HEALTH CONSULTATION
Review of Groundwater Data
SIGMON'S SEPTIC TANK SERVICE FACILITY
(a/k/a SIGMON'S SEPTIC TANK SERVICE)
STATESVILLE, IREDELL COUNTY, NORTH CAROLINA
BACKGROUND AND STATEMENT OF ISSUES
On June 27, 2001, the Agency for Toxic Substances and Disease Registry (ATSDR) received a request from the Environmental Protection Agency (EPA) to determine the public health impact the Sigmon's Septic Tank Service Facility, septage removal business, has on private wells located near the facility [ATSDR 2001a]. ATSDR is also reviewing the migration of chemical constituents to nearby surface water bodies (i.e., streams, creeks, ponds, etc.) and their potential impact on public health through recreational fishing. That evaluation will be released in a separate health consultation.
The Sigmon's Septic Tank Service Facility (CERCLIS
No.: NCD062555792 
)
is located at 1268 Eufola Road, approximately five miles southwest of Statesville,
Iredell County, North Carolina [NC DENR 1998, NC DENR 2000]. The facility is
active under the current name Sigmon's Environmental Services. Septic wastes
are temporarily stored in four cylindrical tanks on the property, and the sludges
are periodically removed and transported to a wastewater treatment plant for
disposal. The business employs five workers. The work area includes the area
surrounding the storage tanks (Figure 2). The owner of the business lives on
the property. Drinking water for the owner's home and the business office is
obtained from a private water well located on the property (Figures 1 and 2).
Public access is not restricted on the south side of the property (i.e., former
lagoon area/waste pile), and unauthorized persons have been reported to enter
the property with recreational vehicles through breaks in the fence.
The site has been listed under several names including: Sigmon's Septic Tank Service, AAA Enterprises, and Sigmon Environmental Services (current name). Services provided by the business owners have included the pumping and removal of septic tank wastes and heavy sludges for various customers (e.g., residential, commercial, and industrial), installation and repair of septic tanks, and a variety of other waste removal services to various industries. The groundwater medium has been investigated for many years and the residents are well aware of the groundwater contamination resulting from activities at the Sigmon's Septic Tank Service Facility [NC DENR 1998, NC DENR 2000]. Below is a chronological list summarizing the events that have occurred at the Sigmon's Septic Tank Service Facility:
| HISTORICAL EVENTS AT SIGMON'S SEPTIC TANK SERVICE FACILITY | ||
| Time Period | Event Summary | |
|
|
||
| 1970 - 1978 | Wastewaters from the Sigmon's Septic Tank
Service Facility are originally discharged to the City of Statesville wastewater
treatment plant. |
|
| 1973 - 1974 | Sludges from the Sigmon's Septic Tank Service Facility are land applied to area farmlands. | |
| 1978 - 1992 | Sigmon's Septic Tank Service begins disposing of septic wastes within ten on-site lagoons. Within time period, the North Carolina Department of Natural Resources and Community Development--Division of Environmental Management (DEM) collected groundwater samples from on-site monitoring wells and from nearby private water wells. Analysis revealed elevated levels of metallic and organic chemicals. | |
| 1992 | DEM and the North Carolina Hazardous Waste Section conduct a site investigation and sampling trip to determine if the wastes in the on-site lagoons are hazardous. Since the chemical constituents of the on-site lagoons did not meet the definition of a hazardous waste, the North Carolina Hazardous Waste Section decides the site does not fall under its jurisdiction and refers it to the North Carolina Solid Waste Section for further evaluation. | |
| 1992 - 1995 | On-site lagoons are non-operational. | |
| 1995 | DEM requires the on-site lagoons to be closed. Lagoon sludges are excavated and piled in lagoon area. | |
| Dec. 1995 | DEM refers site to the North Carolina Superfund Section regarding removal options of the piled sludge in lagoon area. | |
| Dec. 1996 | North Carolina Superfund Section adds the site to the CERCLIS database as one for further investigation. | |
| Jan. 1997 | North Carolina Superfund Section refers the site to the EPA Emergency Response and Removal Branch for removal evaluation. | |
| Apr. 1997 | EPA concludes that the site does not meet their criteria for removal eligibility. | |
| Aug. 1997 | North Carolina Superfund Section initiates its investigation of the Sigmon site by conducting sampling in a combined Preliminary Assessment/Site Inspection (PA/SI). Collected samples from the waste pile, open pits, former lagoon area, storage tank area, surface water pathways, on-site/off-site monitoring wells, and nearby private wells. | |
| Dec. 1999 | North Carolina Superfund Section conducts additional sampling at the site through an Expanded Site Inspection (ESI). Again, collected samples from the waste pile, former lagoon area, surface water pathways, an on-site monitoring well, and nearby private wells. | |
The groundwater pathway appears to be of great concern to nearby private well users. Within one-quarter mile of the Sigmon property, 14 people have some degree of chemical contamination in their well water. Several residents have been advised by the North Carolina Occupational and Environmental Epidemiology Section that water from their private wells should not be used for drinking purposes [NC DENR 1998].
Large amounts of waste still remain on site, which the North Carolina Superfund Section believes is the source of the contamination. They have recommended to EPA that a non-critical removal be conducted to address the remaining source areas at the site. This would result in further mitigating any potential chemical releases to the soil, groundwater, and surface water pathways [NC DENR 2000].
ATSDR reviewed analytical results of collected groundwater samples to determine if chemical releases to the area's groundwater may be impacting the public health of nearby private well users [NC DENR 1998, NC DENR 2000].
In September 1987, four on-site monitoring wells (MW1, MW2, MW3, and MW4) were installed around the lagoon area at depths of 34 to 39 feet. Figures 1 and 2 give an aerial perspective illustrating the location of the on-site monitoring wells (refer to label MW1A in Figures 1 and 2) in respect to the source areas, nearby private wells, and other sampling locations (i.e., an off-site monitoring well). Figure 3 gives a more detailed areal perspective of the on-site monitoring wells in proximity to the closed lagoons.
Between 1987 and 1999, 12 samples were collected from the on-site monitoring wells and subsequently analyzed for chlorides, nitrates, sulfates, metals, volatile organic compounds (VOCs), and semi-volatile organic compounds (SVOCs). Table 1 (see Appendix B) summarizes these analytical results and compares them to drinking water comparison values (CVs, see Appendix A), assuming both short-term (i.e., acute or intermediate) and long-term (i.e., chronic) exposures.
