Abstract
34
35
Introduction— Farmworkers in the United States, especially migrant workers, face unique 36
barriers to healthcare and have documented disparities in health outcomes. Exposure to 37
pesticides, especially those persistent in the environment, may contribute to these health 38
disparities. 39
40
Methods—We queried the National Health and Nutrition Examination Study (NHANES) from 41
1999-2014 for pesticide exposure biomarker concentrations among farmworkers and non-42
farmworkers by citizenship status. We combined this with toxicity assay data from the US 43
Environmental Protection Agency’s (EPA’s) Toxicity Forecast Dashboard (ToxCast). We 44
estimated adverse biological effects that occur across a range of human population-relevant 45
pesticide doses. 46
47
Results—In total, there were 1,137 people with any farmwork history and 20,205 non-48
farmworkers. Of the 14 commonly detectable pesticide biomarkers in NHANES, 2,4-49
dichlorophenol (OR= 4.32, p= 2.01x10-7) was significantly higher in farmworkers than non-50
farmworkers. Farmworkers were 1.37 times more likely to have a bioactive pesticide biomarker 51
measurement in comparison to non-farmworkers (adjusted OR=1.37, 95% CI: 1.10, 1.71). 52
Within farmworkers only, those without U.S. citizenships were 1.31 times more likely to have 53
bioactive pesticide biomarker concentrations compared those with U.S. citizenship (adjusted OR 54
1.31, 95% CI: 0.75, 2.30). Additionally, non-citizen farmworkers were significantly more 55
exposed to bioactive levels of /g2010-hexachlorocyclohexane (BHC) (OR= 8.50, p= 1.23x10-9), p,p-56
DDE (OR= 2.98, p= 3.11x10-3), and p,p’-DDT (OR= 10.78, p= 8.70x10-4). 57
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4
58
Discussion— These results highlight pesticide exposure disparities in farmworkers, particularly 59
those without U.S. citizenship. Many of these exposures are occurring at doses which are 60
bioactive in toxicological assays. 61
62
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1.1 Introduction 63
Pesticide exposure has been linked to a myriad of human health outcomes such as obesity, 64
immune alteration, cancer, neurological conditions, type II diabetes mellitus, and death (Wei et 65
al. 2014; Zong et al. 2018; Medehouenou et al. 2019). More specifically, many pesticides are 66
strong endocrine disruptors because they mimic hormones like estrogens and androgens (Briz 67
et al. 2011; Wong et al. 2019). Persistent pesticides last in the environment and human body for 68
years or even decades and can bioaccumulate and bioconcentrate. Persistent pesticides 69
include organochlorines like dichlorodiphenyltrichlorethane (DDT), Lindane, Chlordane, Dieldrin, 70
Heptachlor and their metabolites. Non-persistent pesticides include organophosphates, 71
carbamates, pyrethroids, chlorinated phenols, acyl alanine fungicides and more chemical 72
groups, and were thought to be the less harmful answer to previously used persistent chemicals 73
(e.g. organochlorines) (Abubakar et al. 2020). However, non-persistent chemicals still affect 74
human health. While pesticides are associated with endocrine disruption, cancers, and motor 75
neuron disorders, there is still a lack of human health data on the dose-response, toxicological 76
mechanisms, or how population exposure concentrations relate to social determinants of health 77
(Mostafalou and Abdollahi 2013; Dhananjayan and Ravichandran 2018). 78
Social determinants of health like occupation or citizenship can alter both exposure and 79
health outcomes related to chemicals like pesticides. Healthcare policy and services are limited 80
to non-existent for immigrants and especially migrant workers residing in the United States 81
(US). For example, many policies that on the surface appear highly beneficial for the American 82
people like the Affordable Care Act of 2010, actually exclude immigrants completely from 83
accessing care (Quesada et al. 2011). In addition, agreements like the North American Free 84
Trade Agreement between the US, Canada, and Mexico limit migrant worker rights (Barnes 85
2013). Moreover, migrant worker health is often unprotected by the law and workplace 86
discrimination leaves migrant workers very vulnerable (Quesada et al. 2011; Ramos et al. 2016; 87
Ramos 2018; Saxton and Stuesse 2018). Prior research on migrant workers in the US Midwest 88
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found factors like economics, logistics, and health significantly affected the mental health of 89
migrant workers (Ramos et al. 2015). Overall, a gap exists in the quantification of pesticide 90
exposure among farmworkers and migrant workers, and specifically how these exposures may 91
differ by worker category or US citizenship status. 92
A major challenge in the field of occupational and environmental health is understanding 93
and predicting the health effects of exposure to chemicals like pesticides. There are currently 94
85,000 chemicals on the global market that Toxic Substances Control Act (TSCA) has listed in 95
its inventory of substances, and there is little to no experimental toxicology or epidemiology data 96
on many of them (Attene-Ramos et al. 2013; Adeola 2021). In 2008, the US Environmental 97
Protection Agency (EPA) collaborated with multiple other federal agencies including the Food 98
and Drug Administration and the National Institute of Environmental Health Sciences to create 99
