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1. ATSDR > Public Health Assessments & Consultations

Health comparison values for ATSDR public health assessments are contaminant concentrations that are found in specific media (air, soil, and water) and that are used to select contaminants for further evaluation. The values provide guidelines that are used to estimate a dose at which health effects might be observed. Health comparison values used in the Environmental Contamination and Other Hazards and the Public Health Implications sections of this public health assessment are listed and described below.

Environmental Media Evaluation Guides (EMEGs) are estimated contaminant concentrations in media where there is no chance for noncarcinogenic health effects to occur. The EMEG is derived from U.S. Agency for Toxic Substances and Disease Registry's (ATSDR) minimal risk level (MRL).

Remedial Media Evaluation Guides (RMEGs) are estimated contaminant concentrations in media where there is no chance for noncarcinogenic health effects to occur. The RMEG is derived from U.S. Environmental Protection Agency's (EPA) reference dose.

Cancer Risk Evaluation Guides (CREGs) are estimated contaminant concentrations that would be expected to cause no more than one excess cancer in a million (10E-6) persons exposed over a lifetime. CREGs are calculated from EPA's cancer slope factors.

Calculation of Exposure Dose from Ingestion of Soil

The exposure doses for soil ingestion were calculated in the following manner. For residents, the maximum concentration for a contaminant was multiplied by the soil ingestion rate for adults, 0.0001 Kg/day; children, 0.0002 Kg/day, or pica children, 0.005 Kg/day. (The habit of ingesting large amounts of soil is called pica.) This product was divided by the average weight for an adult, 70 Kg (154 pounds) or for a child, 10 Kg (22 pounds). For workers, a soil ingestion rate of 0.00005 Kg/day was used. Those calculations assume that there is frequent daily exposure to soil contaminated at the maximum level. A qualitative summary of these results can be found in Table 4 on page 24. The results of the actual calculations are recorded in Table C which is on the following page.

Calculation of Risk of Carcinogenic Effects

Carcinogenic risks from the ingestion of soil were calculated using the following procedure. The exposure doses for ingestion of soil calculated as described previously, were multiplied by the EPA's Cancer Slope Factor for the contaminants of concern (25). For residents, the results represent the maximum risk for cancer after 70 years of exposure to the maximum concentration of the contaminant. For workers, 30 years of exposure is assumed. A cancer slope factor was available only for arsenic. The results of the calculation of carcinogenic risk from exposure to arsenic can be found on Table C which is on the following page. The results are discussed in the Toxicological Evaluation Section.

The actual risk of cancer is probably lower than the calculated number. The method used to calculate EPA's Cancer Slope Factor assumes that high dose animal data can be used to estimate the risk for low dose exposures in humans (42). The method also assumes that there is no safe level for exposure (43). There is little experimental evidence to confirm or refute those two assumptions. Lastly, the method computes the 95% upper bound for the risk, rather the average risk, which results in there being a very good chance that the risk is actually lower, perhaps several orders of magnitude (44).

The public health assessment was available for public review and comment in the Murray library from October 10 to November 29, 1996. The public comment period was announced in local newspapers. In addition, the public health assessment was sent to several individuals, federal, state, and local agencies, and ASARCO. Comments were received only from a consultant for ASARCO, PTI Environmental Services. Those comments and our responses are summarized below.

COMMENT:

The commenter did not agree with ATSDR's assumption on page 27 that ingested arsenic in soil is absorbed as readily as arsenic dissolved in water. Several references were cited that indicate that soil arsenic is less available than arsenic in water.

Response:

The public health assessment was revised to indicate that there is evidence that soil arsenic is less available than arsenic in water.

COMMENT:

The commenter did not agree with the statement on page 27 of the public health assessment that pica children could experience proteinuria from ingesting cadmium-contaminated soil. The commenter concluded that the proteinuria observed in a long-term study would not occur in children because they would not be exposed long enough. The commenter asserted that the ingestion rate cited by ATSDR for pica is too high because that condition is a rare and transient condition.

Response:

We have revised the public health assessment to better alert the reader to these issues. However, whenever there is uncertainty in a toxicological evaluation, ATSDR makes conservative decision to insure that public health is protected. The studies which indicate that many years of exposure to low levels of cadmium will lead to increased occurrence of proteinuria, do not rule out the possibility that children may experience that same condition after a few years of exposure. Therefore, we mention proteinuria as a possible effect. Likewise, while pica is often transient, it is not always.

COMMENT:

The commenter criticized the inclusion in the public health assessment of 15-20 year-old blood lead studies that indicate blood lead levels increase 4-5 µg/dL for every 1000 ppm increase in soil lead concentrations. The commenter suggests that more recent studies, indicating that there is a much lower increase in blood lead levels per 1000 ppm increase in soil lead concentration, should be cited instead.

Response:

In the public health assessment, it is stated, "Blood lead testing was done because there is good evidence from three literature reviews that children's blood lead levels could have been elevated because of soil lead contamination." As the commenter indicates, there are many studies, both recent and in the past, identifying small increases in blood lead. However, the three literature reviews were cited because they provide valid data that blood lead levels of health concern are possible at the soil lead concentrations found in the Murray Smelter area. Because elevated levels were possible, blood lead testing was conducted. This testing indicated that blood lead levels were not elevated and ATSDR used the results as the basis to conclude that the site was not a current public health hazard.

Agency for Toxic Substances and Disease Registry, 1825 Century Blvd, Atlanta, GA 30345
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