In recent years, environmental justice (EJ) researchers have examined the relationship between exposure to air toxics and children’s school performance at the level of the school [1
]. These studies have found a negative association between school-based exposure to air toxics and academic performance, usually measured as aggregated standardized test scores and/or rate of absenteeism. One of the first studies used United States Toxic Release Inventory (TRI) data and 1990 census tract-level estimates of respiratory air toxics risk to predict standardized test scores in Los Angeles Unified School District (LAUSD) schools [6
]. They found that air pollution levels at schools negatively and statistically significantly predicted standardized test scores, adjusting for school demographics. In a follow up study that included all public schools in California, Pastor et al. [7
] found that the general pattern observed in the LAUSD held for the rest of the state. Outside of California, similar findings have been found in Baton Rouge, Louisiana [3
] and Massachusetts [8
]. Using the USEPA’s (United States Environmental Protection Agency’s) 2002 Risk-Screening Environmental Indicators (RSEI) geographic microdata, Lucier et al. [3
] identified twelve known developmental, neurological and respiratory toxicants and found that the levels of these pollutants at school were linked to decreased test scores. Considerable research shows that respiratory and neurological toxins have negative impacts on school performance [3
], as does diesel particulate matter (PM) [9
]. To date, no research has examined the influence of ambient concentrations of metabolic disrupting chemicals on academic achievement.
Metabolic disrupting chemicals, or metabolic disruptors (MDs) as they are henceforth called, are important to consider because of the widespread use of these chemicals and the insufficient knowledge about the health effects of long term, low dose exposures. MDs are a specific class of endocrine disrupting chemicals (EDCs) that remain understudied despite growing evidence linking them to serious negative health consequences. Common MDs are bisphenol A, diethylstilbestrol (DES), phthalates, and organotins. MDs were first known as “obesogens” when scientists discovered that some EDCs predisposed laboratory mice to gain weight. In the last ten years, the term “metabolic disruptors” has become more common than “obesogens” because the effects of these chemicals extend beyond only weight gain and include altered liver and cardiovascular function and altered glucose homeostasis [11
As endocrine disrupting chemicals, MDs interfere with the endocrine hormonal system in human and wildlife species and alter bodily functions [12
]. They have been linked to obesity and also have negative impacts on neurological and thyroid function, insulin, metabolism, and glucose function/regulation, as well as adverse reproductive outcomes such as cancer of reproductive organs, endometriosis, infertility, and birth defects [12
]. MDs, like other ECDs, function to mimic naturally occurring hormones such as estrogens, androgens, and thyroid hormones.
The link between environmental chemicals and weight gain is relatively novel [14
], and evidence confirming the link between exposure to MDs and obesity has mounted in the past two decades [15
]. Understanding the implications of exposure to MDs at the individual and population level is critical, because the use of MDs is widespread and highly unregulated; MDs are in food, pharmaceuticals, household products, and air emissions from a variety of sources. Many of these chemicals bioaccumulate and persist in the environment and have been detected in most people who have participated in biomonitoring studies [18
]. We hypothesize that there may be a link between ambient outdoor air concentrations of MDs and individual children’s academic achievement. Previous studies linking air pollution to academic achievement have focused on explaining the association through respiratory health effects e.g., missing school due to asthma attacks [6
] or through neurological effects on children’s development [2
]. We propose an additional possible explanation, which is that MDs influence children’s weight, which negatively impacts their school performance. This is informed by studies finding a negative association between higher body mass indexes and reduced school performance in children [20
The evidence for this link between children’s weight and school performance is not as well documented as the non-cognitive effects of obesity (e.g., glucose intolerance, high blood pressure, high cholesterol, sleep apnea, depression, and anxiety) [24
]. Findings connecting obesity in childhood to academic outcomes are beginning to appear in the literature and some studies assert that the linkage is neurological as opposed to social/behavioral [26
]. Recently, two studies found that childhood obesity was associated with poorer working memory performance and a decreased ability to regulate one’s cognitive control network, which includes inhibition, working memory, and cognitive flexibility [29
]. This is significant because the relationship between the cognitive control network and school performance is well-documented [31
]. While a neurological link is probable, there are also social and behavioral factors that likely influence the relationship between weight and school performance, such as bullying, discrimination from peers and teachers, and low self-esteem [32
This study on MDs and children’s school performance builds from a large body of environmental health and environmental justice (EJ) scholarship. In the US, the academic EJ literature has shown that environmental risks have disproportionately fallen on minorities and the poor, and that race and class are often the greatest predictors of exposure to hazardous air pollutants in the US [37
]. Previous school-based EJ studies have not examined MDs as an environmental hazard nor have they included body weight as a mediating variable in the relationship between residential exposure to air toxics and academic achievement. Specifically, we examine the impacts of ambient MDs at children’s home sites on grade point averages among a representative sample of 1319 fourth and fifth graders in El Paso, Texas directly and indirectly through body mass index using a mediation model.
