Asthma is a chronic obstructive airway disease that disproportionately impacts racial and ethnic minoritized populations and children living in inner cities in areas of high poverty [1,2]. Since asthma has no cure, effective disease management and control partly focuses on identifying and minimizing exposures to environmental triggers [3]. While studies have demonstrated that air pollution is a recognized risk factor for pediatric asthma morbidity in urban settings, less is known about the role of other contaminants, like pesticides. Prior studies have targeted children living near agricultural areas or with parental occupational exposure to pesticides [4,5], with sparse evidence on inner-city children. Dilapidated housing conditions in disadvantaged neighborhoods increase the likelihood of pest infestations [6–10], where pesticide use is high and promoted to control pests recognized to increase asthma morbidity risk [7,9,11]. Pesticides can also persist indoors long after applications have occurred, posing long-term exposure risks for children [12]. Exposure to pesticides among children in urban settings may occur through inhalation from indoor residential use, consumption of treated food commodities, and from track-in of pesticide residues from outdoors [9,13–15].

Pesticides, including organophosphorus and pyrethroid insecticides, are frequently used in agricultural, residential, and gardening applications [16–18]. Exposure to pesticides is ubiquitous among humans with widespread exposures detected among U.S. children [19–22]. For example, glyphosate, the most commonly used herbicide in the U.S., has been detected in >80% of urine samples from U.S. adults and children [23,24], and among pregnant women in urban locations [25]. Regulatory restrictions have been enacted worldwide to ban select pesticides or applications in efforts to limit human exposure. However, in recent decades, the U.S. has relied heavily on voluntary cancellations from pesticide manufacturers and lagged behind the European Union, Brazil, and China in prohibiting the use of pesticides, including many that are considered hazardous to human health [26]. Exposure to these pesticides has continued in the U.S. population, raising concerns about their potential health risks [27–29], including adverse respiratory health effects [4,5,30].

Limited toxicological studies indicate that pesticides could induce or aggravate asthma through several mechanisms, including modifications of autonomic function and immune response [31–33]. For example, experimental studies in vertebrates have indicated that pyrethroids decrease gene expression related to immune function and induce apoptosis in thymus cells, decreasing immune response. This is hypothesized to result from activation of the glucocorticoid receptor, which can cause immunosuppression [34]. Other mechanisms by which pesticides like pyrethroids and others including organophosphates could impact respiratory health involve oxidative stress [35], a well-accepted mechanism in the pathogenesis of asthma that can initiate and augment inflammatory processes [19,36,37]. Pesticides, including glyphosate, could also induce airway inflammation by increasing neutrophil and eosinophil counts, cytokine production and mast cell degranulation as reported in mice [38].

Despite the potential for increased pesticide exposures among children with asthma in inner cities due to residential uses, track-in of residues from outdoors, and consumption of foods treated with pesticides, studies characterizing pesticide exposure and the role of pesticides on asthma morbidity in this population remain sparse. Here, we aimed to address current knowledge gaps by characterizing exposures to select pesticides and examining their associations with respiratory morbidity in a group of predominantly Black children with established asthma in a low-income urban setting. We hypothesized that exposure to the target pesticides would be positively associated with increased asthma morbidity.

The present study identified positive associations between concentrations of select urinary pesticide biomarkers and respiratory morbidity indicators in a sample of participants with asthma, largely composed of Black children from lower socioeconomic backgrounds. Specifically, we observed consistent positive associations between urinary levels of TCPY (biomarker of chlorpyrifos) and asthma symptoms. Our results highlight the ongoing need for pesticide biomonitoring in non-occupational populations, particularly in low-income and racially or ethnically diverse communities who often face higher exposure burdens to contaminants that may adversely impact respiratory health [59]. Exposure disparities in the study population may have been due to multiple sources, including consumption of treated food commodities, indoor residential uses, and track-in from outdoors though further studies are needed to investigate predominant sources in urban settings [9,13–15].