the Toxicology in the 21st Century (Tox21) program (Thomas et al. 2018). The goal of Tox21 is 100
to develop high throughput testing methods to determine the safety of chemicals such as food 101
additives and pesticides. Additionally, Tox21 quantifies the biological mechanisms that 102
chemicals alter to prioritize the chemicals being tested and generate a wealth of data to predict 103
toxicological responses in the human body (Attene-Ramos et al. 2013; Thomas et al. 2018). 104
These data are a rich, but untapped, resource to characterize the dose-dependent effects of 105
exposure to pesticides in the context of social determinants of health like occupation and 106
citizenship. This data is then presented in the Toxicity Forecast Dashboard (ToxCast). 107
To address these gaps and understand how pesticide exposure and effects vary by 108
occupation and citizenship, this study’s goal is to determine if people residing in the US are 109
exposed to bioactive concentrations of pesticides. This project has the following aims: 1) 110
quantify and compare pesticide biomarkers among farmworkers and non-farmworkers, 2) 111
quantify and compare pesticide biomarkers between citizen and non-citizen farmworkers, 3) 112
compare exposure concentrations to known bioactive benchmark concentrations in the Tox21 113
high throughput toxicity data (ToxCast). We hypothesized that on average farmworkers will have 114
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higher concentrations of pesticides biomarkers than non-farmworkers. Furthermore, among 115
farmworkers, we hypothesize that non-citizens will have higher pesticide biomarker 116
concentrations than US citizens. Additionally, we hypothesize people residing in the US will be 117
exposed to bioactive concentrations of pesticides. Moreover, we hypothesize farmworkers will 118
be exposed to bioactive concentrations of pesticides more frequently than non-farmworkers. 119
120
1.2 Methods 121
Our overall study design involves comparing the distributions of chemical biomarker 122
concentrations in The National Health and Nutrition Examination Survey (NHANES) with the 123
distributions of doses for those chemicals which exhibit bioactivity in ToxCast. In addition, we 124
quantify which cellular target families are most often affected by these pesticides and look to 125
see how these target families differ by history of farmwork and U.S. citizenship status. 126
127
1.2.1 The National Health and Nutrition Examination Survey (NHANES) 128
NHANES is a cross-sectional study representative of the US population with 129
oversampling weights for minoritized populations. NHANES is a cross sectional assessment of 130
the health and nutrition of adults and children residing within the US. The current iteration of the 131
continuous study began in 1999. Study participants are enrolled on a continuous basis, with 132
data analyzed and deposited in two-year windows. NHANES collects extensive information on 133
the study participants such as self-reported occupation, urinary and serum biomarkers, and self-134
reported demographics such as age, gender, citizenship, poverty index ratio, and education. 135
136
1.2.2 Study Population 137
This study included NHANES study participants aged 18 years and older who also had 138
occupation and pesticide exposure data present between 1999 and 2014. This study integrated 139
29 datasets from NHANES laboratory data to understand pesticide exposure, occupation, and 140
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demographics of the study population. From the Industry and Occupation Survey, individuals 141
were coded as “farmworker” or “non-farmworker” using the Current Industry (OCD230=1, 142
OCD231=1), Current Occupation (OCD240=18, OCD241=18), Longest Industry (OCD390=1, 143
OCD391=1), and Longest Occupation (OCD392=18), where all participants who put 144
“Agriculture, Forestry and Fishing” were coded as a farmworker. 145
From the demographics data, DMDEDUC2 (older than 18 years of age) and DMDEDUC3 (l8 146
years of age and younger) were combined to create one education level based on the 147
DMDEDUC2 categories. The US citizenship variable (DMDCITZN) is defined as 1= “Citizen by 148
Birth or naturalization” and 2= “Not a citizen of the US”, and we removed anyone who 149
responded with “Refused”, “Don’t Know”, or skipped the question. 150
151
1.2.3 Biomonitoring Samples and Detectability 152
NHANES performs chemical biomonitoring in study participants urine and blood. 153
Participants provided partial urine void in a sterile sampling cup at the mobile examination 154
center. Blood samples are collected by certified laboratory professionals. Urine and blood 155
samples are then analyzed for chemical metabolites using isotope dilution gas chromatography 156
high-resolution mass spectrometry (GC/IDHRMS). Pesticide biomarkers measured in blood 157
samples and reported as either 1) fresh weight basis (i.e., pg/g serum) and 2) lipid weight basis 158
(i.e., ng/g lipid). The lipid adjusted values account for blood lipid concentrations and are of 159
particular importance for the accurate quantification of lipophilic pesticides (Barr et al. 2005). 160
All urinary biomarker measurements were adjusted for urinary creatinine, and all blood 161
pesticide biomarker measurements were blood lipid adjusted. Detectability percentages were 162
calculated by dividing the total number of measurements above LOD by the total number of the 163
chemical’s measurements in NHANES. To ensure that we included chemicals with values 164