Research Questions and Proposed Model
The research questions and hypotheses are: (1) What is the direct effect of residential MD concentration on children’s grade point average (GPA), controlling for relevant covariates (e.g., economic deprivation, mother’s education, mother’s English proficiency, and teenage motherhood)? We hypothesize that MD concentration will be negatively associated with children’s GPA; (2) What is the direct effect of body mass index (BMI) on children’s GPA, controlling for relevant covariates? We hypothesize that BMI will be negatively associated with children’s GPA; (3) Does BMI mediate the association between residential MD concentration and children’s GPA, controlling for relevant covariates? We hypothesize that while MDs will have a direct effect on GPA, this effect will be mediated by BMI.
This study is novel in that it is the first to address a limitation in the literature on pollution and academic performance, and the EJ literature more generally, with its focus on MDs. It is a first foray into examining the potential impacts of ambient concentrations of MDs on school performance. The relationship between exposure to hazardous air pollutants and academic achievement has been clearly documented [1
], yet the pathways through which this relationship works remain unclear. Researchers have suggested that the relationship between exposure to air pollutants and decreased school performance can be explained in two ways [9
]. First, exposure to air pollutants can be linked to respiratory illnesses and more missed school due to these illnesses. Second, the relationship can be explained through neurological impacts. Exposure to air toxicants may damage critical areas of children’s brains [73
], resulting in reduced capacities to learn and retain information. In the study, we hypothesized that air toxics, specifically MDs, also have negative implications for children’s academic achievement both directly and indirectly through children’s BMI. This hypothesis encompasses three relationships, which we will discuss in turn.
The first relationship is the path connecting the concentrations of MDs at the child’s home to BMI, which was positive and statistically significant, supporting our hypothesis. This finding seems to substantiate the previously hypothesized environmental link between toxics and weight [11
]. This finding is not surprising given that MDs disrupt the body’s endocrine and metabolic functioning, meaning that the children are more susceptible to weight gain, diabetes, and metabolic syndrome [11
The second relationship is the path from BMI to GPA, which was negative and statistically significant, indicating that higher BMI was associated with lower GPA. In terms of why this association was found, there are currently two hypotheses in the literature. The first is a psychosocial explanation, which asserts that heavier children tend to face bullying and discrimination and have lower self-esteem, which translates into generally worse school outcomes [32
]. The second is a biological explanation, which posits that obesity affects brain and memory functions, contributing to poor academic performance outcomes [26
The third relationship is the direct effect of MD concentrations on GPA, which was negative and statistically significant. This reveals that although BMI significantly mediates the relationship between MDs and GPA, there is an additional direct effect whereby MDs predict lower GPAs, apart from their associations with BMI. This is likely because MDs harm bodies in myriad ways beyond promoting weight gain. The impacts of MDs even extend beyond metabolic and endocrine disruption to include, for example, neurological and developmental effects [11
]; it is probable that these chemicals inflict additional damage that may degrade school performance.
These findings are important at both the individual and the population level. At the individual level, they are significant because school achievement predicts both future success in the labor market and health outcomes in adulthood [74
]. The environmental justice literature has documented that poor and minority children are exposed to greater levels of air pollution than their white and more affluent counterparts [5
]. Thus, it stands to reason that the greatest impacts of this insidious association between toxics and school performance are being borne by those who also struggle with other educational challenges. Additionally, it is unlikely that parents or teachers would notice small decreases in GPA and attribute them to either pollutants or BMI, making these findings particularly concerning and difficult to address through behavioral interventions. It suggests that few will take action to prevent exposure to these chemicals, since these associations will almost always go unrecognized.
At the population level, the bodily impacts of chemicals, including MDs, can have catastrophic effects. In addition to the growing cost of obesity and obesity-related diseases within the healthcare sector, Colborn [76
] posited that decreased cognitive abilities due to chemical exposure could place a greater strain on remedial education while leading to the development of fewer people deemed intellectually “gifted.” Ultimately, population-level neurological and cognitive damage stemming from chemical exposures could place a dramatic burden on the healthcare sector and negatively impact the development of human capital, decreasing societal capacities for scientific and technological innovation.