Data from the monitoring wells were not used to determine exposure levels since the most likely points of exposures are private wells. Monitoring well data was reviewed to determine which of the chemicals from the site could potentially impact private wells. Of the 40 chemicals detected in on-site monitoring wells, only 12 (30 percent) were detected at levels above drinking water CVs. Six of the twelve chemicals (i.e., aluminum, arsenic, barium, sodium, benzene, and vinyl chloride) were not detected in private wells at levels above drinking water CVs. Therefore, these six chemicals will not be evaluated any further in this health consultation.
Off-site Monitoring Well
During the August 1997 PA/SI, a groundwater sample was collected from an off-site monitoring well (see Figure 1), located approximately 100 feet southeast of the closed lagoons. Chemical analysis of the collected groundwater sample showed no chemical detects above their respective detection limits except for acetone (31 parts per billion). However, that reading was thought to reflect lab contamination and was below drinking water CVs (refer to Table 2 in Appendix B). As long as several source areas (i.e., closed lagoons, waste piles, and open pits) remain at the site, routine collection and analysis of groundwater samples is advisable.
Private Wells
Chemical analyses of collected samples from the on-site monitoring wells did indicate that the closed lagoons and the waste piles have been, and may still be, sources of contaminants that leach into the area's groundwater. These contaminants may subsequently migrate into nearby private wells. Such migration may account for contamination found in at least two private wells. Samples collected from the two private wells showed chemical detects of mercury, nitrates, and VOCs, that were also detected in the on-site monitoring wells. For these two private wells, chemical constituents were generally detected at higher concentrations than in other nearby private wells. Some of the chemical levels detected in the two private wells also exceeded drinking water CVs. Because source areas still remain at the site and because it is likely that they may have impacted at least two private wells, consideration should be given to removing these source areas from the site to prevent or mitigate any potential migration of chemicals into nearby private wells.
Between 1991 and 1999, groundwater samples were collected from 11 nearby privates wells, with one well being abandoned due to a hazardous waste spill unrelated to the site. Approximately 36 samples were collected and subsequently analyzed for nitrates, sulfates, metals, VOCs, and SVOCs. Tables 3 through 13 (see Appendix B) summarize the analytical results and compares them to drinking water CVs. The analytical results for each well are summarized in the following paragraphs.
Private Well PW2 (see Figures 1 and 2) is topographically down gradient of the Sigmon source areas (i.e., 400 feet southwest of the lagoon area) and serves at least one person. The drilled depth is unknown. Twenty-one of the 23 detected chemicals were also detected on-site (most notably mercury, nitrates, and VOCs). Thus, Private Well PW2 is probably being impacted by the source areas located at the site.
Of the 23 chemicals detected in samples collected from the Private Well PW2, maximum detected concentrations for six chemicals (i.e., nitrates, iron, manganese, mercury, 1,4-dichlorobenzene, and 1,2-dichloroethane) exceeded drinking water CVs (see Table 3).
In an area southeast of the site, a relatively large parcel of property contains a private residence, a business, and four rental houses. Three private wells are also located on the property approximately 1,000 feet east of the lagoon area. One of the private wells (PW1R) provides water for the residence and business. The other two private wells furnish water for the four rental houses, and water samples were collected from one of those wells (PW1S). The private wells furnishing water for the rental houses are approximately 60 feet deep. The owner of the property also installed two monitoring wells, located approximately 100 to 150 feet southeast of the lagoon area. As previously mentioned, the monitoring well approximately 100 feet southeast of the site was sampled during the August 1997 PA/SI. Another well (PW1A) on the property, approximately 200 feet east of the lagoon area, was abandoned in 1994 because a tenant of a house closest to the well reportedly contaminated it with gasoline.
Of the 12 chemicals detected in samples collected from Private Wells PW1A, PW1R, and PW1S (see Tables 4 through 6), the detected levels of only two chemicals (i.e., iron and manganese) measured during a 1992 sampling event from the abandoned well (PW1A) exceeded drinking water CVs (see Table 4). Even though the levels of iron and manganese exceeded drinking water CVs, it is of no public health concern now because the well in question is abandoned and water from that well is not being used for either drinking or non-drinking purposes that could lead to exposure [NC DENR 1998, NC DENR 2000].
Private Well PW33 is located approximately 800 feet northwest of the lagoon area and serves at least two people (see Figures 1 and 2). The bored well is approximately 56 to 62 feet deep. Of the ten chemicals detected in samples collected from the well, only the maximum detected concentration of lead exceeded its respective drinking water CV (see Table 7).
Of the five chemicals detected in a sample collected from Private Well PW5 (east of the site; see Figure 2), drinking water CVs were available for only four. However, these four chemicals were not selected for further public health evaluation because none of the detected levels exceeded their respective drinking water CVs (see Table 8).
Private Well PW3 (see Figures 1 and 2) is topographically down gradient of the Sigmon source areas (i.e., 450 feet southwest of the lagoon area) and serves at least one person. The bored well is approximately 27 feet deep. Sixteen of the 19 chemicals detected from samples were also detected on-site (most notably mercury, nitrates, and VOCs). Thus, Private Well PW3 is probably another well that is being impacted by the source areas located at the site.
Of the 19 chemicals detected in samples collected from Private Well PW3, maximum detected concentrations for seven chemicals (i.e., nitrates, manganese, mercury, bromodichloromethane, chloroform, dibromochloromethane, 1,4-dichlorobenzene) exceeded drinking water CVs (see Table 9).
A private well is located at the facility that is used for drinking purposes. The well is located approximately 1,200 feet north of the lagoon area and serves at least two people (see Figure 1). Of the two chemicals detected in the well, none of the detected levels exceeded any available drinking water CVs (see Table 10).
Private Well PW4 is located approximately 1,100 feet northeast of the lagoon area and serves at least one person (see Figure 1). The bored well is approximately 50 feet deep. The sample collected during the 1997 PA/SI was intended to be used as a background sample.
Of the nine chemicals detected in samples collected from the well, maximum detected concentrations for two chemicals (i.e., lead and 1,4-dichlorobenzene) exceeded drinking water CVs (see Table 11).
The maximum detectable level of 1,4-dichlorobenzene was measured in the well during the 1997 PA/SI. The validity of this measurement is in doubt because the level via the extractable method was used, while the more reliable purgeable method did not record any VOC levels in the well for this chemical or any other volatile chemicals at a detection limit of 5 ppb.
Private Well PW34 is located approximately 600 feet east of the lagoon area and serves at least three people (see Figure 1). The well is 363 feet deep. Of the two chemicals detected in a sample collected from the well, none of the detected levels exceeded any available drinking water CVs (see Table 12).
