HEALTH CONSULTATION
RIO GRANDE AT LAREDO
LAREDO, WEBB COUNTY, TEXAS
BACKGROUND AND STATEMENT OF ISSUES
Development has flourished along the Mexico/U.S. border during the past twenty years. Economic
growth, fueled, in part, by the maquiladoras (product assembly plants) that now exist along the
border, as well as the National Free Trade Agreement (NAFTA), increased the potential for
degradation of water quality in the Rio Grande. The potential exists for contamination of the river
by pesticides from nearby farming operations as well as the threat of contamination with toxic
chemicals 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 Plan
for the Mexican-U.S. Border Area (First Stage, 1992-1994) [1] in 1992, as a cooperative effort to
solve environmental problems in the border area. During the first phase of this cooperative
endeavor, conducted in 1992 and 1993, numerous sites along the Rio Grande were screened to
determine if and to what extent river waters or biota 
had been contaminated by industrial and
agricultural activity [1]. The first study revealed thirty chemicals in fish that exceeded screening
levels and were considered to have the potential of causing adverse human health effects. The second
phase of the study, known as the Rio Grande/Rio Bravo Toxic Substance Study (RGTSS), was
conducted from 1995 through 1997 [2]. Neither study adequately addressed the potential for human
health effects from consumption of contaminated fish. For instance, the earlier investigations relied
heavily upon use of composite fish samples; this and other limitations of the studies made it difficult
to determine the health effects of eating contaminated fish from the Rio Grande. Therefore, the
Texas Natural Resources Conservation Commission (TNRCC) requested in 1999 that TDH evaluate
potential adverse health effects from consumption of fish from the Rio Grande. The first phase of
this assessment, reported in this health consultation, targeted two sites near Laredo, Texas. One site
was 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 United States. Thirteen Texas counties (Cameron, Hidalgo, Starr, Zapata, Webb, Maverick, Kenney, Val Verde, Terrell, Brewster, Presidio, Hudspeth, and El Paso), border the Rio Grande. According to the U.S. 2000 decennial census, the population of the six counties comprising the U.S. Rio Grande Valley is approximately 1,184,000, of which 88.5% are of Hispanic descent [3]. Major U.S. cities in 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 the population living in rural or farming communities. People living in the Rio Grande Valley counties are among the poorest and most disadvantaged in the U.S. This population may be expected to engage in subsistence fishing from the river. The Mexican side of the Rio Grande Valley is bordered by Tamaulipas, Mexico's sixth largest state. Tamaulipas occupies an area of more than 39,000 square miles. Its 2000 population was estimated at 2,732,140, approximately 1.1 million of whom live in Matamoros, Reynosa, and Nuevo Laredo, Tamaulipas' principal border communities. Both subsistence 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 consuming contaminated fish from the Rio Grande may be substantial.
The Seafood Safety Division (SSD) undertook this project in April 1999, funded by a grant to the Texas Department of Health from the TNRCC. SSD collected twenty-four fish samples from the Rio 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 other twelve samples, consisting of three largemouth bass, one striped bass, two channel catfish, three common carp, one freshwater drum, one spotted gar, and one tilapia, were taken from downstream of 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 laboratory assayed 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 near Laredo. Tissue analysis revealed low levels of p,p'-DDE in all twelve samples from downstream of Laredo, while nine of the twelve fish samples from upstream contained low levels of this pesticide. Two fish samples from downstream of the city also contained low levels of PCBs consistent with Aroclor 1254. None of the tissues collected upstream of the city of Laredo contained PCBs. Fish collected from the Rio Grande in the near vicinity of Laredo contained very low levels of several metallic contaminants.
