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The Texas Department of Health (TDH) Seafood Safety Division (SSD) requested that the Health Risk Assessment and Toxicology Program (HRAT) evaluate the potential health risks associated with consumption of fish taken from Lake Houston, located about 18 miles northeast of Houston off of FM 1960. This lake was built in the early 1950’s to serve as a water supply for the City of Houston and is fed by the San Jacinto River. Lake Houston occupies 12,240 acres with a maximum depth of 45 feet reported in 1991.

In 1998, the Texas Natural Resource Conservation Commission (TNRCC) "303D" list reported that water from Lake Houston contained a mercury concentration that exceeded human health criterion for freshwater fish. This criterion was established to protect consumers from bioaccumulation of toxicants in fish tissue. After this information became public, the City of Houston petitioned TDH to collect fish tissue to evaluate potential health risks associated with consumption of Lake Houston fish.

The TDH Seafood Safety Division has established criteria for issuance of fish consumption advisories. This criteria states that in the absence of site specific information, it is assumed that the average consumption rate of fish by recreational fishers is 30 grams per day [1] (approximately one eight-ounce meal per week); and that if an individual can consume more than this amount given the contaminant(s) of concern without exceeding the reference dose for non-carcinogens, an advisory is not recommended.

In April 1998, TDH collected 38 fish, including seven species (eight white bass, four blue catfish, eight largemouth bass, eight freshwater drum, four channel catfish, two black crappie, and four white crappie) from Lake Houston. The specific contaminant of concern that HRAT was asked to evaluate was mercury. All 38 fish were analyzed for mercury and eight of the 38 fish were also analyzed for pesticides, PCB’s, volatile and semi-volatile organic chemicals. The average concentration of mercury found in these fish was 0.315 milligrams mercury/kilogram (mg/kg) fish tissue.


Methylmercury Toxicity
Mercury is a naturally occurring element found throughout the environment in several forms. The mercury in air, water, and soil is predominantly found in the inorganic form. Inorganic mercury released to water can be converted to methylmercury under certain conditions and can be taken up by fish. Concentrations of mercury are generally higher in older and larger predatory fish.

Neurological effects of methylmercury ingestion have been well documented in humans. Clinical manifestations in adults include tingling of the skin, incoordination, difficulty with speech and hearing, tremor, memory loss, and depression. Chronic exposure may result in permanent central nervous system damage. Young children and fetuses are especially at risk for methylmercury poisoning. It can be carried to an infant through breast milk or to the fetus through placental transfer. Neurological effects in children may range from delayed mental and physical development to a severe syndrome similar to cerebral palsy, depending on the extent of exposure [2].

Toxicological Evaluation
To evaluate the current risks associated with eating fish from Lake Houston we used a reference dose (RfD) of 0.0003 mg/kg/day to calculate an upper limit of consumption for the most sensitive subpopulation, women of childbearing age. The reference dose allows a two to four fold margin of safety below the lowest observable adverse effects level (LOAEL) for fetal effects and is equivalent to maternal hair levels of approximately five (5) parts per million (ppm) mercury. According to the World Health Organization (WHO), a prudent interpretation of human exposure data suggests that peak mercury hair levels between 10 to 20 ppm in maternal hair may be associated with a five percent (5%) risk of developmental effects in the fetus [3]. The excess risk associated with a peak maternal hair level of less than five (5) ppm would be considerably less than five percent (5%).

To determine the actual number of meals that could be consumed without exceeding the RfD, we used two values for the concentration term: the arithmetic average and the 95th percent upper confidence interval (95% UCL) of the arithmetic average. The 95% UCL is defined as a value that, when calculated repeatedly for randomly drawn subsets of site specific data, equals or exceeds the true average 95 percent of the time.

We estimated the 95% UCL by defining the distribution of mercury in fish tissue from Lake Houston using the 1998 data and then randomly drawing 1,000 samples of 38 fish from that distribution. Averages were obtained for each of the 1,000 samples and the 950th rank ordered average was defined as the 95th percentile. The arithmetic average should be most representative of the concentration that would be contacted over time; however, the 95% UCL of the average provides a conservative estimate of the average concentration and is useful to account for sampling variations and suspected seasonal variations in fish tissue concentrations. Figure 1 provides a graphical representation of the relationship between the sample distribution and the 95% UCL of the arithmetic average.

Using the arithmetic average, we estimate that adults (body weight 70 kg) could consume approximately 2.2 eight-ounce meals per week and children (body weight 10-30 kg) could consume approximately one-half to two four-ounce meals per week of Lake Houston fish before exceeding the RfD (Table 2). Individuals would have to consume approximately two to four times these amounts before exceeding the lowest level reasonably expected to result in signs or symptoms of toxicity.