Private Well PW6 is located north of the site and was used as the background well for the 1999 ESI (see Figure 2). Of the six chemicals detected in a sample collected from the well, only five had available drinking water CVs. These five chemicals were not selected for further public health evaluation because none of the detected levels exceeded their respective drinking water CVs (see Table 13).
Of the samples collected from 11 nearby private wells surrounding the site, ten chemicals (i.e., Nitrates; Lead; Mercury; Iron; Manganese; 1,4-Dichlorobenzene; 1,2-Dichloroethane; Bromodichloromethane; Chloroform; and Dibromochloromethane) were selected for further public health evaluation because their maximum detect levels exceeded drinking water CVs (see Table 14).
Three additional substances (i.e., calcium; magnesium; potassium) were also selectively screened for further public health evaluation because there are no available drinking water CVs for these elements (see Table 14). All three of these substances are essential elements required for normal human growth and maintenance of health. Calcium is required for the development of strong bones, magnesium is an essential cofactor of many enzymes, and potassium is important in the transmission of nerve impulses.
The estimated maximum daily intake rates of these elements for an adult and a child who consume water containing the maximum chemical levels found in the private wells sampled near the Sigmon's Septic Tank Service Facility, are listed below. These estimated intake rates are compared to the elements' Recommended Daily Allowance (RDA) Ranges that include children and adults of various age groups. The estimated intake rates are too low to even meet the minimal physiological requirements as set by the RDAs. Therefore, these three elements (i.e., calcium, magnesium, and potassium) will not be evaluated any further in this health consultation.
| RDA Comparison | ||||||
| Element | Estimated Daily Intake Rate (mg/day) | RDA Range (mg/day) | ||||
| Adult 1 | Child 2 | |||||
|
|
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| Calcium |
190
|
95
|
800 - 1,200
|
|||
| Magnesium |
24
|
12
|
150 - 350
|
|||
| Potassium |
14
|
7
|
1,600 - 3,500
|
|||
| 1 Assuming the average adult weighs
70 kg (i.e., 150 lbs.) and consumes 2 liters of water per day.
2 Assuming the average child weighs 10 kg (i.e., 20 lbs.) and consumes 1 liter of water per day. |
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Chronic or long- term exposure to chemicals in the groundwater can occur via ingestion, inhalation (i.e., VOCs), and dermal contact, when groundwater is used for drinking, showering, bathing, and other household purposes. Studies indicate that significant exposures to VOCs can occur during these activities as the chemicals volatilize, and are then subsequently inhaled and/or absorbed through the skin. These exposures to VOCs may equal or exceed those from ingestion, usually, by no more than a factor of 2 [Jo et al 1988, Kerger & Paustenbach 2000, Kezic et al 1997, Mattie et al 1994, EPA 1999].
For the purposes of this health consultation, the primary route of human exposure is considered to be ingestion. Inhalation exposure was determined to have no public health implications because of the following: (1) VOCs were estimated to be released into the air at very low levels (i.e., 0.0024 parts per million for 1,2-dichlorobenzene, 0.0022 parts per million for 1,4-dichlorobenzene, 0.0001 parts per million for 1,1-dichloroethane, etc.) during showering and other uses [Andelman 1990], (2) the estimated VOC levels in air (i.e., a conservative approximation assuming the worst case scenario) were well below CVs for inhalation exposures that considered both short-term and long-term exposures, (3) a dermal study indicated that 2% to 5% of organic chemicals (i.e., non-polar compounds) in an aqueous matrix are absorbed through the skin during a 30-minute period [Webster et al 1987]; therefore, the absorption of organic chemicals through dermal exposure from showering under such conditions (i.e., low-level water concentrations) is also considered to be negligible, and (4) the detected metals in water will neither volatilize into the air nor be absorbed through the skin because the detected metals are dissolved in solution and not so readily released.
Based on ATSDR's review of the groundwater sampling and analysis data, the following chemicals were selected for further public health evaluation: nitrates; lead; mercury; iron; manganese; 1,4-dichlorobenzene; 1,2-dichloroethane; bromodichloromethane; chloroform; and dibromochloromethane. With the exception of nitrates, these chemicals were classified as metals (i.e., lead, mercury, iron, manganese) or VOCs (i.e.; 1,4-dichlorobenzene; 1,2-dichloroethane; bromodichloromethane; chloroform; dibromochloromethane).
Nitrates
The toxicity of nitrates is due to its conversion to nitrites by bacteria in the gastrointestinal tract (i.e., intestines). Infants are especially susceptible to methemoglobinemia because the higher pH of their gastric juice is more compatible with the growth of nitrate-reducing bacteria in the gut. Older children, with their more acidic gastric juices, are much less susceptible [Craun et. al. 1981]. Nitrite oxidizes the Fe(+2) of iron in hemoglobin to the Fe(+3) state. The resulting compound (methemoglobin) does not bind oxygen, so that the blood cannot transport as much oxygen from the lungs to the tissues. Infants are the particularly sensitive to nitrate/nitrite toxicity. The characteristic blueness (cyanosis) of lips and mucous membranes, which generally precedes the adverse symptoms of methemoglobinemia, can be produced by methemoglobin levels as low as 10%. Methemoglobin levels under 30% produce minimal symptoms (fatigue, lightheadedness, headache) in healthy children and adults, while levels between 30% and 50% cause moderate depression of the cardiovascular and central nervous systems (weakness, headache, rapid breathing and heartbeat, mild shortness of breath). Levels between 50% and 70% cause severe symptoms (stupor, slow and abnormal heartbeat, respiratory depression, convulsions), and levels above 70% are usually fatal [Ellenhorn, et. al., 1988]. Any levels of methemoglobin that might be associated with the maximum detected nitrate levels in water from private wells at this site are likely to be less than 2%. (See discussion below.)
EPA has developed a chronic oral reference dose for the ingestion of nitrates based on the early clinical signs of methemoglobinemia (cyanosis) in infants ingesting water containing varying concentrations of nitrate-nitrogen. That RfD is set equivalent to the observed NOAEL (i.e., No Observed Adverse Effect Level) of 1,600 µg nitrate-nitrogen/kg/day which is the dose that would be received by a 0-3 month old infant weighing approximately 8.8 pounds (4 kg) and drinking 0.64 liters/day of water (as formula) containing 10,000 ug/L nitrate-nitrogen.
One primary source of organic nitrates is human sewage, which is the type of business conducted at the site (i.e., removal and handling of septic wastes). Due to their high solubility and weak retention by soil, nitrates and nitrites are very mobile in soil and have a high potential to migrate to groundwater. Most nitrogenous materials in natural waters tend to be converted to nitrate, so all sources of combined nitrogen, particularly organic nitrogen and ammonia, should be considered as potential nitrate sources. Because it does not volatilize, nitrate/nitrite is likely to remain in water until consumed by plants or other organisms. Ammonium nitrate will be taken up by bacteria. Nitrate is more persistent in water than the ammonium ion. Nitrate degradation is fastest in anaerobic conditions.