above the limit of detection in most of the study participants, detection frequency percentages of 165
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50% and higher across the population were maintained which resulted in 14 chemicals of 166
interest (Silver et al. 2018). 167
These chemicals included the following: 2,4-Dichlorophenol (24DCP), 2,4-168
Dichlorophenoxyacetic acid (24D acid), 2,5-Dichlorophenol (25DCP), 3,5,6-Trichloropyridinol 169
(TCP), 4-Nitrophenol, β -hexachlorocyclohexane (β -HCH), diethyltoluamide acid (DEET acid), 170
Dieldrin, Heptachlor Epoxide, 3-phenoxybenzoic acid (3-PBA), p,p’-DDE, and p,p’-DDT. 171
Additionally, the measurements of TCP, a chlorpyrifos metabolite, were compared to the 172
ToxCast toxicity data for both CPF and chlorpyrifos-oxon (CPO). 173
174
1.2.4 Toxicity Forecast Dashboard Data 175
The US EPA’s Toxicity Forecast Dashboard (ToxCast) is a collection of publicly 176
available high throughput toxicity data intended to make chemical assessment more accessible 177
by allowing researchers to search which chemicals show toxicological effects more easily within 178
human tissue. High throughput toxicity screening initiatives have been developed to quantify 179
biological effects of chemicals, including pesticides, in vitro. Dose response curves are created 180
for each chemical and assay, and from these curves the activation concentrations and positive 181
hitcalls are defined. ACC is the concentration at which the model reaches the cut-off values for 182
the chemical to be considered active and is based on the levels of significance for the dose 183
curve response. The ACC can be used as a proxy of potency to determine the genes, proteins, 184
enzymes, effects on biological pathway and viabilities at which chemicals are active. 185
186
1.2.5 Comparing NHANES and ToxCast 187
Using the corresponding Chemical Abstracts Service Registry Numbers (CASRNs) 188
obtained from PubChem, data from ToxCast were matched to NHANES. From this new dataset, 189
we created pesticide concentration distribution boxplots by the chemical and farmwork history or 190
U.S. citizenship in the tidyverse using the ggplot2 R package (Wickham 2016). Pesticide 191
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distributions were overlaid unto the same axis to quantify overlap between the pesticide 192
concentration distributions of exposure in NHANES participants and bioactivity in ToxCast. To 193
visualize the distribution of exposure in comparison to pesticide bioactivity concentrations, 194
ToxCast ACCs and NHANES biomarker concentrations were plotted as boxplots using molarity 195
units. 196
197
1.2.6 Statistical Analysis 198
All data management and analysis were completed in R version 4.1.3. All code for our 199
work can be found on our GitHub repository (Millar and Forté 2023). Graphics were created 200
using the ggplot2 package library (Wickham 2016). All NHANES data was downloaded using 201
the RNHANES packaged in R (Susmann 2016). The main outcomes of this project include 1) 202
quantifying the distribution of the pesticide concentrations across NHANES and ToxCast, 2) 203
quantifying the demographics of people with and without bioactive measurements, and 3) 204
investigating how bioactivity differs by chemical, farmwork history, and US citizenship status. 205
These outcomes inform the overarching project question of whether people residing in the US 206
are exposed to bioactive levels of pesticides, how these bioactive pesticides affect the body, 207
and whether the rates of exposure to bioactive pesticide concentrations vary based on 208
sociodemographic factors. 209
We labeled anyone who had at least one chemical measurement equal to or above the 210
minimum ToxCast ACC for that chemical as being “bioactive”. Anyone who did not fit this group 211
was defined as “non-bioactive.” Demographics were quantified by bioactivity status among all 212
study participants and then among farmworkers only. For continuous variables like body mass 213
index (BMI) or age in years, we present the mean and standard error, and for all categorical 214
variables, the stratified frequencies and sub-group percentages are provided. 215
Differences in demographic factors by group or citizenship were tested using a 216
Pearson’s chi-square test, using a Rao and Scott Adjustment where necessary for categorical 217
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variables. Low response was defined as 8 or less respondents within one stratum. And for 218
continuous variables, a Wilcoxon Rank test was used to test group means, with a Kruskall-219
Wallis Correction. All significance testing was completed using the NHANES Full Sample 2 and 220
4 Year MEC Exam Weights. A new weight variable titled “MEC16YR” was created using the 221
weighted MEC 2- and 4-year measurements to represent the weights used from 1999-2002 and 222
each year after, respectively. 223
Non-citizen status was determined by the NHANES variable DMDCITZN. We calculated 224
bioactivity by the chemical and marked measurements as bioactive based on their hitcall 225
equaling 1. For model outcomes this bioactivity status by chemical was used as the outcome 226
variable for logistic regression models used to investigate how the odds of being a farmworker 227
and having at least one bioactive measurement differ from non-farmworkers by the chemical. 228
