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Toxicological Evaluation

         Health assessment comparison values (CVs)
To evaluate non-carcinogenic health effects, the Agency for Toxic Substances and Disease Registry (ATSDR) has developed Minimal Risk Levels (MRLs) and the Environmental Protection Agency (EPA) has developed Reference Doses (RfDs) for contaminants commonly found at hazardous waste sites. The MRL or RfD is an estimate of daily human exposure to a contaminant that is unlikely to cause non-cancer adverse health effects over a lifetime. The MRLs and RfDs used in this assessment are based on chronic exposure to contaminated soil through ingestion.

Health assessment comparison values (CVs) are based on MRLs and RfDs and are concentrations used by health assessors to select environmental contaminants for further evaluation. They should not be used as predictors of adverse health effects or as threshold values for clean-up activities.

Because the MDI site is near an elementary school, a drainage area which is accessible to children who may be playing nearby, and to residences with small children, the health assessment comparison values used in this assessment are based on a child's exposure to contaminated soil. This assumes a body weight of 15 kg and a soil ingestion rate of 200 mg/day.

         Contaminants selected for evaluation
The TNRCC provided results for 23 inorganic contaminants in soil and sediment samples collected on or near the MDI site for toxicological review. Four of the contaminants exceeded health assessment comparison values for one or more of the areas investigated and are discussed below. Four additional contaminants that either did not have CVs or did not exceed CVs are included for toxicological review since the results indicate that these samples are significantly above background concentrations and the contaminants are of significant toxicological importance for exposed populations (Table 4). The remaining 15 constituents were not included in the discussion either because they were not found at elevated levels or the potential for toxicity was too low to be of public health concern. For instance, some contaminants exhibit toxicity as a deficiency of the element and excesses of these contaminants in soil would not be of public health importance. Other contaminants may require such a substantial dose of exposure for public health significance, that the likelihood of adverse health effects resulting from exposure to contaminated soil would be negligible.

Exposure Pathways

         On-site exposure
Because the MDI site is completely enclosed by a locked chain link fence, with upper barbed wire, it is not expected that children will have direct contact with contaminated soil on-site. Three of five soil samples collected on-site exceed health comparison values for arsenic. One of five samples collected on-site exceeds the comparison value for nickel. Although comparison values do not exist for copper or iron, three of five samples collected on-site contains levels of these metals 10-100 times above background levels.

         Drainage area
A primary public health concern results from a drainage area near the site that is highly contaminated with heavy metals. This area is outside of the fenced area and is very accessible to children who may be playing nearby. It is located about 50-75 feet across the street from Bruce Elementary School and is an approximate 10 foot wide by 50 foot long stretch of slick surface material which may increase in size due to rain and runoff conditions. Many of the inorganic contaminants that were found on-site also were found to be concentrated in the drainage area outside of the fenced area. The drainage area, which leads away from the site towards Bruce Elementary School and the surrounding neighborhood, could be a potential source of off-site migration of contaminants found on the MDI site and a source of exposure to on-site contaminants for nearby residents.

The two soil samples collected from the drainage area exceed health comparison values for arsenic, lead, manganese, and nickel. Other contaminants found at levels of public health concern include antimony, cadmium, copper, iron, and thallium.

         Residential soil
All five residential soil samples collected adjacent to the MDI site contain excess levels of lead ranging from 566 ppm to 1,170 ppm; however, excess levels of lead were not found on the MDI site. The source of lead in the residential yards is unknown. One soil sample collected from a residential property adjacent to the site contains 24.4 ppm arsenic, which exceeds the CV of 20 ppm. Other contaminants found at levels warranting further attention include antimony, copper and iron.

         School playground
The two soil samples collected from the Bruce Elementary school yard do not contain levels of contaminants that exceed either the background range or health comparison values (Table 5). According to the Texas Natural Resource Conservation Commission, the soil from this school yard has been removed and replaced within the last ten years

         Buffalo Bayou sediment samples
Three sediment samples were collected upstream and three sediment samples were collected downstream of the MDI discharge area to Buffalo Bayou, approximately two miles away from the site. None of these samples contained elevated levels of inorganic contaminants or were found to exceed CVs.