Nitrate was detected at levels above drinking water CVs in two private wells, Private Wells PW2 and PW3, near the site. The estimated daily dose of nitrate from water containing 23,350 ppb (maximum nitrate detection in Private Well PW2) would be 667 µg/kg/day for a 70 kg (i.e., 150 pounds) adult ingesting two liters of water per day; 2,335 µg/kg/day for a 10 kg (i.e., 20 pounds) child ingesting one liter of water per day; and 3,736 µg/kg/day for a 4 kg (i.e., 8 pounds) infant ingesting 0.64 liters of water (as formula) per day. Although the estimated daily dose for a child is slightly higher than the RfD, non-cancerous health effects are not expected in adults or children older than six months at these dose levels. Although it is not recommended that infants 1-3 months of age be chronically exposed to levels of nitrates that exceed EPA's RfD (in this case, by a factor of 2.3), adverse effects would not be likely to occur in them, either. In one study, oral doses of nitrate ranging from 100 µg/kg/day to 15,500 µg/kg/day in 111 infants less than six months old was associated with methemoglobin levels as high as 5.3% (mean 1.6%), but none of the children had the typical symptoms of methemoglobinemia [Winton, et. al., 1971]. In another study, mean methemoglobin levels were only 1.3% in infants aged 1-3 months who received water containing 11,000-23,000 µg nitrate-nitrogen/L [Simon et al., 1964]. No clinical signs of methemoglobinemia were detected in any of these infants, either. Low levels of methemoglobin (0.5 to 2.0%) occur normally and, due to the large excess capacity of blood to carry oxygen, levels of methemoglobin up to 10% are seldom associated with any clinically significant signs such as cyanosis (IRIS 2001). Most cases of infant methemoglobinemia are associated with exposure to nitrate in drinking water used to prepare infants' formula at levels >20,000 ppb of nitrate-nitrogen. However, cases have been reported at levels of 11,000-20,000 ppb nitrate- nitrogen, especially when associated with concomitant exposure to bacteriologically contaminated water or excess intake of nitrate from other sources. Therefore, if other sources of drinking water are available, well water from Private Well PW2, and possibly Private Well PW3, should not be used for making infant formula.
On July 11, 2001, an ATSDR regional representative learned that a six-month-old baby lived in a home located on the same street as Private Wells PW2 and PW3, probably impacted by the source areas located on site [ATSDR 2001c]. During the 1997 PA/SI, the well for this home was not tested because there was no response to NC DENR messages left at the door. However, the ATSDR regional representative learned from one of the occupants that they used bottle water for both cooking and drinking. The same ATSDR regional representative later learned that the water supplied to the home is from Private Well PW1R [ATSDR 2001e]. Past sampling (i.e., 1997 PA/SI) has indicated that no chemicals from the site have impacted Private Well PW1R. Initially, EPA planned to sample the well supplying water to the home in question during November 2001; however, upon learning that the well water originated from Private Well PW1R, they concluded there was no immediate need to resample the well because it was not contaminated [ATSDR 2001d, ATSDR 2001e]. (Note, detected nitrate levels in Private Well PW1R were no higher than 1,400 ppb as of August 1997.) Even though the water from Private Well PW1R is probably safe for an infant to consume, ATSDR recommends that EPA periodically collect and analyze groundwater samples, especially for nitrates, for households with infants and small children until the sources areas are removed from the site.
Metals
Lead, mercury, and two nutrient metals, iron and manganese, were detected in several private wells near the site. The public health implications of these metals are discussed below.
The EPA Office of Drinking Water has established 15 ppb as an action level for lead in drinking water [EPA 1991]. For children and adults drinking water with lead levels at 1-14 ppb, no action is necessary; however, children and pregnant females, should stop drinking the water if it contains lead levels at 15 ppb or greater. Furthermore, one may want to consider not using the water for cooking, especially if children and pregnant females reside in the household. Adults drinking water with lead levels of 15 to 50 ppb should try to reduce their consumption, and water containing lead levels at 50 ppb or greater should not be used for either drinking or cooking.
Lead was detected in five private wells near the Sigmon's Septic Tank Service Facility; however, only two wells (Private Wells PW33 and PW4) contained lead levels (17 ppb and 28 ppb respectively) that exceeded EPA's Lead Action Level of 15 ppb. EPA's Lead Action Level of 15 ppb was exceeded only once in each well between 1994 and 1999. Intermittent exposures of this type (i.e., limited and infrequent excursions above the action level of 15 ppb) over an extended period of time (e.g., more than a year) are not likely to be associated with any adverse health effects. As estimated by EPA's Integrated Exposure Uptake Biokinetic Model for Lead in Children (IEUBK), a blood lead level increase of 2 µg/dL is expected in children drinking water containing the maximum lead level (28 ppb) found in the Private Well PW4 [EPA 2001]. Inasmuch as average blood lead levels in the U.S. (i.e., levels not associated with toxicity) fell by five times that amount (from 12.8 to 2.8 g/dL) in the 1980s (i.e., between NHANES II and III), an increase of 2 µg/dL is not likely to be of any toxicological significance. However, since it is the total blood lead level, and not some increment, that is associated with health risks, concerned residents should ask their local physicians to determine their blood lead levels.
The health effects of lead are not immediately apparent. Once in the blood, lead is distributed to the soft tissue (kidneys, bone marrow, liver, and brain) and mineralizing tissue (bones and teeth). Bones and teeth contain about 95% of the total body burden of lead [ATSDR 1999b]. It is the level of lead in the blood that is related to the risk of adverse health effects, and the small amounts of lead that are released from the bones over time contribute to those blood levels. Thus, cumulative, low-level exposures, as well as higher acute exposures, can be of potential health concern, especially in pregnant women. However, CDC's current limit of 10 ug/dL is designed to be protective of the public's health and does not constitute an established level of toxicity. Exposure to very high levels of lead can cause anemia and encephalopathy (80-100 µg/dL), kidney damage in adults (40 - 100 µg/dL) and children (35 - 50 µg/dL), and increased blood pressure in adult males (30 µg/dL) [Goyer 1996, Table 23-5, pg 705]. Acute effects of exposure to high lead levels are nausea, vomiting, and headache. High levels of blood lead (40 µg/dL) may affect sperm or damage other parts of the male reproductive system making it difficult for a couple to have children [ATSDR 1999b].