These models were adjusted for BMI, age, poverty index ratio (PIR), survey year, gender, racial 229
ethnicity, U.S. citizenship status, farmwork history, country of birth and education level. After 230
comparing all study respondents’ odds of having a bioactive measure, we created logistic 231
regression models comparing U.S. citizenship status. These models were also adjusted for BMI, 232
age, PIR, survey year, gender, racial ethnicity, country of birth, and education level. 233
Education status was constructed NHANES variables DMDEDUC2 and DMDEDUC3 to 234
include four categories: Less than 9th grade, 9-11th grade (Includes 12th grade with no 235
diploma), High school grad/GED or equivalent, and More than high school. Farmworker status 236
was constructed using NHANES industry or occupation group codes for current job (OCD230, 237
OCD231) or longest job (OCD390, OCD391, OCD392) that included the terms 238
agriculture/agricultural or farming. 239
For lipid adjusted blood measurements, molarity was calculated by multiplying the 240
measurement by serum density of 1.024 g/mL and dividing by molecular weight (Sniegoski and 241
Moody 1979). Urinary measurements were calculated by diving the measurement by molecular 242
weight. All measurements of molarity have units of μ mol/L. 243
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Data from the 1999-2002, 2003-2004, 2005–2006, 2007–2008, 2009–2010, 2011–2012 244
and 2013-2014 data collection cycles were appended, and the sampling weights modified as 245
directed in NHANES documentation. Removal of observations with missing data was done for 246
all analyses. Statistical analysis was done with the R survey package (v4.1-1) to handle 247
complex survey designs present in NHANES. The function survey::svydesign was used to 248
handle sampling weights, with primary sampling units nested within each stratum. 249
Wilcoxon Mann Whitney U test was conducted on individual chemicals in relation to 250
farmworker or non-citizen status using the survey::svyranktest function. The outcome variable 251
for chemicals was calculated as the log molarity for blood measurements and the log of the ratio 252
of the chemical molarity to creatine molarity for urinary measurements. P-values for all tested 253
chemicals were FDR adjusted and AUCs were calculated using the U statistic (Mason and 254
Graham 2002). 255
Both unadjusted and adjusted logistic regression was conducted on individual chemicals 256
in relation to farmworker or non-citizen status using the survey::svyglm function using a quasi-257
binomial model with a logit link. The outcome variable for chemicals was constructed as an 258
indicator variable, with a 1 indicating the measurement was considered chemically bioactive. 259
Adjusted logistic regression included variables for age at screening, race-ethnicity, BMI, 260
education, and survey year for all chemicals, and the additional inclusion of creatine molarity for 261
urinary measurements. P-values for all tested chemicals were FDR adjusted and AUCs were 262
calculated using the WeightedROC R package (v2020.1.31) (Hocking 2020). 263
Initially, the list of pesticides under investigation included 96 different biomarkers present 264
in NHANES, but after removing chemicals with detectability percentages below 50%, we were 265
left with 16 chemicals for analysis (Supplementary Table 1). Assay data for these chemicals 266
from NHANES were then extracted from the ToxCast database. We retrieved the hitcall 267
(representative of an active assay), the activity concentration at cutoff (or ACC), and the 268
intended target family of each ToxCast assay based on the 16 pesticides from NHANES. Using 269
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the hitcall variable, we labeled assays as positive (hitcall==1) or negative (hitcall==0) to mean 270
that an assay did or did not show bioactivity by the pesticide. We created a bioactivity ratio per 271
chemical by dividing the number of positive assays by total number of assays. All chemicals in 272
NHANES were present in ToxCast. However, trans-nonachlor was not maintained in the study 273
because there were only 8 completed assays in ToxCast and none of those assays were active. 274
275
1.3 Results 276
We first assessed demographic features of the study participants based on whether the 277
participant had a history of farmwork or not (Tables 1 and 2). In total, there were 1,137 people 278
who reported any farmwork history, and 20,205 who were categorized as non-farmworkers. The 279
farmworker group was mostly women (N=697, 61.3%), Non-Hispanic White (N=635, 55.8%), 280
U.S. Citizens (N=934, 82.1%) and 26.6% reported some college education or an associate’s 281
degree (N=302). The non-farmworker group had similar mean BMI, age, and poverty index 282
ratio. The non-farmworker group is predominantly men (N=10,187, 50.4%), Non-Hispanic White 283
(N=9,167, 45.4%), had U.S. Citizenship (N=17,626, 87.2%), and 19.2% reported some college 284
or an associate’s degree (N=3,885). 285
To better understand how each of the chemicals relate to each other, Table 3 outlines 286
the pesticides by persistence and frequencies of activity of ToxCast assays. In total, there are 287
15 pesticides that are detectable in NHANES study participants and also assayed in ToxCast. 288
Overall, there were 5 persistent organic pesticides and 10 non-persistent pesticides included in 289
this study. The top three most bioactive pesticides in ToxCast were heptachlor epoxide had the 290
