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Development has flourished along the Mexico/U.S. border during the past twenty years. Economicgrowth, fueled, in part, by the maquiladoras (product assembly plants) that now exist along theborder, as well as the National Free Trade Agreement (NAFTA), increased the potential fordegradation of water quality in the Rio Grande. The potential exists for contamination of the riverby pesticides from nearby farming operations as well as the threat of contamination with toxicchemicals from operation of the maquiladoras and other industries along both sides of the border. Prompted, in part, by these concerns, the U.S. and Mexico issued the Integrated Environmental Planfor the Mexican-U.S. Border Area (First Stage, 1992-1994) [1] in 1992, as a cooperative effort tosolve environmental problems in the border area. During the first phase of this cooperativeendeavor, conducted in 1992 and 1993, numerous sites along the Rio Grande were screened todetermine if and to what extent river waters or biota had been contaminated by industrial andagricultural activity [1]. The first study revealed thirty chemicals in fish that exceeded screeninglevels and were considered to have the potential of causing adverse human health effects. The secondphase of the study, known as the Rio Grande/Rio Bravo Toxic Substance Study (RGTSS), wasconducted from 1995 through 1997 [2]. Neither study adequately addressed the potential for humanhealth effects from consumption of contaminated fish. For instance, the earlier investigations reliedheavily upon use of composite fish samples; this and other limitations of the studies made it difficultto determine the health effects of eating contaminated fish from the Rio Grande. Therefore, theTexas Natural Resources Conservation Commission (TNRCC) requested in 1999 that TDH evaluatepotential adverse health effects from consumption of fish from the Rio Grande. The first phase ofthis assessment, reported in this health consultation, targeted two sites near Laredo, Texas. One sitewas directly upstream of Laredo; the other was downstream of the city.

The Rio Grande forms a major section of the international boundary between Mexico and the UnitedStates. Thirteen Texas counties (Cameron, Hidalgo, Starr, Zapata, Webb, Maverick, Kenney, ValVerde, Terrell, Brewster, Presidio, Hudspeth, and El Paso), border the Rio Grande. According tothe U.S. 2000 decennial census, the population of the six counties comprising the U.S. Rio GrandeValley is approximately 1,184,000, of which 88.5% are of Hispanic descent [3]. Major U.S. citiesin the Valley include Laredo (population 176,576), McAllen (106,414), Edinburg (48,465),Harlingen (57,564), and Brownsville (139,722) [3]. These cities, along with outlying communities,account for approximately 60% of the region's U.S. population, with the other forty percent of thepopulation living in rural or farming communities. People living in the Rio Grande Valley countiesare among the poorest and most disadvantaged in the U.S. This population may be expected toengage in subsistence fishing from the river. The Mexican side of the Rio Grande Valley is borderedby Tamaulipas, Mexico's sixth largest state. Tamaulipas occupies an area of more than 39,000square miles. Its 2000 population was estimated at 2,732,140, approximately 1.1 million of whomlive in Matamoros, Reynosa, and Nuevo Laredo, Tamaulipas' principal border communities. Bothsubsistence and recreational fishing from the Rio Grande are well-documented activities in the U.S.and Mexico. As suggested by population statistics, the public health impacts of consumingcontaminated fish from the Rio Grande may be substantial.

The Seafood Safety Division (SSD) undertook this project in April 1999, funded by a grant to theTexas Department of Health from the TNRCC. SSD collected twenty-four fish samples from theRio Grande at Laredo in Webb County. SSD collected twelve samples from upstream of the city:four largemouth bass, four channel catfish, three common carp, and one longnose gar. The othertwelve samples, consisting of three largemouth bass, one striped bass, two channel catfish, threecommon carp, one freshwater drum, one spotted gar, and one tilapia, were taken from downstreamof Laredo. The United States Geological Survey (USGS) laboratory in Denver analyzed edible fillets(skin off) for many metals, including the "priority" metals arsenic, cadmium, copper, lead, mercury,selenium, and zinc. As part of the USGS's standard analysis for metals in tissues, the laboratoryassayed the tissues for many other metallic elements, including aluminum, barium, boron, iron,nickel, silver, uranium, and vanadium. USGS also analyzed numerous organic compounds,including polychlorinated biphenyls (PCBs), organophosphate and organochlorine pesticides,volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs).