Certain subgroups of the population may be more susceptible to the harmful effects of lead exposure: preschool age children (< 6 years old), pregnant women and their fetuses, and the elderly. Other susceptible people may include those with genetic diseases affecting heme synthesis (a component of the blood), nutritional deficiencies (especially iron and calcium), and neurological or kidney dysfunctions. Smoking cigarettes and drinking alcohol also may increase the risk of noncancerous health effects to lead exposure [ATSDR 1999b].
EPA has concluded that the human data are inadequate to determine if lead exposure could cause cancerous health effects in people. However, based on sufficient evidence in animals and inadequate evidence in humans, EPA has classified lead as a probable (B2) human carcinogen.
A simple medical test is available for screening blood lead levels. People who are concerned about their exposure to lead should see their doctor for more information. In addition, there are a number of short-term remedies that you can take to reduce the lead concentrations in your drinking water and, thus, your exposure to lead.
If the source of lead is the plumbing, let the water run from the tap for from 30 seconds to two minutes before using it for drinking and cooking. The longer water stays in water pipes, the more lead may have dissolved out of the lead pipes. Water that has been in the pipes for more than four hours should be flushed for three to five minutes, for example, first thing in the morning and when you arrive home in the evening. A good indication of when to stop flushing the cold water tap is when the water becomes noticeably colder. Use cold water for cooking or making infant formula because water from the hot water tap tends to dissolve lead more quickly, which will cause lead concentrations to be higher in hot water.
If the source of lead is the groundwater and your tap water contains lead in excess of 15 ppb even after flushing, then you may want to consider using bottled water instead of tap water for drinking or cooking purposes. Alternatively, you may choose to use a water purification system. Purification systems range in size and cost from the water pitcher filtration systems to purification systems for the entire household.
Mercury was selected for further analysis because the average (2.8 ppb) and maximum (7 ppb) levels detected in drinking water wells at the Sigmon site exceeded the MCL of 2 ppb (Table 14), but only marginally so, relative to built-in margins of safety. (The RfD for mercuric chloride contains an uncertainty factor of 1000.) ATSDR currently has no comparison values specific for inorganic mercury. However, concentrations of mercury in groundwater at the Sigmon site did not exceed ATSDR's CVs for either chronic or intermediate duration exposure to mercury chloride via the oral route in adults. More importantly, the dose (0.2 µg/kg/day) that would result if a 70-kg adult (i.e., 150 pound adult) consumed 2 liters of water per day containing the maximum level of mercury (7 ppb) detected near the Sigmon's Septic Tank Facility is at least 1000 times lower than the lowest known LOAELs for inorganic mercury (the predominant form in water) in humans or animals exposed via the oral route [ATSDR 1997, Table 2-2]. This maximum estimated dose is also below all known LOAELs for organic mercury in humans or animals exposed via the oral route. Therefore, none of the levels of mercury detected in groundwater at the Sigmon site would be expected to produce adverse health effects of any kind in exposed residents.
The Environmental Protection Agency (EPA) has established non-enforceable Secondary Drinking Water Guidelines (SDWGs) to maintain the aesthetic quality of water, including its taste and odor. EPA's SDWGs for iron and manganese are 300 and 50 ppb, respectively. At concentrations above the SDWGs, iron and manganese may cause undesirable tastes, deposit on foods during cooking, and leave reddish-brown (iron) or brownish-black (manganese) stains on plumbing fixtures and laundry. The concentrations of iron and manganese in some of the residential wells surrounding the Sigmon's Septic Tank Facility were above these non-health-based standards.
The water samples collected from nearby private wells surrounding the site contained from 14 to 5,500 ppb iron. Water containing about 300 ppb of iron or more may not taste very good, but even at 5,500 ppb iron, no adverse health effects would be expected. Iron is among the most abundant elements present naturally in the Earth's crust, and there is very little risk of toxicity from iron in natural foods and water. This is because (1) iron is an essential element in the human diet and (2) the body has a number of mechanisms specifically designed to maintain a relatively constant blood level in the face of wide variations in dietary intake [Goyer, 1996, pp 715-16]. The recommended daily allowance of iron for adult males and females of reproductive age is 10 and 18 mg, respectively. (As noted previously, the secondary MCL of 0.3 mg/L is based on taste and appearance, and not on any potential for adverse health effects.) The long-term toxic levels of dietary iron seen in most monogastric animals (i.e., those with a single stomach) are generally 340 to 1,700 times greater than the nutritional requirement for humans [NRC 1980, pp 309-12,and Table V-12 on page 320]. By comparison, for women of reproductive age whose iron requirements are met entirely by food alone, and who additionally consume 2L/day of well water containing 5,500 ppb iron, the total daily dietary intake of iron would be little more than twice the nutritional requirement. Thus, while iron toxicity is a possibility under certain circumstances, drinking water is seldom the source of toxic exposures.
Manganese (Mn) is another nutrient metal with very limited toxic potential via the oral route. (Adverse effects in humans resulting from manganese exposure are associated primarily with inhalation exposure in occupational settings such as mining.) The levels of manganese in private wells in the vicinity of the site ranged from 4.2 to 830 ppb. Assuming consumption of 2L/day for adults and 1L/day for children, the highest concentration of manganese detected in wells around the site (830 ppb) would correspond to a daily intake of only 1.66 mg Mn/day for adults and half that for a small child. These amounts are of little consequence when compared to safe normal dietary exposures. The World Health Organization (WHO) estimates that the average daily intake of Mn ranges from 2 to 8.8 mg Mn and that 8-9 mg/day is perfectly safe. A normal diet, especially a vegetarian diet, may contain well over 10 mg Mn/day or 0.14 mg Mn/kg/day for a 70 kg adult [NRC 1980]. The maximum concentration detected does exceed the secondary MCL of 50 ppb, but this MCL, like the one for iron, is based solely on aesthetic considerations. (At concentrations over 2,000 ppb, Mn precipitates upon oxidation and causes undesirable tastes, deposits on foods during cooking, and leaves black stains on plumbing fixtures and laundry.) Therefore, the Mn levels detected in private wells around the site are expected to have no public health implications.
