MOUNT PLEASANT, TITUS COUNTY, TEXAS
The Texas Department of Health Seafood Safety Division (SSD) requested that the Health Risk Assessment and Toxicology Program reevaluate the current advisory limiting consumption of fish taken from Welsh Reservoir, a 1,365 acre cooling reservoir for Welsh Power Plant and operated by Southwestern Electric Power Company (SWEPCO). Welsh Reservoir is located 11 miles southeast of Mount Pleasant in Titus County, Texas. During the late 1970's, several power plant cooling reservoirs in Texas received discharges from ash ponds containing elevated selenium levels, resulting in increased selenium concentrations in fish. An investigation of a series of fish kills in waterbodies near power plants in the early 1980's implicated elevated levels of selenium in fish tissue and water. The Texas Parks and Wildlife Department (TPWD) reported levels of selenium in Welsh Reservoir fish ranging from 2.28 to 4.41 ppm from 1986-1989.
Since the fish kills, the Texas Department of Health (TDH), TPWD, and SWEPCO have workedcooperatively to monitor selenium levels in fish tissue. In 1992, TDH issued a fish consumptionadvisory for the reservoir after it was determined that the levels of selenium in fish could pose apotential health risk. The advisory recommended that adults consume no more than one eight-ounce meal each week and children seven years of age and older consume no more than one four-ounce meal each week. Children six and under, pregnant women, or women who may soonbecome pregnant were advised not to consume the fish. Additionally, persons consuming fishfrom the reservoir were advised not to consume mineral dietary supplements with seleniumexceeding 50 micrograms per day.
In 1998, TDH collected 20 fish (12 largemouth bass, 6 channel catfish, 1 flathead catfish, and 1 common carp) for a re-evaluation of the present fish consumption advisory. The concentration of selenium in the fish ranged from 1.2 to 4.5 ppm with an overall average concentration of 2.7 ppm (Table 1). Selenium levels in fish from Welsh Reservoir continue to be above reported national averages (0.1 to 1.5 ppm ). The average concentration of selenium in fish from Welsh Reservoir ranged between 2.2 and 4.4 ppm from 1986 to 1989 (Figure 1). There is no clear indication of an overall change in selenium concentrations in fish tissue over the 12 years of investigation; however, there is a 9 year period (1989-1998) in which there are no data to represent an overall increase or decrease in average concentrations. The maximum average concentration of 4.4 ppm selenium in fish tissue occurred in 1987.
|Species||# collected||Size||Selenium Range (ppm)||Selenium Average (ppm)|
|Largemouth Bass||12||18 - 20"||1.87 - 3.88||2.97|
|Channel Catfish||6||18 - 23.5"||1.2 - 2.44||1.78|
Selenium is a naturally occurring substance that is widely, but unevenly distributed in rocks andsoils of the earth's crust. It is rarely found in pure form in the environment. When rocksdecompose into soils, selenium often combines with sodium and oxygen to form sodiumselenate, or with water to form sodium selenite, both of which are water soluble. Plants easilytake up inorganic selenium compounds from water and change them to organic seleniumcompounds such as selenomethionine. The major man-made source of selenium in theenvironment is coal burning, especially the fly ash that results from burning coal. Persons livingnear industrial sites may be exposed through ingesting the soil or water, breathing the dust, eatingplants that may have taken up selenium from soil, or ingesting fish that may have taken upselenium from high concentrations in water . The estimated bioaccumulation factor forselenium in fish ranges from 485 to 1,746, depending on the species . In general the moresoluble and mobile forms of selenium (selenite and selenate) dominate under aerobic (highoxygen concentrations) and alkaline (high pH) conditions .
Humans and animals are both capable of absorbing and utilizing both organic or inorganic formsof selenium from food or water sources. Most of the selenium that enters the body is excreted inthe urine within 24 hours. Selenium can build up in the body if exposure is high and occurs overa long period of time. Body burdens of selenium primarily occur in the liver, kidneys, hair, andnails. Selenium is an essential dietary element for both human and animals in either theinorganic or organic form. Selenium has antioxidant effects which help prevent damage totissues caused by oxygen. A selenium deficient diet can result in Keshan disease, the signs andsymptoms of which may include muscle pain, cardiomyopathy, enlargement of the heart,increased red blood cell fragility, and pancreatic degeneration.