We observed widespread exposure to pesticides and exposure disparities, particularly organophosphorus insecticides. Detection frequencies and median urinary concentrations of parent-specific pesticide biomarkers, including TCPY, PNP and IMPY, were higher in our study population compared to U.S. children aged 6–17 years (NHANES 2007–2010, Table S5) [19]. Our results are consistent with previously reported pesticide exposure disparities [19,21,60], indicating that non-Hispanic Black populations experience pesticide exposures up to five times higher than the general U.S. population [59]. Furthermore, the median TCPY concentration (3.6 ng/mL) in our study was twice as high as those reported in more recent studies of Black children in the U.S. [19,61]. Declining exposure trends have been observed in the U.S. general population following regulatory actions like the ban of chlorpyrifos in 2000 for indoor residential uses due to health concerns, particularly among children [9,19,21,62]. Still, urinary TCPY (chlorpyrifos biomarker) concentrations in our study population measured 7–9 years post-ban (2007–2009) were comparable to pre-ban levels in U.S. children (NHANES 1999–2000) and to the highest quartile of exposure most recently estimated among U.S. children (NHANES 2015–2016). Chlorpyrifos is found to persist in indoor environments [9], which may explain the levels observed in our study population. Despite a complete ban for chlorpyrifos use on food commodities in 2021, its use was reinstated for select crops, including apples, strawberries, and citrus, in 2023 [63]. These findings suggest exposures and disparities may continue despite ban efforts.

We also observed higher exposures to the herbicide 2,4,5-T than those observed among U.S. children, ages 6–19 years (NHANES 2007–2010, Table S5) [19] even though its use in the U.S. was discontinued in 1985 [64]. The CDC stopped biomonitoring 2,4,5-T in 2011. Although this is not a frequently measured chemical in epidemiological studies, a study recently (2017) detected 2,4,5-T in U.S. adults [65]. Our finding highlights the need to consider continued biomonitoring of pesticides, including banned substances, among vulnerable and racially diverse communities and children, to evaluate the long-term impacts of policy changes on reducing pesticide exposure disparities.

In contrast, glyphosate, the most heavily applied pesticide globally, continues to increase in use [66], yet there is still limited evidence on human exposure levels in the U.S. [67,68]. The CDC has only started to quantify urinary glyphosate concentrations in the U.S. (NHANES: 2013–14, 2015–16, and 2017–18) and, to date, has not characterized AMPA, a main breakdown product and the primary metabolite of glyphosate [69]. While samples from the present study were collected earlier (2007–2009), the median urinary concentration of glyphosate (0.5 ng/mL) was comparable to levels more recently observed among U.S. children (NHANES 2013–2016, Table S5) [24,68]. The geometric mean urinary AMPA concentration in this study (0.6 ng/mL) was also similar to that observed among 14-year-olds from an agricultural region in California participating in the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) Study sampled in 2014–2015 (0.72 ng/mL) [70]. Notably, glyphosate was linked to increased asthma morbidity in the present study and emerging studies also indicate that exposures to this herbicide may also be linked to other adverse health risks [25,70,71]. Exposure to glyphosate among the general population can occur from consumption of treated food commodities and from use in residential and recreational areas [72], but predominant sources remain understudied in urban settings. Our findings justify ongoing biomonitoring efforts that are comprehensive and inclusive of all racial and ethnic groups.

While previous studies report positive associations between pesticide exposures with asthma prevalence and incidence and allergic effects in children [4,5,30], few have focused on respiratory morbidity in children with established asthma. To our knowledge, our findings are consistent with the only existing study to date that has also assessed pesticide exposures and respiratory morbidity in children with established asthma. In a sample of predominantly Black children (7–12 years) with asthma living in low-income, urban housing in Boston, Cincinnati, and New Orleans, Werthmann et al. reported positive associations, albeit not statistically significant, between urinary concentrations of PNP, a biomarker of parathion, and asthma-related healthcare utilization [61]. While we observed significant associations between urinary TCPY concentrations and asthma-related hospitalizations, instead of PNP, both biomarkers reflect exposure to organophosphorus (OPs) insecticides. As a class of insecticides, OPs may contribute to asthma through multiple mechanisms, including cholinergic overstimulation, airway hyperreactivity, inflammation, and oxidative stress [73,74]. Emerging evidence suggests both acute and chronic effects, with potential long-term consequences for respiratory health, particularly in vulnerable populations like children [73–76]. Although non-significant, Werthmann et al. also reported suggestive evidence that a mixture of multiple pesticide biomarkers (TCPY, 2,4-D, 3PBA, and PNP), which were assessed in our study, was positively associated with asthma-related healthcare utilization [61], but did not assess exposure to glyphosate, which was positively associated with emergency department visits for asthma in our study. While we did not observe consistent associations for the other pesticide biomarkers, there is still the potential for associations, as larger sample sizes may be needed to elucidate these relationships. Higher prevalence and higher exposures to select pesticides in our sample may have provided more variation in exposure as well as more asthma exacerbation events, thereby increasing our power to identify the observed associations [77].

