Regulatory Toxicology and Pharmacology 44 (2006) 111–124 www.elsevier.com/locate/yrtph 0273-2300/$ - see front matter  2006 Published by Elsevier Inc. doi:10.1016/j.yrtph.2005.10.003 A review of the reference dose for chlorpyrifos Qiyu Zhao ¤ , Michael Dourson, Bernard Gadagbui Toxicology Excellence for Risk Assessment (TERA), 2300 Montana Avenue, Suite 409, Cincinnati, OH 45211, USA Received 9 June 2005 Available online 15 December 2005 Abstract Chlorpyrifos is an inhibitor of cholinesterase (ChE) and inhibition of ChE is believed to be the most sensitive eVect in all animal spe- cies evaluated and in humans from previous evaluations. Recent literature, in particular epidemiology studies reporting associations between chlorpyrifos levels and fetal birth weight decreases, suggest the need to reevaluate the basis of the reference dose (RfD) for chlor- pyrifos, however. In this paper, we evaluated newly available publications regarding chlorpyrifos toxicity and discuss the choice of critical eVect—whether cholinesterase inhibition or developmental eVect, the choice of appropriate species and study, the appropriate point of departure, and choice of uncertainty factors—including a discussion of the FQPA safety factor. We conclude that RBC cholinesterase inhibition is the critical eVect, that human studies form the best choice of species—supported by a wealth of experimental animal data, that a NOAEL of 0.1 mg/kg/day is the most appropriate point of departure, and that a 10-fold factor for within human variability is suY- cient to characterize the overall uncertainty in this rather large database. The resulting RfD is 0.01 mg/kg/day.  2006 Published by Elsevier Inc. Keywords: Chlorpyrifos; Fetal development; ChE inhibition; Risk assessment; Reference Dose 1. Introduction Chlorpyrifos, an irreversible inhibitor of cholinesterase (ChE) including acetylcholine esterase (AChE), is one of the most widely used organophosphate insecticides in the US. SuYcient inhibition of AChE in the central and periph- eral nervous systems causes excessive accumulation of ace- tylcholine which in turn results in neurotoxicity in animals and humans. Inhibition of ChE is believed by many groups to be the most sensitive eVect in all animal species evaluated and in humans, regardless of route or duration of exposure (e.g., ATSDR, 1997; US EPA, 2000a; UK ACP, 2003). Recent developments in the epidemiology database of chlorpyrifos prompted a revisit of the information on this chemical’s overall toxicity and corresponding issues in the judgment of its critical eVect. For example, Whyatt et al. (2004) reported an association between umbilical cord plasma chlorpyrifos levels and fetal birth weight decreases among minority women living in New York City during pregnancy, and this Wnding raised a concern on whether impaired fetal development could be the critical eVect rather than the inhibition of AChE as had been believed so far. Zhao et al. (2005) investigated this association through an analysis of other epidemiology and experimental animal studies and showed that the positive association was not consistent with observations from other epidemiology investigations and was also not supported by data from experimental animal studies. SpeciWcally for the experimen- tal animal work, a direct comparison of neonatal informa- tion showed that cholinesterase inhibition was a more sensitive indicator of an eVect than reduced body weight, and that neonates were equally, or perhaps less, sensitive to cholinesterase inhibition than their maternal parent. Although other health organizations and investigators have concluded that cholinesterase inhibition is chlorpyri- fos’ critical eVect, not all of these positions have been informed by the recent epidemiology associations. For example, Table 1 summarizes critical eVects that were con- cluded and used by various international health organiza- tions as the basis for developing chlorpyrifos safe doses. Most of these groups feel that inhibition of AChE either in * Corresponding author. Fax: +1 513 542 7487. E-mail address: zhao@tera.org (Q. Zhao).