Sample Collection and Data Analysis
To evaluate potential health risks to recreational and subsistence fishers who consume environmentally contaminated seafood, the Texas Department of Health (TDH) collects and analyzes samples 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 collects samples from several locations within a water body to characterize the distribution of contaminants in seafood from that water body. It is important to note that people eating contaminated seafood are most likely exposed over the long term through consumption of one or more species contaminated with low concentrations of environmental pollutants. Consequently, people exposed to environmental contaminants through consumption of seafood are unlikely to display acute or overt toxicity. Instead, subtle, delayed, or chronic adverse health effects may be more commonly expected. Thus, the main purpose of TDH contaminant studies is to examine human exposure to species commonly consumed over time. TDH may use average concentrations of chemical contaminants across species and/or sites to assess the probability of adverse health outcomes from low-level, long-term exposure. Despite the possibility that using average concentrations to estimate risk may lead to over- or underestimates of actual exposures or risks, use of averages is a reasonable approach to predicting long-term exposure to low levels of contaminants. Although TDH routinely uses average concentrations for determining external exposure doses, the agency has used and continues to use other statistical procedures to assess the likelihood of adverse health effects from consumption 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 and downstream of Laredo, Texas by comparing average concentrations of chemical contaminants with health-based assessment comparison (HAC) values for non-cancer and cancer endpoints. TDH used the U.S. Environmental Protection Agency's (USEPA) reference doses (RfDs) or the Agency for Toxic Substances and Disease Registry's (ATSDR) minimal risk levels (MRLs) to derive the noncancer HAC values. RfDs and MRLs are estimates of daily exposures to contaminants that are unlikely to cause adverse noncancer health effects, even if exposure occurs over a lifetime. The cancer 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 assumptions for 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 to calculate noncancer HAC values have safety margins built into them. Thus, adverse health effects will 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 a sharp dividing line between "safe" and "unsafe" exposures, in practice, TDH finds it unacceptable when 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 strict demarcation between acceptable and unacceptable exposures or risks is primarily a tool used by risk managers 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 seafood tissue 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, TDH calculates a hazard index (HI), which is the sum of the ratios of the estimated exposure dose for each contaminant divided by its respective RfD or MRL. A HI of less than 1.0 suggests that exposure to combined contaminants at specified exposure levels is unlikely to cause adverse noncancer health effects, even if that exposure continues for many years. On the other hand, while a HI greater than 1.0 does not necessarily mean exposure to the contaminants will result in adverse health effects, it does suggest that the agency might consider some public health intervention. To estimate the potential excess lifetime cancer risk from simultaneous exposure to multiple carcinogens, TDH calculates the cumulative risk by adding the estimated risk for each contaminant. Based on such calculations, TDH recommends limiting consumption of seafood contaminated with carcinogenic chemicals to amounts resulting in an estimated theoretical lifetime cancer risk of not more than 1 excess 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 toxic chemicals, and that any such vulnerabilities demand special attention. Windows of vulnerability, i.e., critical periods, exist during development. These critical periods are particularly evident during early 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]. Unique childhood vulnerabilities result, at least in part, from the fact that, at birth, many organs and body systems, including the lungs, immune, endocrine, reproductive, and nervous systems, have not achieved structural or functional maturity; these organ systems continue to develop throughout childhood 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 target organs. Children's exposures to toxicants may be more significant because children consume more food and liquids in proportion to their body weight than do adults [6]. Children can also ingest toxicants through breast milk -- often unrecognized as an exposure pathway. Thus, children can experience 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]. Children can also be more prone to developing certain cancers from chemical exposures. Therefore, in accordance with ATSDR's Child Health Initiative [7] and EPA's National Agenda to Protect Children's Health from Environmental Threats [6], TDH evaluated the potential public health hazards 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 in the near vicinity of Laredo, Texas.
Assessing Noncancer Health Effects
Fish collected from the Rio Grande upstream of the city of Laredo Texas in 1999 contained Aroclor 1254. 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 neither DDE nor Aroclor 1254 exceeded its HAC value, similar noncancerous liver toxicity profiles suggest that hepatic effects may be cumulative (i.e., additive). The TDH evaluated these two chemicals for their potential additive effects on the liver by calculating a hazard index (HI). The HI was less than 1.0, suggesting that simultaneous exposure to both Aroclor 1254 and DDE is unlikely to increase the risk of noncancerous adverse hepatic effects in persons exposed through consumption of fish from the Rio Grande near Laredo, Texas.
