductive epidemiology at the Harvard T.H. Chan School of Public Health and study coauthor, predicts the laws will have an effect beyond state borders."Those laws will really help push industries to make safer products here in the United States," she says.