Drinking-water s t a n k & Risks for chemicals and radionuclides
By Nancy B. Munro and Curtis C. Travis EPA is in the process of revising the primary and secondary drinking-water regulations in accordance with the Safe Drinking Water Act. Rimary drinkingwater standards consist of two values for each contaminant: a nonenforceable recommended contaminant level (RMCLI. .. which is based on health considerations only, and a maximum contaminant level (MCL), which EPA enforces. The latter is to be. set as close to the RMCL as possible and takes feasibility and cost into account, as well as health protection. Consideration of risk is playing an increasing role in setting environmental
standards, including those for drinking water. One rationale for such a move is mreased consistency across standards. Analysis of the proposed standards indicates that consistency has not yet been achieved. Questions have been raised regarding the adequacy of human health protection afforded by some of the existing and prowsed standards.
Nancy B. Munm
careinogenie risk Much discussion has taken place in
Curtis C. Travis
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recent years about what constihltes a p e r a l l y acceptable level of individual risk in our society. A consensus exists for substances to which large numbers of people are exposed involuntarily via medii such as food and water. Most regnlatory sources place this level at 1V6,meaning one chance in one million that a chronically exposed individual will develop cancer. EPA targets 1V5to 1P6 for drinking water. Table 1 contains estimated excess individual lifetime cancer risks from drinking 2 L daily of water that contains each contaminant at a level equal to its present or proposed MCL or RMCL. The excess lifetime cancer risks associated with these. standards vary over 4 orders of magnitude, from 2 x lVZto2 x 10-6. Contaminants with individual lifetime cancer risks of less than 1V6 a p pear to be adequately regulated. Past experience indicates that when individual lifetime cancer risks are in the range of 10-6 to W, population risks (cancers per year in the exposed population) and economic cost-effectiveness calculations are the primary determinants of regulation. Several chemicals in lhble 1 fall in this range, and it would be useful in fuhlre decisions on drinking-water standards to have sufficient data to dewmine population risk. There are several substances for
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which the level of risk from drinking 2 Uday of water containing the contaminant at the MCL would be greater than lo4. These include arsenic (2 X W), 1.1-dichloroethylene (I x toxaphene (2 x and trihalomethanes (THMs) (5 X le4). These numbers appear high, but it is not clear that they all pose unacceptable hazards. The lifetime cancer risk level associated with both the present MCL and the proposed RMCL for arsenic (a known carcinogen) is extremely high. A review of the Federal Register indicates that EPA has never failed to regulate a substance identified as having an individual lifetime risk greater than 4 x However, several factors must be taken into consideration in evaluating this risk. Arsenic is a widely distributed, naturaUy occurring contaminant. The type of cancer caused by ingestion of arsenic is readily curable if detected early but may not be later. Also, arsenic at low levels may be an essential nutrient for humans, although this has not yet been proven. A comparison with past EPA regulatory actions indicates that the decision on arsenic may be an anomaly. Given the high individual risk levels associated with the arsenic standards and the fact that approximately 112,000 people are receiving water from public water supplies with arsenic levels above the
current MCL of 0.05 mg/L, EPA should consider total population risk in determining if the proposed standards are adequate. Because 1, I-dichloroethylene is a possible carcinogen, the proposed MCL was actually based on the acceptable daily intake (ADD with an additional safety factor of 10. The interim MCL (0.005 mg/L) for toxaphene, a known carcinogen, may be revised in the next stage of developing and updating standards. The proposed RMCL is zero; whether the MCL will be revised downward depends largely on the practical quantitation limit and available treatment techniques for this substance. lkihalomethanes are reaction products resulting from the chlorination process that is used to disinfect water supplies. In setting standards, EPA has attempted to strike a balance between the risks attendant on inadequate disinfection of water and the potential cancer risks associated with the resulting THM content of drinking water. Noncarcinogenic risk The intakes of noncarcinogenic compounds at levels equivalent to the proposed drinking water standards range from1.2Wto100WoftheADI.Where the intake is equal to or less than the ADI, the risk to health should be acceptable if the extent of exposure from
sources other than drinking water is not greater than that assumed in calculating the standards. The risks associated with the compounds having ADIs appear to be reasonable, but the risk associated with lead, for which there is no currently agreed-on ADI, warrants additioni adysis. The orowsed RMCL of 0.02 mnlL for lea2 is’based on a level of concern identified by EPA of 15 pg/dL of lead in blood for infants. EPA has stated that it is the intent of the Safe Drinking Water Act that if there is no safe threshold for a contaminant, the RMCL should be set at zero. Increasing evidence shows that lead has a number of adverse effects that either do not have thresholds or whose thresholds are lower than 15 pg/dL. These effects include reduced birth weight, early neurobehavioral deficiencies, increases in minor congenital anomalies, and reduced growth in young children. The evidence for the first two types of effects comes from current longitudinal studies of infants and young children beginning with prenatal exposure. These and other studies are starting to produce evidence of deleterious effects of lead at levels that even recently were thought not harmful to health.
Prognosis Wide variations exist in the individual lifetime carcinogenic risk associated with the primary drinking-water standards. The risks associated with several contaminantsappear to be high, but others may be regulated more stringently than necessary from a health standpoint. EPA could place the drinking-water standards on a more rational basis by including both individual risk and population risk in its regulatory decision framework. From the standpoint of noncarcinogenic risk, the proposed RMCL for lead is liiely not to allow an MCL to be set low enough to protect human health adequately. An RMCL set at zero would allow more tlexibility in revising the MCL. Nancy B. Munro is a research staff member working with the @ice of Risk Analysis of the Oak Ridge National Labaratory in Oak Ridge, Rnn. She has a Ph. D. in mMMnlian physiologyfrom the University of Kentucky at Lexington and has been working in risk analysisfor four years. She is interested in the interface between risk analysis and publicpolicy C u d C. %vis is director of the Ofice of Risk Analysis at the Oak Ridge National Labomtory. He has a Ph.D. in mathematics from the Universiry of California at Davis, har been involved in thejeld of risk analysis for 10 years, and is editor of the journal, Risk Analysis. Envimn. Sci. Technol., Vol. 20, No. 8. 1986 769
