Widespread use of organochlorine (OC) pesticides, such as dichlorodiphenyltrichloroethane (DDT), hexachlorobenzene (HCB), and hexachlorocyclohexane (HCH), has been drastically curtailed since the 1970s because of concerns about their environmental persistence, tendency to bioaccumulate and biomagnify, and possible adverse effects on humans and wildlife. The U.S. banned use of DDT (1972), HCH (1976), and HCB (1984), except for public health applications, and Mexico subsequently took similar action by banning HCB (1992) and phasing out both DDT and HCH (2000) [1,2,3,4]. Although levels in the environment have declined over time, concentrations of many OC pesticides or their metabolites (e.g., DDE—dichlorodiphenyldichloroethylene and DDD—(dichlorodiphenyldichloro-ethane), which are breakdown products of DDT) continue to be found in human tissue specimens from both U.S. [2,4,5,6,7,8] and Mexico [9,10,11].

Exposure to OC pesticides for U.S. residents occurs primarily from ingestion of contaminated food, including oily fish, shellfish, fatty meats, dairy products and fruits and vegetables. Contaminated drinking water (e.g., runoff from tainted soil) and air (e.g., long-range airborne transport) are comparatively minor exposure pathways [1,2,3]. Once inside the human body, OC pesticides and their metabolites tend to accumulate in adipose tissue, where they can remain for years. They also circulate in the lipid portion of blood serum [1,2,3,8]. Among the diverse toxic effects attributed to OC pesticides are carcinogenicity, male and female reproductive disorders, infertility, adverse developmental effects, neurotoxicity and immunotoxicity [1,2,3,12,13,14].

Research has documented that environmental chemicals in maternal blood can cross the placenta and produce fetal exposures [4,9,11,15,16,17,18,19]. The developing fetus and neonates are particularly prone to the adverse consequences of these exposures because they have higher rates of cell proliferation, decreased capability to activate and detoxify toxic compounds, and lower immune-response capacity [19,20,21]. Organochlorine pesticides or their metabolities have been reported in the blood of pregnant women [4,6,7,8,9,11] and in umbilical cord blood [9,11,15,16,17]. Research suggests that prenatal exposure to OC pesticides is associated with a variety of harmful outcomes [13], including delayed neurodevelopment [22], poor attention in early infancy [23], reduced birth size, weight, and head circumference [24,25], rapid weight gain in the first 6 months and elevated BMI later in infancy [26], obesity and type 2 diabetes later in life [27,28,29,30], and delayed age at menarche [31].

In this article, we summarize results of OC pesticide measurements in matched-pairs of maternal and cord blood from pregnant Hispanic women residing in Brownsville, Texas. Participants in the study were volunteers recruited from the patient population at a private clinic in Brownsville, which is located in south Texas along the U.S.-Mexico border, in a region known as the Lower Rio Grande Valley (LRGV). According to the U.S. Census Bureau [32], the city has a population of 172,434, of whom 92% are Hispanic. The 2006 American Community Service Survey ranked Brownsville as the most impoverished city in the nation based on average-annual household income [32]. More than a third of residents are 18 years old or younger, and 45% live in poverty; the highest proportion of any city in the U.S. with a population over 100,000 [32].

A summary of demographic and socioeconomic characteristics for the 35 women enrolled in the study is presented in Table 1. All the women were Hispanic, between the ages of 18 and 38 years old, and had lived in the Brownsville area for several years. Only one woman identified herself as an active smoker. The mean height was 5.2 ft and mean weight was 161 lbs. The women had an average of 2.6 children at home and the average number of previous pregnancies was 2.8. Approximately 63% were born in the U.S., 6% in Mexico, and 31% elsewhere or unknown. Twenty percent graduated from high school or completed a GED, and an additional 49% graduated from a college or university. Seventy-one percent were married, 26% had never been married, and 3% were separated. Forty percent worked in an office, business, or shopping mall, 20% were housewives, and 20% were teachers, administrators, or students.