highest percentage of assays which were “active” (39.85%), followed by p,p’-DDT (35.73%) and 291
p,p’- DDE (26.78%). The bioactivity threshold is the lowest ACC of the active assays for a given 292
chemical. These values ranged from 6.5nM (2,4-Dichlorophenoxyacetic acid) to 1.45μM 293
(chlorpyrifos). 294
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Next, we wanted to compare the concentrations of chemicals required to activate the 295
ToxCast assays to the biomarker concentrations measured in people in NHANES. Figure 1 296
presents the distribution of pesticide concentrations among people residing in the United States 297
in orange (retrieved from NHANES), and in blue, the ACCs of active assays retrieved from 298
ToxCast. In this figure, where the pesticide distributions of exposure and bioactivity overlap 299
represents pesticide exposures among the US population that are “bioactive”. Additionally, 4-300
nitrophenol is the only pesticide biomarker in NHANES that does not have human 301
measurements that overlap with the bioactive distribution in NHANES. 302
We present the Mann-Whitney-U Rank Test outcomes by chemical in Supplementary 303
Table 2 to test for differences in biomarker concentration by farmworker status, or within 304
farmworkers, comparing between farmworkers with and without US citizenship. When 305
quantifying the odds of having a bioactive measurement (unadjusted outcomes in Supplemental 306
Table 3, fully adjusted outcomes presented in Figure 2 and Supplementary Tables 4 and 5), we 307
found farmworkers were 4.3 times more likely to have a bioactive measurement in comparison 308
to non-farmworkers for 2,4-D (p=2.0x10-7) while farmworkers were significantly less likely to 309
have a bioactive measurement of 4-Nitrophenol (p= 2.7x10-4). Next, we narrowed our analyses 310
to farmworkers only and found farmworkers living without U.S. citizenships were significantly 311
more likely to be exposed to a bioactive measurement of BHC (OR=8.4, p-value=1.2x10-9, 312
U=13.95), p,p’-DDE (OR=3.0, p-value=3.1x10-3, U=9.43), p,p-DDT (OR=10.8, p-value. =8.7x10-313
4, U=6.56). 314
When trying to understand what intended target families are most affected by these 315
chemicals, Supplementary Table 6 provides the frequency of intended target families by the 316
pesticide. Based on individual intended assay target count, cell cycle (N=487), nuclear receptor 317
(N=318), cytokine (N=143), DNA binding (N=172), and cell adhesion molecules (N=65) were the 318
most frequent targets of the pesticides. Overall, p,p’-DDE (N=305) had the most intended target 319
family counts based on positive assays, followed by p,p’-DDT (N=278), heptachlor epoxide 320
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(N=259), and chlorpyrifos (N=126). Heptachlor epoxide had the highest number of positive 321
assays targeting the cell cycle (N=123) and p,p’-DDT had the second most (N=120). 322
Additionally, for p,p’-DDE had mostly nuclear receptor targeting positive assays (N=102), 323
followed by the cell cycle (N=74) and DNA binding (N=64). 324
325
1.3.1 Discussion 326
When looking at individuals who have pesticide biomarker concentrations at these 327
bioactive levels, demographics statistically differed based on bioactivity, farmwork history and 328
citizenship status. We found NHANES participants are broadly exposed to bioactive 329
concentrations of pesticides. Heptachlor epoxide, p,p’-DDT, and p,p’-DDE were the most 330
bioactive pesticides in ToxCast based on overall percent of positive assays. Disproportionate 331
exposures to bioactive concentrations of pesticides were particularly evident in farmworkers 332
without U.S. citizenship, particularly for persistent pesticides. 333
Pesticide exposures have been associated with increased mortality due to cancer, 334
diabetes mellitus, poisonings, and tuberculosis and other lung infection (Mills et al. 2006; Fry 335
and Power 2017). Pesticide exposure throughout the life course has been associated with 336
breast cancer and dysregulated mammary gland development. For example, mothers with the 337
highest p,p-DDT concentrations were 3.7 times more likely to have daughters who developed 338
cancer by the age of 52 in comparison to mothers with the lowest p,p-DDT blood concentrations 339
(Cohn et al. 2015). Women who are farmworkers and not US citizens could be at increased risk 340
of exposure-associated diseases like breast cancer – these findings warrant further 341
investigation in this area. 342
Citizenship status is also a known barrier to health insurance and treatment (Guadamuz et 343
al. 2020; Chasens et al. 2020), potentially compounding adverse effects of exposure to toxic 344
chemicals like pesticides. In a study of 2,702 participants living with diabetes, non-citizens had a 345
greater risk for poor glycemic management (OR=5.16, 95% CI: 3.73, 6.04) in comparison to 346
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citizens by birth (Chasens et al. 2020). Additionally, citizens by naturalization were also at an 347
increased risk of poor glycemic management (OR=1.95, 95% CI: 1.49,2.55) (Chasens et al. 348
2020). Additionally, this study found that individuals with diabetes and without health insurance 349
were almost twice as likely to have poor glycemic management compared to insured people 350
(OR=1.99, 95% CI: 1.53-2.59). Similar outcomes have also been noted in cardiovascular 351
disease. Using NHANES, researchers retrieved data from 2011 to 2016 to investigate 352