         Background samples
Three background samples were collected from residential yards approximately one-half mile upwind of the MDI site. These samples were analyzed both for metals and organic contaminants. Three organic contaminants were found at the following levels: endrin aldehyde 5.8 ug/g, DDE 19 ug/g, chlordane 2.5 ug/g. None of the metals or organic chemicals analyzed exceed CVs or are present at levels warranting further attention.

Contaminants Exceeding Health Comparison Values

Lead was found at elevated levels in the residential yards and in the drainage area; however, it was found at background levels on-site. Lead is naturally present in most soils and is widespread in the human environment as a result of industrialization. It is generally found in higher concentrations in urban environments, principally as a result of automobile emissions and the use of lead-based paint. The natural lead content of soil derived from crustal rock typically ranges from <10 to 30 parts lead per million parts soil (ppm).

Preschool-age children and fetuses are usually the most vulnerable segments of the population for exposure to lead. This increased vulnerability results from a combination of factors which include the following: 1) the developing nervous system of fetuses and neonates are more susceptible to the neurotoxic effects of lead; 2) young children are more likely to play in dirt and to place their hands and other objects in their mouths, increasing the opportunity for soil ingestion; 3) the efficiency of lead absorption from the gastrointestinal tract is greater in children than in adults.

Infants often are born with some lead in their bodies due to their mother's past exposure to lead. Infants and children are exposed to lead mainly through diet and ingestion of non-food materials associated with normal early hand-to-mouth behavior. The degree to which hand-to-mouth behavior contributes to blood lead levels depends on the levels of lead in house dust, soil, and paint. In the United States, leaded paint continues to cause most of the severe lead poisoning in young children because it is the most widespread source and has the highest concentration of lead per unit of weight [1]. The overall half-life of lead in blood is estimated to be 36 days ± 5 days [2].

The most serious effects of acute high dose lead exposure is encephalopathy, characterized initially by headache and drowsiness, and in more severe cases by coma, convulsions, and death. Virtually all children who recover from acute lead encephalopathy exhibit residual reduction in intelligence and behavioral dysfunction. Acute encephalopathy is usually associated with high blood lead levels (over 150 µg/dL). Another effect of acute high dose lead exposure is the Fanconi syndrome, an acute injury to the renal tubules, characterized by spillage of glucose, protein, amino acids, and phosphates into urine.

Chronic exposure to low lead levels has been shown to cause subtle effects on the central nervous system which manifest as deficits in intelligence, behavior, and school performance [3]. Recent information indicates that children with blood lead levels as low as 10 µg/dL can develop neurological and cognitive deficits [4]. Available evidence is not sufficient to determine whether lead-associated deficits are irreversible [5].

Lead is especially harmful to unborn children. Exposure to lead during pregnancy has been correlated with premature births, low birth weight infants, and spontaneous abortions. While the impact of maternal and cord blood lead levels below 10 µg/dL have not been well-defined, reduced gestational age and reduced birthweight have been associated with blood lead levels of 10 to 15 µg/dL [1]. In addition, lead has been found to lower intelligence quotient (I.Q.) scores, slow growth, and cause hearing problems in children. These adverse effects can persist and lead to decreased performance in school.

Anemia is the most serious effect of lead on the hematologic system. Lead-induced anemia occurs primarily by the lead-induced inhibition of several enzymes involved in the production of hemoglobin. Exposure to lead has been associated with hypertension, renal failure, and gout. Lead has not been shown to be carcinogenic in humans; however, high doses of lead have been found to produce kidney tumors in laboratory studies of rats and mice. The extremely high cumulative doses of lead used in animal studies are difficult to extrapolate to low-level exposure in humans, and do not provide a sufficient basis for quantitative risk assessment. Based on animal data, EPA currently classifies lead as a B2 carcinogen (probable human carcinogen).