Cord and maternal blood were analyzed for 30 OC analytes, including four isomers (α, β, γ, and ∆) of hexachlorocyclohexane (HCH) and several compounds that are components of commercial DDT (dichlorodiphenyltrichloroethane) and/or metabolites of DDT (i.e., 2,4’-DDT and 4,4’-DDT, 2,4’-DDE and 4,4’-DDE (1,1’-(2,2-dichloroethenylidene)-bis[4-chlorobenzene]), 2,4’-DDD and 4,4’-DDD (1,1’-(2,2-dichloroethylidene)-bis[4-chlorobenzene])). As shown in Table 2, 14 compounds (47%) were not found at all and 11 compounds (37%) were not measurable in more than 75% of the specimens. These 25 compounds were excluded from further statistical analysis, but are included (if applicable) as part of the “total HCH, total chlordane, and total DDT” values (sum of all measureable concentrations above zero) reported in Table 3.

Summary statistics for the other five compounds (heptachlor-epoxide, trans-nonachlor, 4,4’-DDE, 4,4’-DDT, and hexachlorobenzene) and for total HCH, chlordane, and DDT (sum of all concentrations above zero for relevant isomers and/or metabolites of each compound) are provided in Table 3. Mean and standard deviation for each compound are presented for cord blood, maternal blood, and the ratio of cord-to-maternal blood (C/M ratio). The geometric mean C/M ratio was <1 for trans-nonachlor, 4,4’-DDE, 4,4’-DDT, hexachlorobenzene, total chlordane and total DDT, and slightly >1 for heptachlor-epoxide and total HCH. Based on calculated z-scores, differences between matched cord and maternal blood were statistically significant (p < 0.05) only for 4,4’-DDE and total DDT.

Although concentrations of individual analytes were comparatively low overall, total maternal DDT levels were comparatively higher for those in the upper tail of the distribution. For the four highest maternal specimens (approximately the upper tenth percentile), total DDT concentrations ranged from 4.29–4.74 ng/mL, while the four lowest values were less than 0.30 ng/mL (a spread of more than 14X). Total HCH and total chlordane concentrations were uniformly low, with maternal values ≤0.15 ng/mL for HCH and ≤0.55 ng/ml for chlordane.

Results from this study, which involved women in higher-than-average socioeconomic strata, show that pregnant Hispanic women and their fetuses residing in Brownsville are differentially exposed to a diversity of OC pesticides, and concentrations in maternal blood are typically similar to or higher than in cord blood. Previous studies have used a variety of analytical techniques to monitor disparate populations, and relatively few measurements are available from Mexican-American women and their fetuses. It is not straightforward, therefore, to put measured OC concentrations into perspective by relating them to comparable statewide or national distributions. The Centers for Disease Control and Prevention’s (CDC’s) Fourth Report on Human Exposure to Environmental Chemicals [2] summarizes blood OC data from several hundred adult Mexican Americans (both male and female) collected between 1999 and 2004. Similar to the data from Brownsville, CDC’s geometric mean for heptachlor-epoxide, DDT, and HCH tended to be below the LOD, while for trans-nonachlor it was approximately 0.65 ng/mL (compared to 0.01 ng/mL in Brownsville) and for DDE it was about 3.5 ng/mL (compared to 0.22 ng/mL for 4,4’-DDE in Brownsville).

Although mean values for Brownsville appear to be relatively low, high-end concentrations (upper 10%) for total DDT are relatively high. There was considerable spread in the Brownsville data for total DDT—the range was from <0.15 ng/mL for both cord and maternal specimens to 1.47 ng/mL for cord blood and 4.74 ng/mL for maternal blood. The four highest total DDT values (roughly the top 10th percentile) measured in Brownsville varied from 0.54 to 1.47 ng/mL for cord blood and from 4.29 to 4.74 ng/mL for maternal blood. The maternal concentrations were approximately four times higher than 95th percentile values reported by the CDC [2], while cord blood concentrations were similar.

Based on current scientific knowledge and understanding, the human health effects from these blood levels of OC pesticides are unknown. Keeping in mind that our cohort is a relatively-small convenience sample, and that quantifiable OC levels were often below the nominal limit of detection, our findings, nevertheless, document both maternal and in utero exposure to a variety of OC pesticides, many of which are known or suspected neurodevelopmental toxicants, human carcinogens, endocrine disrupters, and obesogens [1,2,3,21,22,23,24,25,26,27,28,29,30]. This raises questions about the potential harmful effects of observed OC exposures on the developing fetus, especially given its inherent biological vulnerability, the potential for synergistic interactions between multiple chemical and nonchemical stressors, and the likelihood that susceptibility is enhanced by socioeconomically-difficult and environmentally-demanding living conditions [18,19,20,21,27,28,29,30,38,39,40,41,42].