prevalence, treatment, and control of hypercholesterolemia, included 11,680 US-born citizens, 353
2,752 foreign born citizens, and 2,554 non-citizens (Guadamuz et al. 2020). In that study, over 354
half of non-citizens did not have health insurance (52.2); which was significantly more than US-355
born citizens (13.6%, p<0.001) (Guadamuz et al. 2020). 356
Non-citizens also had significantly higher prevalence of diabetes (15.7% vs. 12.8%, 357
p<0.001) (Guadamuz et al. 2020). Treatment percentages were also significantly lower among 358
non-citizens than US-born citizens with hypercholesteremia (16.4% vs 45.5%), hypertension 359
(60.3% vs. 81.1%), and diabetes (51.2% vs. 69.5%) (p<0.001) (Guadamuz et al. 2020). Among 360
noncitizens, those without a usual source of health care or health insurance had lower treatment 361
percentages for hypercholesteremia (2.7% and 8.1%), hypertension (22.2% and 39.1%), and 362
diabetes (15.5% and 28.6%) (Guadamuz et al. 2020). It is very important to understand that 363
overall, environmental risk factors of the many pesticides on the global market are still poorly 364
characterized across the literature. 365
366
1.3.2 Limitations and Strengths 367
Our research shows that NHANES respondents are exposed to multiple pesticides and 368
pesticide types. Quantifying chemical mixtures across a population is complex and methodology 369
for understanding these mixtures is still an emerging area of research. However, there is still 370
plenty of research to be done in understanding chemical mixtures. Much of the research on 371
chemical health outcomes focuses on one chemical at a time, including our study, but people 372
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17
are often exposed to more than one chemical, chemicals can interact with each other to create 373
new chemicals and once chemicals are in the environment, they can also react with the ambient 374
air or be degraded by the sun’s rays. All these changes to chemicals in relation to mixtures and 375
being in the environment create nuanced exposures and further research is needed to 376
understand how these mixtures may uniquely affect the human body. 377
Some pesticides which did not meet our inclusion criteria could have different exposure 378
based on farmwork occupational status. Oxypyrimidine (7.88% vs. 13.76%, 0.033), desethyl 379
hydroxy DEET (17.37% vs. 11.30%, p =0.015), and DEET (9.17% vs 6.25%, p = 0.036) were 380
significantly different between farmworkers and non-farmworkers, respectively. However, all of 381
these chemicals had detectability percentages below the cutoff for inclusion in our study. It is 382
possible that by restricting the chemicals included we are missing some important differences in 383
pesticide exposure between farmworkers and non-farmworkers. Studying exposures and effects 384
of these less commonly detected pesticides could be an important area of investigation. 385
One of the major limitations of this project is that while NHANES is thorough, reliable, 386
and valid study, it is still cross-sectional. This means the measurements within it are a single 387
measurement in time and cannot be fully representative of chronic exposures or chronic 388
symptomology due to exposures. Another limitation includes most farmworkers being recruited 389
between 1999 and 2004 (N= 1,775, 69.6%), which is of importance since the recruitment and 390
laboratory methods have been updated since 2003. Newer methods for quantifying chemicals 391
from blood and urine samples are more sensitive and can detect lower quantities of chemicals. 392
Additionally, farmworkers living without citizenship had significantly lower BMI as well, which 393
may impact metabolism and accumulation of chemicals in the body. 394
An additional limitation of this study is that not every chemical is measured in every 395
participant, and that not every assay is completed in each chemical. This limitation makes direct 396
comparisons impossible and therefore our results are somewhat limited to group means. There 397
are some known limitations to the ToxCast dataset such as interference of cytotoxicity. Non-398
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18
specific cell stress can interfere with the frequency reading since the cell is overworking to re-399
gain homeostasis after chemical exposure. ToxCast assays are often assessing effects in a 400
single tissue cell type, which may not accurately reflect chemical sensitivity across organ 401
systems or within particularly susceptible individuals. Moreover, while ToxCast maintains a 402
robust suite of assays measuring effects across a broad spectrum of potential toxic outcomes, 403
not every chemical is tested for every assay and not all potential biological outcomes following 404
chemical exposure are captured. 405
Other limitations inherent to interpreting bioactivity also exist. For starters, urine and serum 406
concentrations reflect excreted or circulating concentrations, respectively, but may not be 407
representative of concentrations in target organs like fat, liver, kidneys, or brain. This is 408
important because many chemicals target specific organs (e.g., organochlorines targeting the 409
central nervous system) or bioaccumulate in specific tissue types like lipids. There are also 410
challenges to being able to relate metabolites to their parent compounds since some chemicals 411