Table 1 is a summary of the organic compounds detected in fish samples from the Rio Grande nearLaredo. Tissue analysis revealed low levels of p,p'-DDE in all twelve samples from downstreamof Laredo, while nine of the twelve fish samples from upstream contained low levels of thispesticide. Two fish samples from downstream of the city also contained low levels of PCBsconsistent with Aroclor 1254. None of the tissues collected upstream of the city of Laredo containedPCBs. Fish collected from the Rio Grande in the near vicinity of Laredo contained very low levelsof several metallic contaminants.


Sample Collection and Data Analysis

To evaluate potential health risks to recreational and subsistence fishers who consumeenvironmentally contaminated seafood, the Texas Department of Health (TDH) collects and analyzessamples from the state's public waters. These samples are representative of available species,trophic levels, lipid content, and legal sizes. When it is appropriate and practical, TDH collectssamples from several locations within a water body to characterize the distribution of contaminantsin seafood from that water body. It is important to note that people eating contaminated seafood aremost likely exposed over the long term through consumption of one or more species contaminatedwith low concentrations of environmental pollutants. Consequently, people exposed toenvironmental contaminants through consumption of seafood are unlikely to display acute or overttoxicity. Instead, subtle, delayed, or chronic adverse health effects may be more commonlyexpected. Thus, the main purpose of TDH contaminant studies is to examine human exposure tospecies commonly consumed over time. TDH may use average concentrations of chemicalcontaminants across species and/or sites to assess the probability of adverse health outcomes fromlow-level, long-term exposure. Despite the possibility that using average concentrations to estimaterisk may lead to over- or underestimates of actual exposures or risks, use of averages is a reasonableapproach to predicting long-term exposure to low levels of contaminants. Although TDH routinelyuses average concentrations for determining external exposure doses, the agency has used andcontinues to use other statistical procedures to assess the likelihood of adverse health effects fromconsumption of contaminated seafood when these procedures are appropriate and necessary.

Deriving Health-based Assessment Comparison Values (HACs)

TDH evaluated chemical contaminants in fish from the Rio Grande directly upstream anddownstream of Laredo, Texas by comparing average concentrations of chemical contaminants withhealth-based assessment comparison (HAC) values for non-cancer and cancer endpoints. TDH usedthe U.S. Environmental Protection Agency's (USEPA) reference doses (RfDs) or the Agency forToxic Substances and Disease Registry's (ATSDR) minimal risk levels (MRLs) to derive thenoncancer HAC values. RfDs and MRLs are estimates of daily exposures to contaminants that areunlikely to cause adverse noncancer health effects, even if exposure occurs over a lifetime. Thecancer risk comparison values in this health consultation are based on the USEPA's chemical-specific cancer slope factors (SF), an estimated lifetime risk of 1 excess cancer in 10,000 (1 x 10-4)people exposed for a lifetime, and an exposure period of 30 years. TDH used standard assumptionsfor body weight (70 kilograms, adult; 35 kilograms, child) and fish consumption (30 grams per day,adult; 15 grams per day, child) to calculate the HAC values [4]. Many of the constants used tocalculate noncancer HAC values have safety margins built into them. Thus, adverse health effectswill not necessarily occur simply because concentrations of toxicants in seafood exceed HAC values. Moreover, although health-based assessment comparison values (HAC values) do not represent asharp dividing line between "safe" and "unsafe" exposures, in practice, TDH finds it unacceptablewhen consumption of less than one meal per week would result in exposures that exceed the RfD,the MRL, or, for multiple contaminants, a hazard index of 1. Thus, in the final analysis, the strictdemarcation between acceptable and unacceptable exposures or risks is primarily a tool used by riskmanagers to ensure protection of public health.