More than 90% of Brownsville residents are Hispanic (primarily Mexican American), and many exhibit socioeconomic characteristics associated with poor health outcomes, including poverty, illiteracy, inadequate housing, unemployment, English as a second language, substandard diet, lack of access to health care and generally more stressful and less healthful lives [43,44,45]. Residents along the U.S.-Mexico border often lack adequate sewage treatment facilities, cannot afford air conditioning, experience frequent flooding, and do not have sufficient knowledge about healthy lifestyles [43,44]. Among environmental hazardous routinely encountered by this population are (a) contaminated drinking and recreational water from agricultural runoff, municipal waste, and factory discharges; (b) polluted air caused by emissions from motor vehicles, industrial facilities, agricultural operations, and open burning of trash; (c) tainted food related to subsistence farming and fishing, and (d) contaminated soil resulting from pesticide use, waste disposal, and illicit scrap yards and tire dumps [46]. Moreover, Brownsville has a higher-than-average prevalence of many chronic diseases to which environmental exposures contribute, including tuberculosis, cardiovascular disease, obesity and diabetes, [43,44,45], and children living along the Texas-Mexico border are hospitalized with asthma at a 36% higher rate than non-border children [47]. The evidence suggests that pregnant women (and their fetuses) living in Brownsville are likely to experience varying exposure to a complex mixture of chemical and nonchemical stressors [43,44,45,46,47,48]; a situation that could make them more susceptible to the adverse effects of OC pesticides and other environmental chemicals [38,39,40,41,42].

Because OC pesticides are toxic to humans, persist in the environment for years, bioaccumulate in the body and have a tendency to biomagnify in the food chain, their use has been banned or restricted in most developed countries for more than 30 years [1,2,3,4]. The 2001 Stockholm Convention, a global treaty signed by more than 150 countries, called on governments to eliminate the production and use of 13 OC pesticides (aldrin, chlordane, chlordecone, dieldrin, endrin, heptachlor, HCB, alpha- and beta-HCH, lindane, mirex, pentachlorobenzene, toxaphene) and reduce the manufacture and application of DDT [49]. Nevertheless, as a result of residues from past applications, continued use by some countries, and long-range environmental transport, OC pesticides and related by-products are still routinely detected in air, water, soil, sediment, fish, birds, and mammals from around the World [1].

Human biomonitoring studies have documented OC pesticides and their metabolites in blood from people worldwide [15,16,17,26], including those living in Mexico [9,10,11] and the U.S. [2,4,5,6,7,8]. Maternal exposures and related cross-placental transport are a particular public health concern because the developing fetus is intensely sensitive to xenobiotic chemicals during certain time windows of vulnerability when even minute amounts of exogenous substances can produce harmful consequences [7,13,18,19]. The data on hand indicate that fetal exposures to OC pesticides/metabolites occur regularly [9,10,11,15,16,17,19] and may be associated with neurodevelopmental problems [13,22,23], decreased birth size, weight and head circumference [24,25], quick weight gain and elevated BMI in infancy [26], and subsequent obesity, pre-diabetic conditions, and type 2 diabetes [27,28,29,30].

Future research needs to involve a larger, statistically-based sample, which includes pregnant women from lower socioeconomic strata, and more detailed information should be collected on participants’ diet, housing characteristics, living situation, neighborhood environment, and activity patterns. Follow-up efforts ought to be aimed at identifying exposure pathways and routes for those at the high-end of the exposure distribution. Because of the elevated prevalence of obesity and diabetes in this population, particular attention should be directed towards investigating the relationship between fetal exposure to chemical obesogens, such as DDT, and consequent development of childhood obesity and pre-diabetic conditions. The cumulative effect of fetal exposure to multiple chemical and nonchemical stressors needs to be examined in order to identify those individuals at highest comparative risk for adverse health outcomes.