Although no threshold level for adverse health effects has been established, evidence suggests that adverse effects occur at blood lead levels at least as low as 10 µg/dL. The Centers for Disease Control and Prevention (CDC) has determined that a blood lead level greater than or equal to 10 µg/dL in children indicates excessive lead absorption and constitutes the grounds for intervention. The 10 µg/dL level is based on observations of enzymatic abnormalities in the red blood cells at blood levels below 25 µg/dL and observations of neurologic and cognitive dysfunction in children with blood lead levels between 10 and 15 µg/dL [5].

A number of studies are available relating blood lead levels in children to levels of lead in the environment. In general, blood lead levels rise 3-7 µg/dL for every 1,000 ppm increase in soil or dust lead concentration. Regression models for the correlation between blood lead levels and soil lead levels predict that soil lead concentrations between approximately 500 and 1000 ppm would result in blood lead levels below 10 µg/dL [6]. Soil lead levels in the residential yards and drainage canal range from 566 ppm ot 1880 ppm. Based on available models correlating blood lead levels with lead in soil, elevated blood lead levels could result in children exposed to the contaminated soil.

Arsenic was found to be slightly above the CV of 20 ppm in soil on-site and in one of five residential yards. However, the comparison value for arsenic is exceeded by more than five times in the drainage area. Arsenic occurs naturally in the earth's crust and is usually found in the inorganic form in the environment. Background levels in soil range from 1-40 ppm with an average value of about 5 ppm. If arsenic contaminated soil comes in contact with skin, only a small amount is absorbed. Most of the arsenic absorbed into the body is excreted in the urine within several days, although some may remain longer than a few months [7].

Oral ingestion of low levels of inorganic arsenic may cause irritation of the stomach and intestines, decreased production of red and white blood cells, abnormal heart rhythm, blood vessel damage, and impaired nerve damage. The single most characteristic effect of long term oral exposure to inorganic arsenic is a pattern of skin changes, including darkening of the skin and appearance of small corns or warts on the palms, soles, and torso. A small number of these may develop into skin cancer [7]. The EPA has classified arsenic as a known human carcinogen based on increased incidence of lung cancer in populations exposed primarily through inhalation and based on increased skin cancer in several populations consuming drinking water with high arsenic concentration [8].

An oral reference dose (RfD) of 0.0003 mg/kg/day has been established by EPA for arsenic. However, there is in general no consensus among scientists on the oral RfD. Values range from 0.0001 to 0.0008 mg/kg/day based on increased incidence of hyperpigmentation and keratosis that increases with dose and age [9]. Children are considered the most sensitive subpopulation to arsenic exposure since they are most likely to ingest contaminated soil. Assuming a body weight of 15 kg for small children and an average soil ingestion rate of 200 mg/day, a concentration greater than 22.5 ppm arsenic in soil would result in exceedance of the RfD for this sensitive subpopulation. Considering the uncertainties associated with the RfD, it is not expected that children ingesting arsenic in soil from the five residences identified would experience adverse health effects.

The level of manganese in the drainage area is four times above the CV. Manganese is only slightly elevated on-site and is below comparison values in the yards. Manganese is a natural component in the environment. Higher than average levels may be found near hazardous waste sites where manganese metal is used to make steel or other products.

There is only limited evidence that oral exposure to excessive levels of manganese can result in adverse health effects in humans. Manganese ingestion may lead to neurological effects similar to those seen following inhalation exposure. Symptoms include weakness, abnormal gait, ataxia, muscular hypotonicity, and a fixed emotionless face. Based on the available literature, it is possible that other factors may contribute to the neurological effects. Ingestion of manganese through contaminated soil typically results in gastrointestinal absorption of about 3-5% [10].

