| Fundamental Flaws of Hormesis for Public Health Decisions Kristina A. Thayer,1 Ronald Melnick,1 Kathy Burns,2 Devra Davis,3 and James Huff1 1National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, USA; 2Sciencecorps.org, Lexington, Massachusetts, USA; 3H. John Heinz III School of Public Policy & Management, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA Abstract
Hormesis (defined operationally as low-dose stimulation, high-dose inhibition) is often used to promote the notion that while high-level exposures to toxic chemicals could be detrimental to human health, low-level exposures would be beneficial. Some proponents claim hormesis is an adaptive, generalizable phenomenon and argue that the default assumption for risk assessments should be that toxic chemicals induce stimulatory (i.e., "beneficial") effects at low exposures. In many cases, nonmonotonic dose-response curves are called hormetic responses even in the absence of any mechanistic characterization of that response. Use of the term "hormesis," with its associated descriptors, distracts from the broader and more important questions regarding the frequency and interpretation of nonmonotonic dose responses in biological systems. A better understanding of the biological basis and consequences of nonmonotonic dose-response curves is warranted for evaluating human health risks. The assumption that hormesis is generally adaptive is an oversimplification of complex biological processes. Even if certain low-dose effects were sometimes considered beneficial, this should not influence regulatory decisions to allow increased environmental exposures to toxic and carcinogenic agents, given factors such as interindividual differences in susceptibility and multiplicity in exposures. In this commentary we evaluate the hormesis hypothesis and potential adverse consequences of incorporating low-dose beneficial effects into public health decisions. Key words: biphasic dose response, hormesis, individual susceptibility, low-dose exposures, nonmonotonic dose response, nonlinear dose response, public health, regulation, risk assessment. Environ Health Perspect 113: 1271-1276 (2005) . doi:10.1289/ehp.7811 available via http://dx.doi.org/ [Online 15 June 2005]
Address correspondence to K. Thayer, NIEHS, MD A3-01, PO Box 12233, Research Triangle Park, NC 27709 USA. Telephone: (919) 541-5021. Fax: (919) 541-0295. E-mail: thayer@niehs.nih.gov We thank N. Walker and S. Taylor for their thoughtful review of the manuscript. The authors declare they have no competing financial interests. Received 1 December 2004 ; accepted 14 June 2005.
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