Limitations and Directions of Future Research
This study has several limitations. One relates to the measurement of MDs. There is not yet an official list of endocrine or metabolic disrupting chemicals, which impacts this study and others seeking to examine the effects of MDs on human and ecological well-being. Researchers with The Endocrine Disruption Exchange and the Institute for Environment and Health have provided lists of known and suspected MDs, but these are not monitored, regulated, or regularly updated by agencies such as the USEPA, US Centers for Diseases Control, or World Health Organization. Additionally, as of 2016, over 1000 EDCs have been identified [60
]; the 2005 NATA includes data on only 177 chemicals which are all regulated by the Clean Air Act (Environmental Protection Agency 2014), many of which may have metabolic disrupting effects that are currently unknown. Because many chemicals in industrial use and on the consumer market are not regulated, many such chemicals now in circulation (or in development) may be identified as MDs for generations to come.
There are limitations with the USEPA’s NATA data. First, we paired our 2012 survey data with 2005 NATA estimates, which were the most recent estimates available when the study was conducted. Despite the time lag, we believe that the concentrations between 2005 and 2011 have remained relatively constant given that all major freeways, roads, factories, refineries, airports, train stations, and ports of entry within the EPISD have remained in the same locations since 2005. Secondly, this study examined only MDs that are emitted in the air and regulated under the Clean Air Act, and hence included in the NATA. Airborne emissions do not fully encapsulate the multiple forms of exposure to MDs, as MDs and EDCs are found in household and personal care products [77
], in indoor air and dust [79
], and food products and packaging [80
], in addition to ambient air emissions.
Lastly, there are limitations associated with our survey data. We lack variables related to children’s health behaviors (e.g., exercise and diet), which would be important to control for in future studies. The effects of exposure to MDs, and EDCs more generally, are mediated by sleep patterns, the microbiome, preexisting health conditions, and other social and genetic determinants of health not included in our model [11
]. The data used in our BMI calculations were reported by caretakers as opposed to collected through in-person measurements of the children. Research has shown that bias in height and weight reporting is greatest among parents of young children (ages 2–5) and reduced when they have older children [82
]. Given that the children under study are ages 10–11, reporting bias is likely less of an issue than if we were studying preschoolers. Note also that researchers have concluded that parent-reported height and weight measures are accurate enough to be applied in studies on obesity in children ages 3–17 [83
]. The use of cross-sectional survey data means that we examined associations at only one point in time and cannot determine if levels of MDs preceded decreased school performance.
The findings from this study report a disturbing association between probable chemical exposures to a new and relatively unregulated class of chemicals and academic performance in elementary school children. Children are more susceptible to the adverse effects of exposure to air toxics because they have larger lung surface in relation to their body weight compared to adults, meaning that they breathe in more air per kilogram of body weight. Additionally, children spend more time outside than adults do, and often engage in activities such as playing sports and running that require them to breathe in more air so their body weight burden is significantly greater than that of adults [84
]. They also have more open pathways of environmental exposure (e.g., through hand-to-mouth contact) than do adults. These children, the majority of whom are ages 10–11, are in the pre-pubescent development stage. According to the developmental origins of disease theory, children at this stage of life are in a critical window whereby the body is much more sensitive to environmental exposures and stressors than it is at older ages; environmental exposures during these critical windows can radically alter gene development and, in turn, affect organ and tissue function [11
]. Additionally, these environmental exposures are more likely to result in adverse health outcomes in children due to their rapidly developing systems. Exposure to MDs during this critical window of vulnerability may impact children’s abilities to learn and retain information.
These results contribute to discussions about the environment and obesity and the growing literature on children’s weight and academic achievement. Findings corroborate previous studies linking exposure to air toxics to academic achievement and demonstrate that, even after controlling for economic and demographic factors, MDs have negative impacts on children’s academic performance both directly and indirectly through BMI. The relevance of research on MDs is difficult to overstate, since rates of childhood obesity are expected to double in the next 20 years [26
Research on MDs is in its infancy; in the future, more chemicals will be found to have metabolic disrupting properties, making such studies increasingly important. In terms of chemical-testing policy in the US, there is an urgent need to move beyond the cancer paradigm (i.e., a focus solely on cancer when testing a single chemical) and to develop innovative ways to test entire classes of chemicals (e.g., MDs) for multiple health effects at the same time. MDs have insidious effects on the endocrine system, which serves as the body’s “biological highway” [76
]. Non-cancer outcomes including memory loss, decreased neurological capabilities, and behavioral disorders, such as autism and attention deficit/hyperactivity disorder, have serious consequences for human well-being. In order to reduce exposure to MDs and reduce the body burdens of toxic chemicals in humans, chemical manufactures could be required by US law to show “proof of safety” before chemicals are sold, as opposed to the current model, which allows chemicals to be sold until they are proven to cause adverse health effects. Researchers should also develop methods of monitoring both personal and community exposure to MDs across generations. The effects of environmental exposures may have a greater impact on health and non-health outcomes (including school performance) than genetic determinants, even though environmental determinants receive less focus and research funding.