Addressing the Potential for Cumulative Effects

When multiple chemicals affect the same target organ, or when several chemicals present in seafoodtissue may be carcinogens, TDH assumes adverse health effects are cumulative (i.e., additive) [5]. To evaluate the potential public health impact of additive noncancerous health effects, TDHcalculates a hazard index (HI), which is the sum of the ratios of the estimated exposure dose for eachcontaminant divided by its respective RfD or MRL. A HI of less than 1.0 suggests that exposure tocombined contaminants at specified exposure levels is unlikely to cause adverse noncancer healtheffects, even if that exposure continues for many years. On the other hand, while a HI greater than1.0 does not necessarily mean exposure to the contaminants will result in adverse health effects, itdoes suggest that the agency might consider some public health intervention. To estimate thepotential excess lifetime cancer risk from simultaneous exposure to multiple carcinogens, TDHcalculates the cumulative risk by adding the estimated risk for each contaminant. Based on suchcalculations, TDH recommends limiting consumption of seafood contaminated with carcinogenicchemicals to amounts resulting in an estimated theoretical lifetime cancer risk of not more than 1excess cancer in 10,000 thousand persons exposed for a lifetime to the chemicals through seafood.

Addressing the Unique Vulnerabilities of Children

TDH recognizes that fetuses, infants, and children can be uniquely vulnerable to the effects of toxicchemicals, and that any such vulnerabilities demand special attention. Windows of vulnerability,i.e., critical periods, exist during development. These critical periods are particularly evident duringearly gestation, but also appear throughout pregnancy, infancy, childhood, and adolescence - indeed,at any time when toxicants may permanently impair or alter structure or function [6]. Uniquechildhood vulnerabilities result, at least in part, from the fact that, at birth, many organs and bodysystems, including the lungs, immune, endocrine, reproductive, and nervous systems, have notachieved structural or functional maturity; these organ systems continue to develop throughoutchildhood and adolescence. Children can also differ from adults in absorption, metabolism, storage,and excretion of toxicants, any of which could result in higher biologically effective doses to targetorgans. Children's exposures to toxicants may be more significant because children consume morefood and liquids in proportion to their body weight than do adults [6]. Children can also ingesttoxicants through breast milk -- often unrecognized as an exposure pathway. Thus, children canexperience toxic effects at a lower exposure level than would affect adults. Stated differently,children could react more severely than would adults to an equivalent exposure dose [6]. Childrencan also be more prone to developing certain cancers from chemical exposures. Therefore, inaccordance with ATSDR's Child Health Initiative [7] and EPA's National Agenda to ProtectChildren's Health from Environmental Threats [6], TDH evaluated the potential public healthhazards to children who eat fish from the Rio Grande in the near vicinity of the city of Laredo,Texas. TDH found no public health hazard to children who consume fish from the Rio Grande inthe near vicinity of Laredo, Texas.

Characterizing the Risk

Assessing Noncancer Health Effects

Fish collected from the Rio Grande upstream of the city of Laredo Texas in 1999 contained Aroclor1254. Aroclor 1254 concentrations did not exceed the noncancer HAC value for this compound(Table 1). All samples upstream and downstream of Laredo contained small quantities of p,p'-DDE,which is usually considered a metabolic or degradation product of the ubiquitous legacy insecticide,p,p'-DDT. DDE concentrations did not exceed HAC values for this compound. Although neitherDDE nor Aroclor 1254 exceeded its HAC value, similar noncancerous liver toxicity profiles suggestthat hepatic effects may be cumulative (i.e., additive). The TDH evaluated these two chemicals fortheir potential additive effects on the liver by calculating a hazard index (HI). The HI was less than1.0, suggesting that simultaneous exposure to both Aroclor 1254 and DDE is unlikely to increase therisk of noncancerous adverse hepatic effects in persons exposed through consumption of fish fromthe Rio Grande near Laredo, Texas.