VOCs
Based on the private well sampling data, the concentrations for none of the detected VOCs exceeded EPA's MCLs or any CVs for non-cancer effects. The maximum concentrations of five VOCs ( bromodichloromethane, chloroform, dibromochloromethane, 1,4-dichlorobenzene, and 1,2-dichloroethane) did exceed their respective cancer-based CVs for drinking water. These CVs were based on studies in which laboratory animals were force-fed very high, single, daily doses of the substance in oil over most of the animals' lifetimes, an experimental practice that maximizes the instantaneous assault on the animals' defense systems and increases the likelihood that toxic effects will be produced. However, humans are exposed to chloroform and other chlorination by-products in drinking water (not in gavage oil) and substances like chloroform and dibromochloromethane do not cause cancer in laboratory animals when they are administered in drinking water. This is probably because (a) the substances are less soluble in water than in oil and (b) the total daily dose in humans is spread out over the entire day so that each individual dose is much smaller (and, therefore, more easily detoxified) than the single daily dose administered to laboratory animals. Therefore, ATSDR considers that neither cancer nor non-cancer effects would be expected to occur as a result of site-specific exposures to VOCs in drinking water at the Sigmon site, even with a lifetime of exposure.
The following issues should be noted regarding the groundwater contamination found near the Sigmon's Septic Tank Service Facility:
ATSDR'S CHILD HEALTH INITIATIVE
As part of ATSDR's Child Health Initiative, ATSDR considers children in the evaluation for all environmental exposures and uses health guidelines that are protective for children. When evaluating any potential health effects via ingestion, children are considered a special population because their lower body weight causes an increased body burden (i.e., higher exposure doses), which can make them more susceptible to adverse health effects via chemical exposure. Average body weight differences, as well as average differences in child-specific intake rates for various environmental media, are taken into account by ATSDR's child Environmental Media Evaluation Guides (EMEGs).
The maximum levels of nitrate detected (23,350 ppb) could pose an increased risk of higher methemoglobin levels to very young infants (less than six months of age) drinking formula prepared with private well water. However, the information available to ATSDR suggests that no infants lived in the homes serviced by the wells containing high levels of nitrate. Nevertheless, as a matter of prudent public health policy, ATSDR recommends that households with infants and small children, have their well water tested periodically to assure that the concentrations of nitrates and lead are within safe drinking water standards.
Environmental Health Scientist:
David S. Sutton, PhD
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry
Toxicologist:
Frank C. Schnell, PhD, DABT
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry
Writer/Editor:
Kathryn D. Harmsen, MPH
Office of Policy and External Affairs
Agency for Toxic Substances and Disease Registry
Reviewed by
Chief:
John E. Abraham, PhD, MPH
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry
Chief, Health Consultation Section:
Susan Moore, MS
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry
Regional Representative, Region IV:
Benjamin Moore
Office of Regional Operations
Agency for Toxic Substances and Disease Registry
(*) Andelman JB. 1990. Total Exposure to Volatile Organic Compounds in Potable Water (in textbook entitled "Significance and Treatment of Volatile Organic Compounds in Water Supplies"). Lewis Publishers. Chelsea, MI. 485-504.
*Andres P. 1984. IgA-IgG Disease in the Intestine of Brown Norway Rats Ingesting Mercuric Chloride. Clin. Immunol. Immunopathol. 30: 488-494.
Agency for Toxic Substances and Disease Registry. December 1989. Toxicological Profile for Bromodichloromethane. US DHHS, Public Health Service; Atlanta, GA.
Agency for Toxic Substances and Disease Registry. December 1990. Toxicological Profile for Chlorodibromomethane. US DHHS, Public Health Service; Atlanta, GA.
Agency for Toxic Substances and Disease Registry. September 1997. Toxicological Profile for Chloroform (Update). US DHHS, Public Health Service; Atlanta, GA.
Agency for Toxic Substances and Disease Registry. December 1998. Toxicological Profile for 1,4-Dichlorobenzene (Update). US DHHS, Public Health Service; Atlanta, GA.
*Agency for Toxic Substances and Disease Registry. March 1999a. Toxicological Profile for Mercury (Update). US DHHS, Public Health Service; Atlanta, GA.
*Agency for Toxic Substances and Disease Registry. July 1999b. Toxicological Profile for Lead (Update). US DHHS, Public Health Service; Atlanta, GA.
Agency for Toxic Substances and Disease Registry. August 1999c. Toxicological Profile for 1,2-Dichloroethane (Draft Update-Public Comment). US DHHS, Public Health Service; Atlanta, GA.
Agency for Toxic Substances and Disease Registry. September 2000. Toxicological Profile for Manganese (Update). US DHHS, Public Health Service; Atlanta, GA.
*ATSDR Electronic Mail, To: Susan Moore, Section Chief, Consultations Section, Exposure Investigations and Consultations Branch, Division of Health Assessment and Consultation, ATSDR, Atlanta, GA, From: Benjamin Moore, Office of Regional Operations, ATSDR, Region 4, Atlanta, GA, Date: June 27, 2001a.
Agency for Toxic Substances and Disease Registry. "Drinking Water Comparison Value Table." US DHHS, Public Health Service; Atlanta, GA. June 30, 2001b.
*Agency for Toxic Substances and Disease Registry. ATSDR Record of Activity. Site Visit Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville, Iredell County, North Carolina. Date: July 11, 2001c.
*Agency for Toxic Substances and Disease Registry. ATSDR Record of Activity. Site Visit Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville, Iredell County, North Carolina. Date: September 26, 2001d.
*ATSDR Electronic Mail, To: Benjamin Moore, Office of Regional Operations, ATSDR, Region 4, Atlanta, GA, From: Giezelle Bennett, EPA, Region 4, Atlanta, GA, Date: November 19, 2001e.
*Bernaudin JF, Druet E, Druet P, and Masse R. 1981. Inhalation or Ingestion of Organic or Inorganic Mercurials Produces Auto-Immune Disease in Rats. Clin. Immunol. Immunopathol. 20: 129-135.
*Craun, GF, DG Greathouse and DH Gunderson. 1981. Methemoglobin levels in young children consuming high nitrate well water in the United States. Int. J. Epidemiol. 10(4): 309-317.
*Druet P, Druet E, Potdevin F, and Sapin C. 1978. Immune Type Glomerulonephritis Induced by HgCl2 in the Brown Norway Rat. Ann. Immunol. 129C: 777-792.
*Ellenhorn MJ and Barceloux DG. 1988. Medical Toxicology: Diagnosis and treatment of human poisoning. Elsevier Science Publishing Company, Inc., New York, NY, pg. 849
Environmental Protection Agency. Guidelines for Carcinogenic Risk Assessment. Fed. Reg., 51: 33997-33998, September 24, 1986.
*EPA. 1991. U.S. Environmental Protection Agency. National Primary Drinking Water Regulations. Code of Federal Regulations.
Environmental Protection Agency. October 1996. Drinking water regulations and health advisories. Office of Water. EPA 822-B-96-002.
Environmental Protection Agency. May 5, 1998a. URL:
http://www.epa.gov/iris/subst/0076.htm "Safe Drinking Water Fact Sheet for
Nitrate."