can have more than one parent compound (e.g. the pyrethroid metabolite 3-PBA). This can 412
make ascertaining what active ingredient is bioactive in the human body difficult, and even if 413
considering a limited number of chemicals, there is no way to calculate a direct contribution of 414
each parent compound to a non-specific metabolite. 415
A strength of our study is that it is the first to provide a comprehensive quantification of all 416
the pesticide exposure concentrations within the US population using NHANES from 1999 to 417
2014 and to then stratify these concentrations by social determinants of health with a focus on 418
farmwork, fishing, and forestry work history and U.S. citizenship. By considering all the 419
pesticides within NHANES and narrowing down to those with at least 50% detectability, we find 420
that even within NHANES a small portion (15%) of these chemicals are detected in a majority of 421
NHANES participants. ToxCast & NHANES are both validated, reliable study datasets created 422
by the US government to assess chemical bioactivity and examine the health of people residing 423
in the US. By integrating these two datasets, the results are more generalizable to the U.S. 424
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19
population. Additionally, this study is one of few to consider health disparities associated with 425
occupation or citizenship and how they may affect pesticide exposure and potential resultant 426
health effects. This project can inform evidence-based guidelines and policies that are focused 427
on reducing pesticide exposure concentrations among people residing within the United States. 428
429
1.3.3 Future Directions 430
While NHANES quantifies many chemical biomarker concentrations for each study 431
participant, these measures do not fully capture how many chemicals each person may be 432
exposed to since every chemical is not tested for in every person. Moreover, toxicological 433
research should continue to focus on novel methods for assessing toxicity of chemical mixtures 434
and interactions to better understand population pesticide exposure and bioactivity of combined 435
pesticide exposures in at-risk individuals. Currently, research looks at predominantly the active 436
ingredients of pesticides, but inactive ingredients used to create pesticides may also influence 437
human health, this is currently being missed in many toxicological studies. Future research can 438
also include temporal data on pesticide exposure. Both NHANES and ToxCast include singular 439
exposure time points in humans and in vitro, respectively. However, for many farmworkers, 440
pesticide exposure is chronic and happens over multiple exposure incidents. 441
Expanding this research to disease biomarkers, symptoms, and diagnoses will also be an 442
important future direction. This way we can better connect target families of ToxCast assays to 443
health outcomes and then stratify findings by occupation and social determinants of health like 444
income, gender, citizenship, and country of birth. In this same vein of understanding social 445
determinant effects on health, more research on how these biomarker concentration 446
distributions differ based on residing or working in a low versus high income country will be 447
important because laws within a nation can alter the health and exposure for many. 448
449
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24
Statements and Declarations 542
543
Funding: The researchers included on this study were supported by the National Institute of 544
Occupational Safety and Health Education Research Center (Grant# T42 OH 008455), the 545
National Institute of Environmental Health Sciences Environmental Toxicology and 546
Epidemiology Program (Grant# T32 ES007062), the National Science Foundation Graduate 547
Research Program (Grant # DGE-1256260), and the National Institutes of Health (R01 548
ES028802, P30 ES017885, R01AG072396). 549
550
Competing Interests: The authors do not declare any financial conflicts of interest. 551
552
Author Contributions: Justin Colacino and Chanese Forté contributed to the study conception 553
and design. Material preparation and data collection were performed by Chanese Forté. 554
Analysis was performed by Jess Millar and Chanese Forté. The first draft of the manuscript was 555
written by Chanese Forté and all authors commented on subsequent versions of the 556
manuscript. All authors read and approved the final manuscript. 557
558
Data Availability: The NHANES and ToxCast datasets analysed during the current study are 559
available from the CDC, https://wwwn.cdc.gov/nchs/nhanes/, and the EPA, 560
https://www.epa.gov/chemical-research/exploring-toxcast-data. 561
562
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25
Tables 563
564
Table 1. Stratified Demographics of NHANES Participants, by Farmwork Category 565
Non-Farmworker Farmworker
Variable
Mean
Standard
Error
Mean
Standard
Error p-value
Body Mass Index 28.4 6.7 28.32 6.08) 0.584
Age in years 45.88 19.5 48.63 18.81 2.15x10-4
Poverty Index Ratio 2.5 1.63 2.82 1.72) 5.99x10-5
Survey Year N=20,205 Percent N=1,137 Percent
< 2.2x10-
16
1999-2000 1,404 6.9 159 14
2001-2002 1,691 8.4 219 19.3
2003-2004 2,890 14.3 358 31.5
2005-2006 1,654 8.2 32 2.8
2007-2008 3,626 17.9 87 7.7
2009-2010 3,831 19 154 13.5
2011-2012 3,278 16.2 96 8.4
2013-2014 1,831 9.1 32 2.8
Gender 1.76x10-10
Men 10,187 50.4 440 38.7
Women 10,018 49.6 697 61.3
Racial Ethnicity
< 2.2x10-
16