While evidence synthesized from NHANES (1999–2008) reported generally null associations between asthma prevalence and exposure to organophosphorus insecticides among U.S. children, positive associations were observed among non-Hispanic Blacks, who experienced the highest rates of asthma prevalence [78]. Studies of children from agricultural areas, where pesticide exposures are often high, have also reported positive associations between organophosphorus pesticide exposures and decreased lung function [79], respiratory symptoms [80], and urinary concentrations of leukotriene E4, a marker of the cysteinyl leukotriene pathway implicated in asthma pathophysiology [81]. Although we did not observe statistically significant changes in lung function or inflammation in the present study, our results are consistent with previous studies that reported positive associations between endocrine disrupting chemicals and asthma symptoms [46,58]. Still, it is possible that pesticide exposures may potentiate asthma morbidity through other mechanisms, which do not directly correspond to increases in FeNO or changes in spirometry [73,74].

The present study has some limitations. First, we were able to conduct longitudinal assessment of pesticide exposures using urinary biomarkers, which are considered the gold standard, but the relatively short half-lives of these biomarkers limit the precision of exposure assessment over longer time periods [16,17,64,85]. Although analysis of biobanked samples serves as an efficient use existing resources, our exposure assessment was constrained to the timing of clinical visits, which occurred every three months in the parent study. For highly detected pesticide biomarkers, low ICCs confirmed the high temporal variability of urinary concentrations within children over the study period. While we collected repeated urine samples to characterize acute exposures, it is also possible that our acute exposures do not directly correspond to the outcome periods assessed (e.g., symptoms in the prior two weeks, healthcare utilization in the prior three months). However, we expect that non-differential measurement error (independent of asthma morbidity status) would attenuate the effect estimates observed [86], making it plausible that the true magnitude of associations identified are larger. We also observed consistent association trends across different outcomes and in our sensitivity analyses with TCPY and AMPA, providing suggestive evidence that our findings are not coincidental. Other limitations include our sample size, which restricted our capacity to examine sexually dimorphic effects. Additionally, the potential for spurious findings and unmeasured confounding remains possible, and the large number of biomarkers examined raises the opportunity of random variation causing the statistical significance of some associations. Finally, our study captures exposures from 2007–2009, and there is the potential that pesticide exposures have declined over time. However, we still observed associations with asthma morbidity at levels comparable to recent estimates for select pesticide biomarkers. Additionally, we observed detectable concentrations of select biomarkers despite multiyear bans on their parent pesticides, revealing persistent exposures and the need to investigate present day exposure disparities.

While acknowledging its limitations, the present study has several strengths. Notably, our efforts focused on an understudied population of predominantly urban, Black children with established asthma. To our understanding, the current study is the first to characterize exposure variability for glyphosate and AMPA, in addition to several other pesticide biomarkers, among children. Our prospective study design leveraged repeated measures to enhance the accuracy of both exposure and outcome characterization while decreasing the risk of recall bias. Few studies have examined associations between pesticides and asthma morbidity and our findings highlight the importance of conducting epidemiologic studies among populations with pre-existing health conditions to identify potential points of intervention. Finally, we were able to evaluate the robustness of our results by adjusting for meaningful confounders (e.g., race, education, season), and co-exposures to other contaminants previously linked to asthma morbidity (PM_2.5, NO_2, BPA, and phthalates) when examining the independent effects of target pesticide biomarkers on asthma morbidity.

This study on a group of predominantly Black children in a low-income setting provides evidence of pesticide exposure disparities and positive associations with asthma symptoms in children. We observed elevated concentrations of select pesticides comparable to pre-ban levels for biomarkers of chlorpyrifos and 2,4,5-T, in our population of Black children with asthma. We also detected concentrations of the most widely used herbicide globally, glyphosate, at a higher frequency than the general population of US children. These findings suggest that exposures to select pesticides are associated with asthma morbidity among children with established asthma, but the existing body of evidence remains quite limited. Future studies should continue to characterize pesticides as exposures may vary over time, especially in racially and ethnically diverse populations to elucidate pesticide exposure disparities and inform interventions aimed at reducing asthma morbidity in children.