The human recommended daily allowance (RDA) of selenium for maintenance of good health is 55 micrograms per day (ug/day) for women and 70 micrograms per day for men. Estimates of the average intake of selenium from food for the U.S. population range from 71 to 152 micrograms of selenium per person per day . Welsh et al. reported that about three percent of Maryland residents consumed diets which contained over 200 µg of selenium per day . The Food and Nutrition Board of the National Research Council has estimated the safe and adequate daily dietary intake of selenium in adults to be 50 to 200 µg . However, when eaten in amounts that are not much higher than required for good nutrition, selenium becomes harmful to humans and animals  (Figure 2). Signs and symptoms of sub-acute and chronic toxicity in humans include brittleness and loss of hair and nails; blisters, eruptions and skin mottling; pitting and excessive decay of the teeth; a garlic or sour-milk breath odor; periodic episodes of nausea and vomiting; and increasing fatigue.
No human populations in the U.S. have been reported to have chronic selenium toxicity,including populations in the western part of the country with naturally high levels of selenium inthe soil and water. In a study of 142 subjects living in a seleniferous area of South Dakota andWyoming, there was no evidence of toxicity from selenium in subjects whose intake was as highas 724 ug per day . There have been a limited number of case reports of individuals whodeveloped minimal signs of selenium toxicity following chronic dietary intakes of 850-900 ugper day. In areas of China with selenium intakes of 3,200 to 6,690 ug per day, clinically apparentselenium toxicity was observed, while persons with daily intakes in the range of 42-750 ug perday did not produce any signs of toxicity or other adverse health effects .
The Agency for Toxic Substances and Disease Registry (ATSDR) recently has established achronic oral Minimum Risk Level (MRL) for selenium of 5 µg/kg/day . For a 70 kg adult thisis equivalent to an intake of 350 µg/day. The MRL is an estimate of the daily human exposure toa hazardous substance that is likely to be without appreciable risk of noncancer health effectsover a specified duration of exposure. The MRL was derived from a study involving a Chinesepopulation with high selenium intake. Blood selenium concentrations and degree of selenosiswere determined for 349 adults. The endpoint used to derive the MRL was nail disease,specifically brittleness of nails. By regression analysis the corresponding selenium dietaryintakes were determined. A "no observable adverse effects level" (NOAEL) of 15 ug/kg/day,was determined. The NOAEL is the highest dose at which no statistically or biologicallysignificant adverse effects were observed. For a 70 kg adult this dose is equivalent to a seleniumintake of 1,050 µg/day. To establish the MRL, ATSDR divided the NOAEL by a factor of threeto account for human variability. It is important to note that the MRL based on the Chinesestudy may be conservative because only dietary exposure was considered and it has beensuggested that inhalation exposure to selenium in smoke was significant. The population studiedcooked their meals on open fires of coal containing high concentrations of selenium.
The EPA and the National Toxicology Program have determined that selenium is not classifiableas a carcinogen and it is rated as a Group D chemical. In fact, studies of cancer in humanssuggest that lower than normal selenium levels in the diet may increase the risk of cancer. Thismay be due to the fact that selenium is used in the body in enzymes that protect against oxidativedamage to tissues which may be responsible for cancer promotion. One specific form ofselenium, called selenium sulfide, is a probable human carcinogen. Selenium sulfide is notfound in foods, and is a very different chemical from the organic and inorganic seleniumcommonly found in the environment. Because it is not absorbed through the skin, its primary usein the anti-dandruff shampoo Selsun Blue, is considered safe .