Mexican American 3,517 17.4 278 24.5
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26
Other Hispanic 1,577 7.8 36 3.2
Non-Hispanic White 9,167 45.4 635 55.8
Non-Hispanic Black 4,435 22 135 11.9
Other Race 1,509 7.5 53 4.7
Country of Birth 0.538
Born in 50 US states or
DC 606 90.2 0 -
Born in Mexico 30 4.5 71 74
Born elsewhere 36 5.4 25 26
U.S. Citizenship 4.04x10-4
Non-Citizen 2,579 12.8 203 17.9
Citizen 17,626 87.2 934 82.1
Education Level
< 2.2x10-
16
Less than 9th grade 2,004 9.9 233 20.5
9-11th grade 4,021 19.9 147 13
Highschool 4,707 23.3 210 18.5
Graduate/GED 5,566 27.6 243 21.4
Some College or AA 3,885 19.2 302 26.6
566
P-values are derived from a chi-square test, using a Yate's Correction where necessary, and for 567
continuous variables, a Wilcoxon Rank Test was used with a Kruskall-Wallis Correction (as 568
needed). Percentages are out of the total number of respondents for that specific question. In 569
this table, other race includes multi-racial. In this study, 9-11 grad includes 12th grade 570
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27
completion without a high school diploma. All values in this dataset are weighted and stratified 571
according to NHANES guidelines. 572
573
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Table 2. Stratified Demographics of NHANES Participants with a History of Farmwork, by Citizenship 574
Variable
Citizen Non-Citizen
Mean Standard Error Mean Standard Error p-value
Body Mass Index 28.52 6.33 27.37 4.66 0.038
Age in years 49.9 18.9 42.74 17.07 7.57x10-5
Poverty Index Ratio 3.13 1.68 1.41 1.07 < 2.2 x10-16
Variable N=1,007 % N=237 %
Survey Year 1999-2000 139 14.9 20 9.9 9.41x10-05
2001-2002 188 20.1 31 15.3
2003-2004 325 34.8 33 16.3
2005-2006 20 2.1 12 5.9
2007-2008 66 7.1 21 10.3
2009-2010 105 11.2 49 24.1
2011-2012 68 7.3 28 13.8
2013-2014 23 2.5 9 4.4
Gender Men 378 40.5 62 30.5 0.013
Women 556 59.5 141 69.5
Racial Ethnicity Mexican American 125 13.4 153 75.4 < 2.2 x10-16
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Other Hispanic 23 2.5 13 6.4
Non-Hispanic White 622 66.6 13 6.4
Non-Hispanic Black 129 13.8 6 3
Other Race 35 3.7 18 8.9
Country of Birth Born in 50 US states or DC 606 90.2 0 0 < 2.2 x10-16
Born in Mexico 30 4.5 71 74
Born elsewhere 36 5.4 25 26
Education Level Less than 9th grade 110 11.8 123 60.6 < 2.2 x10-16
9-11th grade 115 12.3 32 15.8
Highschool 183 19.6 27 13.3
Graduate/GED 234 25.1 9 4.4
Some College or AA 290 31.1 12 5.9
575
P-values are derived from a chi-square test, using a Yate's Correction where necessary, and a Wilcoxon Rank Test was completed 576
with a Kruskall-Wallis Correction. Percentages are out of the total number of respondents for that specific question. In this table, 577
other race includes multi-racial. In this study, 9-11 grad includes 12th grade completion without a high school diploma. 578
579
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30
Table 3. Bioactivity of pesticides cross-listed between NHANES and ToxCast, by pesticide and 580
persistence 581
Common Name
CAS-RN
Total
Assays
Positive
Assays
Bio-active
Assay
Percentage
Bioactivity
Threshold (µM)
2,4-Dichlorophenol
120-83-
2
678 27 3.98
0.34
2,4-
Dichlorophenoxyacetic
acid 94-75-7
807 18 2.23
6.49x10-3
2,5-Dichlorophenol
583-78-
8
599 14 2.34
0.33
3-Phenoxybenzoic acid
3739-
38-6
622 11 1.77
0.23
3,5,6-Trichloropyridinol
6515-
38-4
433 21 4.85
1.35
4-Nitrophenol
100-02-
7
682 43 6.30
8.63x10-3
ß-
hexachlorocyclohexane
a
319-85-
7
654 24 3.67
0.03
Chlorpyrifos
2921-
88-2
639 126 19.72
1.45
Chlorpyrifos-oxon
5598-
15-2
693 132 19.05
0.04
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31
DEET Acid
134-62-
3
1025 13 1.27
0.17
Dieldrin a 60-57-1 549 121 22.04 0.32
p,p'-DDE a 72-55-9 1139 305 26.78 0.31
p,p'-DDT a 50-29-3 778 278 35.73 0.43
Heptachlor Epoxide a 76-44-8 650 259 39.85 1.31
582
aPersistent Organic Pollutant. 583
A positive assay is defined as hitcall==1. The bioactivity assay percentage is created by dividing 584
the total number of positive assays by the total number of assays and multiplying by 100%. 585
Bioactivity ratio per chemical was calculated by dividing the count of positive assays by the total 586
number of assays within the US Environmental Protection Agency’s Toxicity Forecast 587
Dashboard database. 588
589
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32
Figures 590
591
592
Figure 1. Comparing the chemical molarity of of NHANES subjects with bioactivity threshholds 593
taken from chemical assays. ACC is the activity concentration at cut-off for a specific assay 594
where a chemical is considered active. 595
596
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted January 28, 2023. ; https://doi.org/10.1101/2023.01.24.23284967doi: medRxiv preprint
33
597
Figure 2. Comparing the odds of having a bioactive pesticide biomarker concentration by 598
farmwork history and for farmworkers only by citizenship. This figure presents the outcomes of 599
the regression model of farmworker and non-farmworker health outcomes. Bioactive was 600
defined as having at least one pesticide biomarker concentration that was the same or higher 601
concentration than the minimal concentration needed to see an effect. The data for this table 602
was retrieved from the U.S. EPA’s Toxicity Forecast Dashboard and the National Health and 603
Nutrition Examination Survey. 604
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted January 28, 2023. ; https://doi.org/10.1101/2023.01.24.23284967doi: medRxiv preprint