Environmental Protection Agency. December 16, 1998b. National Primary Drinking Water Regulations: Disinfectants and Disinfection By Products; Final Rule. Federal Register: Vol. 63, No. 241, pp 69389-69476.
*Environmental Protection Agency. 1999. "Risk Assessment Guidelines for Dermal Assessment." Washington, DC.
*Environmental Protection Agency. 2001. URL:
http://www.epa.gov/superfund/programs/lead/ieubk.htm "The IEUBK."
Environmental Protection Agency, Region III Office. "Risk-Based Concentration Table." Philadelphia, Pennsylvania. May 8, 2001.
*Goyer, Robert A. Toxic Effects of Metals. Chap. 23 of Casarett and Doull's TOXICOLOGY: The basic Science of Poisons. McGraw-Hill, New York, N.Y., 1996, pp 691-736.
*Integrated Risk Information System. U.S. Environmental Protection Agency, Office of Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH.
*Jo WK, Weisel CP, Lioy PJ. 1988. "Routes of Chloroform Exposure and Body Burden from Showering with Chlorinated Tap Water." Risk Anal. 10:575-580.
*Kerger B and Paustenbach D. 2000. "Exposure to 1,1,1-TCE Vapors in a Home Due to Contaminated Groundwater." Risk Anal. in press.
*Kezic S, Mahieu K, Monster AC, de Wolff FA. 1997. "Dermal Absorption of Vaporous and Liquid 2-Methoxyethanol and 2-Ethoxyethanol in Volunteers." Occup. Environ. Med. 54:38-43.
*Mattie DR, Bates GD (Jr.), Jepson GW, Fisher JW, McDougal JN. 1994. "Determination of Skin-Air Partition Coefficients for Volatile Chemicals: Experimental Method and Applications." Fundam. Appl. Toxicol. 22:51.
*North Carolina Department of Environment and Natural Resources-Division of Waste Management-Superfund Section. Combined Preliminary Assessment/Site Inspection Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville, Iredell County, North Carolina, Reference No. 06611. September 1998.
*North Carolina Department of Environment and Natural Resources-Division of Waste Management- Superfund Section. Expanded Site Inspection Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville, Iredell County, North Carolina, Reference No. 0406611. March 2000.
*National Research Council, Safe Drinking Water Committee, National Academy Press, Washington, D.C. Drinking Water and Health. Volume 3. 1980.
*National Toxicology Program. 1993. "Toxicology and Carcinogenesis Studies of Mercuric Chloride (CAS No. 7487-94-7) in F344/N Rats and B6C3F1 Mice (gavage studies)." U.S. Department of Health and Human Services, Public Health Service, National Institutes of Health, Research Triangle Park, North Carolina. NTP TR 408, NIH Publication No. 91-3139.
Public Health Assessment Guidance Manual. US DHHS, Public Health Service; Atlanta, GA. March, 1992.
Salvato JA. 1992. Environmental Engineering and Sanitation. 4th edition. Chapter 3: Water Supply.
*Simon CH, Manzke HK and GM. 1964. "Occurrence, Pathogenesis, and Possible Prophylaxis of Nitrite Induced Methemoglobinemia. Zeitschr. Kinderheilk. 91:124-138. (German)
*Webster RC, Mobayen M, Maibach HI. 1987. "In Vivo and In Vitro Absorption and Binding to Powdered Stratum Corneum as Methods to Evaluate Skin Absorption of Environmental Chemical Contaminants from Ground and Surface Water." J. Toxicol. Eviron. Health 21:367-374.
ATSDR comparison values (CVs) are media-specific concentrations that are considered to be "safe" under default conditions of exposure. They are used as screening values in selecting site-specific chemicals for further evaluation of their public health implications. Generally, a chemical is selected for further public health evaluation because its maximum concentration in air, water, or soil at the site exceeds at least one of ATSDR's CVs. This approach is conservative by design. ATSDR may also select detected chemical substances for further public health evaluation and discussion because ATSDR has no CVs or because the community has expressed special concern about the substance, whether it exceeds CVs or not.
It cannot be emphasized strongly enough that CVs are not thresholds of toxicity. While concentrations at or below the relevant CV are generally considered to be safe, it does not automatically follow that any environmental concentration that exceeds a CV would be expected to produce adverse health effects. In fact, the whole purpose behind highly conservative, health-based standards and guidelines is to enable health professionals to recognize and resolve potential public health problems before they become actual health hazards. For that reason, ATSDR's CVs are typically designed to be 1 to 3 orders of magnitude lower (i.e., 10 to 1,000 times lower) than the corresponding no-effect levels or lowest-effect levels on which they are based. The probability that adverse health outcomes will actually occur depends, not on environmental concentrations alone, but on several additional factors, including site-specific conditions of exposure, and individual lifestyle and genetic factors that affect the route, magnitude, and duration of actual exposures.
Listed below are the abbreviations for selected CVs and units of measure used within this document. Following this list of abbreviations are more complete descriptions of the various comparison values used within this document, as well as a brief discussion on one of ATSDR's most conservative CVs.
CREG = Cancer Risk Evaluation Guide EMEG = Environmental Media Evaluation Guide LTHA = Drinking Water Lifetime Health Advisory MCL = Maximum Contaminant Level MCLA = Maximum Contaminant Level Action. MRL = Minimal Risk Level RBC = Risk-Based Concentration RfD = Reference Dose RMEG = Reference Dose Media Evaluation Guide
Units of Measure: ppm = Parts Per Million [e.g., mg/L (water), mg/kg (soil)] ppb = Parts Per Billion [e.g., µg/L (water), µg/kg (soil)] kg = kilogram (1,000 grams) mg = milligram (0.001 gram) µg = microgram (0.000001 gram) L = liter (1000 mL or 1.057 quarts of liquid, or 0.001 m3 of air) m3 = cubic meter (a volume of air equal to 1,000 liters)
Cancer Risk Evaluation Guides (CREGs) are derived by ATSDR. They are estimated chemical concentrations theoretically expected to cause no more than one excess cancer in a million people exposed over a lifetime. CREGs are derived from EPA's cancer slope factors and therefore reflect estimates of risk based on the assumption of zero threshold and lifetime exposure. Such estimates are necessarily hypothetical for, as stated in EPA's 1986 Guidelines for Carcinogenic Risk Assessment, "the true value of the risk is unknown and may be as low as zero."
Drinking Water Equivalent Levels (DWELs) are lifetime exposure levels specific for drinking water (assuming that all exposure is from that medium) at which adverse, noncarcinogenic health effects would not be expected to occur. They are derived from EPAs RfDs by factoring in default ingestion rates and body weights to convert the RfD dose to an equivalent concentration in drinking water.