Assessing Cancer Health Effects

Neither Aroclor 1254 nor DDE from the Rio Grande upstream or downstream of Laredo exceededits respective cancer HAC value (Table 1). However, the USEPA classifies these chemicals asprobable human carcinogens (Group B2) based on an increase in the incidence of tumors inlaboratory animals [8]. Because simultaneous exposure to both chemicals can occur, TDH evaluatedthe possibility of cumulative carcinogenic effects. Using EPA's chemical-specific cancer slopefactors and the average concentration for each compound, TDH calculated the theoretical increasein cancer risk from consumption of fish from the Rio Grande near Laredo that could contain bothAroclor 1254 and p,p'-DDE. For consumption of seafood, TDH has set an acceptable risk level ofone excess cancer in 10,000 persons exposed for a lifetime. The estimated theoretical increase inrisk associated with consumption of fish from the Rio Grande near Laredo was approximately 1excess cancer in 254,770 persons exposed for 30 years. Qualitatively, TDH interprets these findingsas no apparent increase in the risk for cancer.


The TDH concludes that regular consumption of fish from the Rio Grande in the near vicinity ofLaredo, Texas (upstream and downstream) should not result in exposure to doses of PCBs or DDEor combinations of the two chemicals that would exceed risk management guidelines. Therefore,eating fish from the Rio Grande near Laredo poses no apparent public health hazard.


TDH has established criteria for issuing fish consumption advisories that are based, in part, onconsumption guidelines from the USEPA [4]. When analysis of seafood contaminant data indicatethat eating less than one meal per week (adults: eight ounces; children: four ounces) results inexposures exceeding health-based assessment guidelines, risk managers may recommend that theCommissioner of Health issue a consumption advisory or ban possession of seafood from an affectedwater body. Based on the results of this health consultation, the Seafood Safety Division and theEnvironmental Epidemiology and Toxicology Division recommend that:

  1. TDH presently take no public health action for that portion of the Rio Grande in the directvicinity of the city of Laredo, Texas.

  2. TDH reviews data from fish and crabs taken from the Rio Grande in the vicinity of Laredo,Texas as resources allow.


The public can obtain information about TDH fish consumption advisories and bans from throughTDH Seafood Safety Division (512-719-0215) or on the World Wide Web at Health consultations dealing with contaminants in seafood fromTexas waters may also be available to the public from the Agency for Toxic Substances and DiseaseRegistry ( The Texas Department of Healthprovides this information to the U.S. Environmental Protection Agency (, the TexasNatural Resource Conservation Commission (TNRCC and to the TexasParks and Wildlife Department (TPWD Each year, the TWPD informsthe fishing and hunting public of closure areas in an official hunting and fishing regulations booklet [9] that is available at some state parks and at establishments that sell fishing licenses.

If questions or concerns arise about the scientific information presented in this in this healthconsultation, readers may telephone the Seafood Safety Division (512-719-0215) or theEnvironmental Epidemiology and Toxicology Division (512- 458-7269) at the Texas Departmentof Health. Toxicological information is also available from the Agency for Toxic Substances andDisease Registry (ATSDR), Division of Toxicology, at the toll-free number (800-447-1544) inAtlanta, Ga.

Table 1.

Organic contaminants (mg/kg) detected in fish from the Rio Grande upstream and downstream of Laredo, Texas, 1999.
# Sampled
(Reporting Limit)
Average Concentration
Health Assessment Comparison Value2 Basis for Comparison Value
Downstream of Laredo
Aroclor 1254  2/12
.0113 (nd3-0.069)  0.047 EPA chronic oral RfD for Aroclor 1254: 0.00002 mg/kg/day
0.272 EPA slope factor for PCBs: 2.0
p,p=-DDE 12/12
0.037 (0.006-0.208) 1.6 EPA slope factor: 0.34 (mg/kg/day)-1
Upstream of Laredo
Aroclor 1254   Not Detected   Not Applicable  


EPA chronic oral RfD for Aroclor 1254:


EPA slope factor for PCBs: 2.0 (mg/kg/day)-1


.022 (nd-0.115)