Nickel was found to be slightly above the CV in the drainage area and in one of five samples on-site. It was well below the CV in the residential yards. Nickel normally occurs at very low levels in the environment. Nickel may be released to the environment from stack furnaces, trash incinerators, and metal processing facilities. It strongly adheres to soil particles and cannot easily be taken up by humans to cause adverse health effects. If nickel is eaten, it quickly leaves the body through the feces and urine. The most common adverse health effect of nickel in humans is an allergic reaction, producing a skin rash at the site of contact. Large amounts of nickel must be consumed before adverse health effects are observed. This type of exposure through the environment is highly unlikely near the MDI site. Symptoms may include gastrointestinal distress, an increase in red blood cells, and increased protein in the urine [11].

Other Contaminants of Public Health Concern

Antimony enters the environment during the mining and processing of its ores and the production of antimony metal and alloys. Small amounts are also released to the environment by incinerators and coal burning power plants. Antimony is usually found at very low levels in the environment. Soil usually contains less than 1 ppm; however, concentrations close to 9 ppm have been found in some soils. The soils in the residential yards, the drainage area and on-site contain between 0.4 and 14 ppm antimony. These are below the CV of 20 ppm, but well above the background level of between 0.4 and 2 ppm.

Much of the antimony found in soil and sediment is so strongly attached to dirt particles or buried in minerals that it cannot cause adverse health effects from exposure. There is no information on human health effects from ingesting antimony for more than 14 days. People who were exposed to over 19 ppm in drinking water had severe gastrointestinal effects, including nausea, vomiting, and diarrhea [12].

Copper was found significantly above background levels on-site, in the drainage area, and in the residential yards. Long-term exposure to copper dust can irritate the nose, mouth, and eyes. Copper is a naturally occurring metal that is extensively mined and processed in the United States. Most copper compounds found in air, water, sediment, and soil are so strongly attached to soil or imbedded in minerals that they cannot easily cause adverse health effects. Copper found at hazardous waste sites is usually in this form. Soil generally contains between 2 and 250 ppm copper. The soil in one residential yard near MDI contained 8,470 ppm and the two samples collected from the drainage area contained 826 and 940 ppm copper.

The primary toxicological effects of consuming high levels of copper in humans are gastrointestinal. Large daily intakes of copper can cause headaches, dizziness, nausea, stomach cramps, and diarrhea. Very young children are sensitive to copper, and long-term exposure may cause enlargement of the liver and spleen and alterations of liver enzymes. Ingested copper typically leaves the body through the feces and urine over a period of several days after exposure [13].

Iron was found at up to 50 times above the background level in the drainage area, the residential yards, and on-site. Iron is an essential element that may occur at elevated levels in the environment primarily as a result of the manufacture of iron and steel castings of alloys. The health effects of acute iron toxicity are nearly always associated with accidental ingestion of iron-containing medicines and most often occur in children. Chronic toxicity in adults is a more common problem. Toxicological effects include bloody diarrhea, ulceration of the gastrointestinal tract, metabolic acidosis, liver damage, and coagulation defects [14].

Thallium is a metal that is widely distributed in trace amounts in the earth's crust. It is found in the environment as a result of the manufacture of electronic devices or its use in power plants, cement factories, and smelters. Thallium is naturally found in soil at levels from 0.3 to 0.7 ppm. An average of 1.7 ppm of thallium in soil has been found at hazardous waste sites. Thallium was found at levels ranging from 0.47 to 23.4 ppm in soil from the MDI site and in the drainage area.

When thallium is swallowed, it is rapidly absorbed and distributed to various body parts, especially the liver and kidney. It leaves the body slowly and can be found in urine as long as two months after exposure. The significant routes of exposure near hazardous waste sites are through swallowing contaminated soil or dust, drinking contaminated water, and skin contact with the contaminated soil. Acute ingestion of thallium can cause cardiovascular damage, gastroenteritis, liver necrosis, necrosis of the kidney cortex, extensive hair loss, and nervous system degeneration [15].

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