Using the 95% UCL of the arithmetic average, we estimate that adults weighing 70 kg could consume approximately 1.6 eight-ounce meals per week and children weighing 10-30 kg could consume approximately one-half to one and one-half four-ounce meals per week of Lake Houston fish before exceeding the RfD (Table 2).

Table 1.

Summary of Results for Fish Taken from Lake Houston
Species # affected Average concentration (mg/kg)
White Bass 8/8 0.39
Blue Catfish 4/4 0.148
Channel Catfish 2/4 0.086
Largemouth Bass 8/8 0.574
Freshwater Drum 6/8 0.249
White Crappie 4/4 0.268
Black Crappie 2/2 0.123
Overall Fish Concentration 0.315

Table 2.

Recommended Limitations on Long-Term Fish Consumption by Body Weight
Body Weight Estimates based on an average mercury level of 0.31 mg/kg and a UCL of 0.45 mg/kg in the 38 fish samples collected from Lake Houston
(kg) (lb)

Reference Dose (µg/day)

(BW × 0.3 ug/kg/day)

Quantity of fish that can be consumed without exceeding MRL
Meals per week
(0.31 mg/kg)
95% UCL
(0.45 mg/kg)
Assumes a child body weight and an average meal size of 4 ounces each
10 22 3 0.6 0.4
20 44 6 1.3 0.9
30 66 9 1.9 1.3
Assumes an adult body weight and an average meal size of 8 ounces each
40 88 12 1.3 0.9
50 110 15 1.6 1.1
60 132 18 1.9 1.3
70 154 21 2.2 1.6
80 176 24 2.6 1.8
90 198 27 2.9 2
100 22 30 3.2 2.2

Figure 1. Probability Density Distribution for Mercury in Fish from Lake Houston


The TDH has prepared this consult under a Cooperative Agreement with the Agency for Toxic Substances and Disease Registry (ATSDR). TDH has included the following information in accordance with ATSDR's Child Health Initiative.

ATSDR’s Child Health Initiative recognizes that the unique vulnerabilities of infants and children demand special emphasis in communities faced with contamination of their water, soil, air, or food. Children are at greater risk than adults from consumption of mercury contaminated fish. Infants may be exposed by breast milk or fetuses may be exposed through placental transfer. Children are also smaller, resulting in higher doses of chemical exposure per body weight. The developing body systems of children can sustain permanent damage if toxic exposures occur during critical growth stages. Most importantly, children depend completely on adults for risk identification and management decisions, housing decision, and access to medical care. TDH evaluated the likelihood for children (body weight 10-30 kg) to be exposed to levels of mercury that exceed the reference dose and determined that approximately one-half to two four-ounce meals per week of Lake Houston fish could result in levels that exceed the reference dose (assuming an average mercury concentration in fish of 0.31 mg/kg).


  1. The 38 fish collected from Lake Houston contain an average level of 0.3l mg/kg mercury.

  2. Considering all of the fish species combined, for adults (body weight 70 kg) a consumption level of approximately two eight-ounce meals per week would be required to exceed the reference dose; and for children (body weight 10-30 kg) a consumption level of approximately one-half to two four-ounce meals per week would be required to exceed the reference dose. Approximately two to four times these amounts would be required to exceed the lowest level reasonably expected to result in signs or symptoms of toxicity (LOAEL).

  3. Based on available information, the estimated low risks for potential adverse health effects do not exceed the TDH Seafood Safety’s criteria for issuance of a fish consumption advisory. Average levels of fish consumption (approximately one meal per week) present no apparent public health hazard.


  1. TDH does not recommend limitations on consumption of fish from Lake Houston.


  1. EPA, 1994. Guidance for assessing chemical contaminant data for use in fish advisories Volume II. Risk Assessment and Fish Consumption Limits. Environmental Protection Agency, Office of Water, Washington, D.C.

  2. ATSDR, 1994. Agency for Toxic Substances and Disease Registry. Toxicological Profile for Mercury (Update). Atlanta Ga., May 1994.

  3. WHO, 1990. World Health Organization. Environmental Health Criteria, 101. Methylmercury. Geneva, 1990.


Lisa R. Williams, M.S.
Health Risk Assessment and Toxicology Program

John F. Villanacci, Ph.D.
Health Risk Assessment and Toxicology Program


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


William Greim
Environmental Health Scientist
Division of Health Assessment and Consultation
Superfind Site Assessment Branch


The Lake Houston 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.

William Greim
Technical Project Officer, SPS, SSAB, DHAC

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

Richard Gillig

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