1.6 EPA slope factor: 0.34 (mg/kg/day)-1

1 Minimum detected concentration to maximum detected concentration. (The range is computed by subtracting the minimum concentration from the maximum concentration).
2 derived from the MRL or RfD for noncarcinogens or the EPA slope factor for carcinogens; assumes a body weight of 70 kg and a consumption rate of 30 grams per day, and assumes a 30-year exposure period for carcinogens and an acceptable excess lifetime cancer risk of 1x10-4
3 nd-not detected at concentrations exceeding the laboratory's reporting limit


  1. Governments of Mexico and the United States of America. Binational Study Regarding thePresence of Toxic Substances in the Rio Grande/Rio Bravo and its Tributaries Along theBoundary Portion Between the United States and Mexico. 1994.

  2. Governments of Mexico and the United States of America. Second Phase of the BinationalStudy Regarding the Presence of Toxic Substances in the Rio Grande/Rio Bravo and itsTributaries Along the Boundary Portion Between the United States and Mexico, Vol. II. Final Report. 1997.

  3. [USCB] United States Census Bureau. 2000 Decennial Census. Information available atURL:

  4. [USEPA] U.S. Environmental Protection Agency. Guidance for assessing chemicalcontaminant data for use in fish advisories, Vol. 2. Risk assessment and fish consumptionlimits, 3rd ed. Office of Science and Technology, Office of Water. Washington, D.C.: 2000. Information available at URL:

  5. [USEPA] U.S. Environmental Protection Agency. Guidelines for the health risk assessmentof chemical mixtures. Office of Research and Development. Washington, D.C.: 1986.

  6. [USEPA] U.S. Environmental Protection Agency. Strategy for research on environmentalrisks to children, section 1.2. Office of Research and Development, Washington D.C.: 2000.

  7. [ATSDR] Agency for Toxic Substances and Disease Registry. Child health initiative. Officeof Children's Health. Atlanta, Ga.: 1995.

  8. [IRIS] Integrated Risk Information System (IRIS). USEPA, Office of Research andDevelopment, National Center for Environmental Assessment, Washington, D.C.: 2001. Information available at URL:

  9. [TPWD] Texas Parks and Wildlife. Texas Parks and Wildlife 2000-2001 outdoor annual: official huntingand fishing regulations. Texas Monthly Custom Publishing. 2000.


Jerry Ann Ward, Ph.D.
Seafood Safety Division
Bureau of Food and Drug Safety

Eric Fonken, D.V.M., M.P.Aff.
Assistant Director
Seafood Safety Division
Bureau of Food and Drug Safety

Susan Bush, B.S.
Survey Branch Chief
Seafood Safety Division
Bureau of Food and Drug Safety

G. Kirk Wiles, R.S.
Seafood Safety Division
Bureau of Food and Drug Safety

Lisa Williams, M.S.
Environmental Epidemiology and Toxicology Division
Bureau of Epidemiology

John F. Villanacci, Ph.D.
Environmental Epidemiology and Toxicology Division
Bureau of Epidemiology


George Pettigrew, P.E.
Senior Regional Representative
ATSDR - Region 6


Alan W. Yarbrough
Environmental Health Scientist
Division of Health Assessment and Consultation
Superfund Site Assessment Branch
State Programs Section


This Rio Grande at Laredo Health Consultation was prepared by the Texas Department of Healthunder a cooperative agreement with the Agency for Toxic Substances and Disease Registry(ATSDR). It is in accordance with approved methodology and procedures existing at the timethe health consultation was initiated.

Alan W. Yarbrough
Technical Project Officer, SPS, SSAB, DHAC, ATSDR

The Division of Health Assessment and Consultation, ATSDR, has reviewed this healthconsultation and concurs with its findings.

Roberta Erlwein for Richard Gillig
Chief, State Programs Section, SSAB, DHAC, ATSDR

Table of Contents The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
Contact CDC: 800-232-4636 / TTY: 888-232-6348

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