LETTERS Setting health risk benchmarks Dear Editor: Two recent ES&T articles contained misstatements and errors regarding risk assessment guidelines and data developed by U.S. agencies. The Policy Analysis article titled "Comparison of Hazardous Air Pollutants Health Risk Benchmarks" (Dec. 1997, pp. 568A573A) misstated the Agency for Toxic Substances and Disease Registry's (ATSDR) minimal risk levels (MRLs) and presented a message that is inconsistent widi the extensive coordination and collaboration that exist between government agencies. For example, the last two sentences in the abstract are inaccurate and misleading. The author states that "Discrepancies arise because of different uncertainty factors and differences in the selection and interpretation of studies used to assess healdi risks. Government agencies should seek to further harmonize risk assessment guidelines, data, and interpretations." Representation from EPA, the National Institute of Environmental Health Sciences (NIEHS), and ATSDR on health guidance review committees has existed for over five years and has served a critical, positive role in minimizing potential conflicting conclusions regarding health guidance issues. The MRL values reported by the authors for manganese (0.003 mg/ m3) and inorganic mercury (0.000014 mg/m3) in Table 1 are not the most current values. The current values are 0.00004 mg/m 3 and 0.0002 mg/m3, respectively. In addition, the MRL reported in Table 1 for hexamethylene-1 -6-diisocyanate (0.0007 mg/m3) was incorrecdy calculated by the authors; the correct value is 0.00007 mg/m3. Contrary to the implications of this article, ATSDR, EPA, and other agencies have collaborated extensively to harmonize risk assessment approaches. As a result, the methods
used and conclusions drawn by the agencies are remarkably consistent and complementary. For example, as reported at the 1995 Society of Toxicology annual meeting, based on a review of 84 MRLs and 345 reference doses (RfD), both chronic oral RfDs and chronic oral MRLs were available for 21 substances. For 14 of the 21 substances, the chronic oral RfD and chronic oral MRL were the same value; for the remaining seven substances, the values fell within an order of magnitude of each other. In most instances, these differences arose due to the availability of more recent data sets on which to base health guidance values. ATSDR and EPA continue to work together with other agencies to provide appropriate health guidance values based on the most current data. Coordination in the development of health guidance values between federal agencies is an ongoing process mat serves a critical role in the interest of public health. In the news article "Mercury Health Standard Eased by U.S. Agency" (Jan. 1, 1998, p. 8A), ATSDR's MRL for methylmercury (MeHg) (0.5 pg/kg/day) in the draft toxicological profile is characterized as "by far the least restrictive of any U.S. federal agency." In fact, ATSDR's draft chronic oral MRL for MeHg is essentially the same as the World Health Organization's acceptable daily intake (ADI) level of 0.47 ug/ kg/day [Science, 278, 1997, pp. 19041905) and the U.S. Food and Drug Administration's ADI of 30 pg/day (0.5 pg/kg/day for a 60-kg female), which was used to derive the FDA action level of 1 ppm in fish, shellfish, crustaceans, and other aquatic animals. ATSDR is currently reviewing comments received on its draft profile on mercury and will release its final MRLs later this year. It should also be noted that EPA's RfD for MeHg is 0.1 pg/kg/day and not 0.01 pg/kg/day as stated in this article.
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It is our hope that this communication will serve to reduce the potential for misinterpretation of significant efforts and foster cooperation and collaboration among federal agencies. CHRISTOPHER T. DE ROSA Agency for Toxic Substances and Disease Registry Atlanta, GA 30333 Response De Rosa is correct in stating that the MRL values for manganese and inorganic mercury presented in our article are not the current ATSDR values. MRL values presented in Table 1 were found at the ATSDR Web site (http://atsdrl.atsdr.cdc.gov:8080/ mrls.html) and were those publicly available at the time we prepared the article in March 1997. These two values have since been replaced with the values presented by De Rosa. In addition, the MRL reported in Table 1 for hexamethylene-1,6-diisocyanate was incorrecdy stated in the article, widi die correct value being 0.00007 mg/m3. SUSAN B. GOLDHABER and ROBERT L. CHESSIN Research Triangle Institute Research Triangle Park, NC 27709
Linking monitoring to modeling Dear Editor: In his editorial tided "Too Litde Data, Too Many Models" (May 1,1998, p. 207A), William Glaze cautions that in recent years there has been an overreliance on modeling in environmental decision making and that more monitoring (data) are needed to restore good science principles. The National Council of the Paper Industry for Air and Stream Improvement, Inc. (NCASI), agrees with this view and offers over 20 years of environmental modeling research as support. Our study on the Ouachita River in die early 1980s gave rise to what is now the QUAL2E model. In es-