In 1992, when TDH estimated the risks associated with eating selenium contaminated fish, anMRL for this contaminant was not available. Using available information, TDH estimated 400µg/day (5.71 µg/kg/day assuming a 70 kg adult) to be a safe and acceptable dietary seleniumintake. This estimated safe daily intake is remarkably similar to the MRL (5 µg/kg/day)established by ATSDR and provided approximately a 2.6-fold safety factor below the currentNOAEL of 1,050 ug/day, or the lowest level reasonably expected to result in signs or symptomsof toxicity. In performing the risk assessment, TDH determined the acceptable daily intake ofselenium from fish by subtracting an assumed daily intake of selenium from other sources (200µg/day [2.86 µg/kg/day assuming a 70 kg adult]) from the estimated safe and acceptable dailyintake. Thus, for a 70 kg adult, TDH estimated that 200 µg (2.86 µg/kg/day) was an acceptabledaily intake of selenium from fish.
To evaluate the current risks associated with eating fish from Welsh Reservoir, we used the sameparadigm used in the previous risk assessment except that we used the MRL established byATSDR. Assuming a background daily intake of selenium from other sources of 200 µg weestimate that approximately 150 µg/day (2.14 µg/kg/day assuming a 70 kg adult) is an acceptabledaily intake of selenium from fish.
To determine the actual number of meals that could be consumed without exceeding the MRL,we used two values for the concentration term; the arithmetic average and the 95th percent upperconfidence interval (95% UCL) of the arithmetic average. The 95% UCL is defined as a valuethat when calculated repeatedly for randomly drawn subsets of site specific data, equals orexceeds the true average 95 percent of the time. The arithmetic average should be mostrepresentative of the concentration that would be contacted over time; however, the 95% UCL ofthe average provides a conservative estimate of the average concentration and is useful toaccount for sampling variations and suspected seasonal variations in fish tissue concentrations.
We estimated the 95% UCL by defining the distribution of selenium in fish tissue from Welsh Reservoir using the 1996-97 data and then randomly drawing 1,000 samples of 20 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. Figure 3 provides a graphical representation of the relationship between the sample distribution, the arithmetic average, and the 95% UCL of the arithmetic average.
Using these assumptions we estimate that adults (70 kg) could eat approximately one and one-half to two (eight ounce) meals per week and children (body weight 10-30 kg) could consume approximately one-half to one and one-half (four ounce) meals per week of Welsh Reservoir fish before exceeding the MRL (Table 2). Individuals would have to consume approximately three times these amounts before exceeding the lowest level reasonably expected to result in signs or symptoms of toxicity.
|Body Weight||Estimates Based on Average and Upper 95% UCL Selenium Levels for the 20 fish samples collected|
(BW x 2.86 ug/kg/day)
Minimum Risk Level
(BW x 5 ug/kg/day)
Acceptable Daily Intake
((b) - (a))
|Quantity of fish that can be consumed without exceeding MRL|
|Meals per week|
|95% UCL (3.05 ppm)|
|Assumes a child body weight and an average meal size of 4 ounces each|
|Assumes an adult body weight and an average meal size of 8 ounces each|
The TDH has prepared this consult under a Cooperative Agreement with the Agency for ToxicSubstances and Disease Registry (ATSDR). TDH has included the following information inaccordance with ATSDR's Child Health Initiative.
ATSDR's Child Health Initiative recognizes that the unique vulnerabilities of infants andchildren demand special emphasis in communities faced with contamination of their water, soil,air, or food. Children are at greater risk than adults from certain kinds of exposures to hazardoussubstances emitted from waste sites and emergency events. They are more likely to be exposedbecause they play outdoors and they often bring food into contaminated areas. They are shorterthan adults, which means they breathe dust, soil, and heavy vapors close to the ground. Childrenare also smaller, resulting in higher doses of chemical exposure per body weight. The developingbody systems of children can sustain permanent damage if toxic exposures occur during criticalgrowth stages. Most importantly, children depend completely on adults for risk identificationand management decisions, housing decisions, and access to medical care.
Special consideration was given to children's health in this consultation. Children consuming one -half to one and one-half (four ounce) meals of Welsh Reservoir fish per week would not exceed minimal risk levels for adverse health effects from selenium in fish.