Assessing Cancer Health Effects
Neither Aroclor 1254 nor DDE from the Rio Grande upstream or downstream of Laredo exceeded its respective cancer HAC value (Table 1). However, the USEPA classifies these chemicals as probable human carcinogens (Group B2) based on an increase in the incidence of tumors in laboratory animals [8]. Because simultaneous exposure to both chemicals can occur, TDH evaluated the possibility of cumulative carcinogenic effects. Using EPA's chemical-specific cancer slope factors and the average concentration for each compound, TDH calculated the theoretical increase in cancer risk from consumption of fish from the Rio Grande near Laredo that could contain both Aroclor 1254 and p,p'-DDE. For consumption of seafood, TDH has set an acceptable risk level of one excess cancer in 10,000 persons exposed for a lifetime. The estimated theoretical increase in risk associated with consumption of fish from the Rio Grande near Laredo was approximately 1 excess cancer in 254,770 persons exposed for 30 years. Qualitatively, TDH interprets these findings as no apparent increase in the risk for cancer.
CONCLUSIONS AND PUBLIC HEALTH IMPLICATIONS
The TDH concludes that regular consumption of fish from the Rio Grande in the near vicinity of Laredo, Texas (upstream and downstream) should not result in exposure to doses of PCBs or DDE or 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, on consumption guidelines from the USEPA [4]. When analysis of seafood contaminant data indicate that eating less than one meal per week (adults: eight ounces; children: four ounces) results in exposures exceeding health-based assessment guidelines, risk managers may recommend that the Commissioner of Health issue a consumption advisory or ban possession of seafood from an affected water body. Based on the results of this health consultation, the Seafood Safety Division and the Environmental Epidemiology and Toxicology Division recommend that:
The public can obtain information about TDH fish consumption advisories and bans from through
TDH Seafood Safety Division (512-719-0215) or on the World Wide Web at http://www.tdh.state.tx.us/bfds/ssd . Health consultations dealing with contaminants in seafood from
Texas waters may also be available to the public from the Agency for Toxic Substances and Disease
Registry (http://www.atsdr.cdc.gov/HAC/PHA/region_6.html). The Texas Department of Health
provides this information to the U.S. Environmental Protection Agency (http://fish.rti.org ), the Texas
Natural Resource Conservation Commission (TNRCC http://www.tnrcc.state.tx.us ) and to the Texas
Parks and Wildlife Department (TPWD http://www.tpwd.state.tx.us ). Each year, the TWPD informs
the 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 health consultation, readers may telephone the Seafood Safety Division (512-719-0215) or the Environmental Epidemiology and Toxicology Division (512- 458-7269) at the Texas Department of Health. Toxicological information is also available from the Agency for Toxic Substances and Disease Registry (ATSDR), Division of Toxicology, at the toll-free number (800-447-1544) in Atlanta, Ga.
Table 1. Organic contaminants (mg/kg) detected in fish from the Rio Grande upstream and downstream of Laredo, Texas, 1999.
| CONTAMINANT | # Detected/ # Sampled (Reporting Limit) |
Average Concentration |
Health Assessment Comparison Value2 |
Basis for Comparison Value |
| Downstream of Laredo | ||||
| Aroclor 1254 | 2/12 (0.04) |
.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 (mg/kg/day)-1 |
|||
| p,p=-DDE | 12/12 (.005) |
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 |
0.047 |
EPA chronic oral RfD for Aroclor 1254: |
|
0.272 |
EPA slope factor for PCBs: 2.0 (mg/kg/day)-1 | |||
|
p,p=-DDE |
9/12 |
.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
..Jerry Ann Ward, Ph.D.
Toxicologist
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.
Director
Seafood Safety Division
Bureau of Food and Drug Safety
Lisa Williams, M.S.
Toxicologist
Environmental Epidemiology and Toxicology Division
Bureau of Epidemiology
John F. Villanacci, Ph.D.
Co-Director
Environmental Epidemiology and Toxicology Division
Bureau of Epidemiology
ATSDR REGIONAL REPRESENTATIVE
George Pettigrew, P.E.
Senior Regional Representative
ATSDR - Region 6
ATSDR TECHNICAL PROJECT OFFICER
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 Health under 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 time the 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 health consultation and concurs with its findings.
Roberta Erlwein for Richard Gillig
Chief, State Programs Section, SSAB, DHAC, ATSDR
