Toxicological Review for 2-Methylnaphthalene (CAS No. 91-57-6 ...

1. INTRODUCTION
This document presents background and justification for the hazard and dose-response
assessment summaries in EPA’s Integrated Risk Information System (IRIS). IRIS Summaries may
include an oral reference dose (RfD), inhalation reference concentration (RfC) and a carcinogenicity
assessment.
The RfD and RfC provide quantitative information for noncancer dose-response assessments.
The RfD is based on the assumption that thresholds exist for certain toxic effects such as cellular
necrosis but may not exist for other toxic effects such as some carcinogenic responses. It is expressed
in units of mg/kg-day. In general, the RfD is an estimate (with uncertainty spanning perhaps an order of
magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to
be without an appreciable risk of deleterious noncancer effects during a lifetime. The inhalation RfC is
analogous to the oral RfD, but provides a continuous inhalation exposure estimate. The inhalation RfC
considers toxic effects for both the respiratory system (portal-of-entry) and for effects peripheral to the
respiratory system (extrarespiratory or systemic effects). It is generally expressed in units of mg/m 3 .
The carcinogenicity assessment provides information on the carcinogenic hazard potential of the
substance in question and quantitative estimates of risk from oral exposure and inhalation exposure.
The information includes a weight-of-evidence judgment of the likelihood that the agent is a human
carcinogen and the conditions under which the carcinogenic effects may be expressed. Quantitative
risk estimates are presented in three ways. The slope factor is the result of application of a low-dose
extrapolation procedure and is presented as the risk per mg/kg/day. The unit risk is the quantitative
estimate in terms of either risk per µg/L drinking water or risk per µg/m 3 air breathed. Another form in
which risk is presented is a drinking water or air concentration providing cancer risks of 1 in 10,000; 1
in 100,000; or 1 in 1,000,000.
Development of these hazard identification and dose-response assessments for 2-
methylnaphthalene has followed the general guidelines for risk assessment as set forth by the National
Research Council (1983). EPA guidelines that were used in the development of this assessment may
include the following: Guidelines for the Health Risk Assessment of Chemical Mixtures (U.S. EPA,
1986a), Guidelines for Mutagenicity Risk Assessment (U.S. EPA, 1986b), Guidelines for
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