Minimal Risk Levels (MRLs) are ATSDR's estimates of daily human exposure to a chemical that are unlikely to be associated with any appreciable risk of deleterious noncancer effects over a specified duration of exposure. MRLs are calculated using data from human and animal studies and are reported for acute (< 14 days), intermediate (15-364 days), and chronic (> 365 days) exposures. MRLs for oral exposure (i.e., ingestion) are doses and are typically expressed in mg/kg/day. Inhalation MRLs are concentrations and are typically expressed in either parts per billion (ppb) or ug/m3. The latter are identical to ATSDR's EMEGs for airborne contaminants. ATSDR's MRLs are published in ATSDR Toxicological Profiles for specific chemicals.
Environmental Media Evaluation Guides (EMEGs) are media-specific concentrations that are calculated from ATSDR's Minimal Risk Levels by factoring in default body weights and ingestion rates. Different EMEGs are calculated for adults and children, as well as for acute (<14 days), intermediate (15-364 days), and chronic (365 days) exposures.
EPA's Reference Dose (RfD) is an estimate of the daily exposure to a contaminant unlikely to cause any non-carcinogenic adverse health effects over a lifetime of chronic exposure. Like ATSDR's MRL, EPA's RfD is a dose and is typically expressed in mg/kg/day.
Reference Dose Media Evaluation Guide (RMEG) is the concentration of a contaminant in air, water, or soil that ATSDR derives from EPA's RfD for that contaminant by factoring in default values for body weight and intake rate. RMEGs are calculated for adults and children. RMEGs are analogous to ATSDR's EMEGs.
Risk-Based Concentrations (RBCs) are media-specific values derived by the Region III Office of the Environmental Protection Agency from EPA's RfDs, RfCs, or cancer slope factors, by factoring in default values for body weight, exposure duration, and ingestion/inhalation rates. These values represent levels of chemicals in air, water, soil, and fish that are considered safe over a lifetime of exposure. RBCs are calculated for adults and children. RBCs for noncarcinogens and carcinogens are analogous to ATSDR's EMEGs and CREGs, respectively.
Lifetime Health Advisories (LTHAs) are calculated from the DWEL (Drinking Water Equivalent Level) and represent the concentration of a substance in drinking water estimated to have negligible deleterious effects in humans over a lifetime of 70 years, assuming 2 L/day water consumption for a 70-kg adult, and taking into account other sources of exposure. In the absence of chemical-specific data, LTHAs for noncarcinogenic organic and inorganic compounds are 20% and 10%, respectively, of the corresponding DWELs. LTHAs are not derived for compounds which are potentially carcinogenic for humans.
Maximum Contaminant Levels (MCLs) are drinking water standards promulgated by the EPA. They represent levels of substances in drinking water that EPA deems protective of public health over a lifetime (70 years) at an adult exposure rate of 2 liters of water per day. They differ from other protective comparison values in that they are legally-enforceable and take into account the availability and economics of water treatment technology.
Maximum Contaminant Level Action (MCLA) are action levels for drinking water set by EPA under Superfund. When the relevant action level is exceeded, a regulatory response is triggered.
When screening individual chemical substances, ATSDR staff compare the highest single concentration of a chemical detected at the site with the lowest comparison value available for the most sensitive of the potentially exposed individuals (usually children). Typically the cancer risk evaluation guide (CREG) or chronic environmental media evaluation guide (cEMEG) is used. This "worst-case" approach introduces a high degree of conservatism into the analysis and often results in the selection of many chemical substances for further public health evaluation that will not, upon closer scrutiny, be judged to pose any hazard to human health. However, in the interest of public health, it is more prudent to use an environmental screen that identifies many chemicals for further evaluation that may be determined later to be "harmless," as opposed to one that may overlook even a single potential hazard to public health. The reader should keep in mind the conservativeness of this approach when interpreting ATSDR's analysis of the potential health implications of site-specific exposures.
As ATSDR's most conservative comparison value, the CREG, deserves special mention. ATSDR's CREG is a media-specific contaminant concentration derived from the chronic (essentially, lifetime) dose of that substance which, according to an EPA estimate, corresponds to a 1-in-1,000,000 cancer risk level. Note, this does not mean that exposures equivalent to the CREG actually are expected to cause 1 excess cancer case in 1,000,000 people exposed over a lifetime. Nor does it mean that every person in that exposed population has a 1-in-1,000,000 risk (i.e., 1x10-6) of developing cancer from the specified exposure. Although commonly misinterpreted in precisely this way, cancer risk assessment methodology can only provide conservative estimates of population risk which do not, in fact, apply to any particular individual. Even for populations, cancer risk estimates do not necessarily constitute realistic predictions of the risk. As EPA stated in its Guidelines for carcinogen Risk Assessment, "the true value of the risk is unknown and may be as low as zero" [EPA 1986].
Unlike non-cancer comparison values which correspond to "safe" levels that include specified margins of safety, ATSDR's CREGs (and the risk estimates on which they are based) correspond to purely hypothetical (and unmeasurable) 1-in-a-million cancer risk levels that include unspecified margins of safety (i.e., relative to the lowest known cancer effect levels) which often range from thousands to millions or more. In the U.S., these hypothetical risk levels are based on the zero-threshold assumption according to which any non-zero dose of a carcinogen must be associated with some finite increment of risk, however small. Using linear models based on this assumption, it is actually possible to "quantify" undetectable/non-existent cancer risks that are (hypothetically) associated with even immeasurably small doses. EPA uses such risk estimates as regulatory tools in, for example, the ranking of contaminated sites for cleanup. ATSDR uses them as screening values. However, once ATSDR has screened a substance and selected it for further evaluation, the CREG, like all other screening values, becomes irrelevant in subsequent stages of analysis. Further evaluation of the public health implications of site-specific exposures must, necessarily, be based on the best medical and toxicologic information available [PHAGM 1992].
References
(**)Public health Assessment Guidance Manual. U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Atlanta, GA 30333, March 1992.
**Environmental Protection Agency. Guidelines for Carcinogenic Risk Assessment. Fed. Reg., 51: 33997-33998, September 24, 1986.
Williams, Gary M., and Weisburger, John H. 1991. "Chemical Carcinogenesis". Chapter 5 in: Casarett and Doull's TOXICOLOGY: The Basic Science of Poisons. (Mary O Amdur, John Doull, and Curtis Klaassen, Editors.) Pergamon Press pp 127-200. [See section entitled "Quantitative Aspects of Carcinogenesis," pp152-155.
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