US EPA in their non-cancer health assessments. In this context, a RfD is not itself an action level, nor does it establish an acceptable daily dose. Their primary use is to evaluate increments of exposure from specific sources above background when background exposures are low and insignificant. Reference material (RM) Material or substance, one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials [ISO Guide 30[43]]. NOTE: A reference material may be in the form of a pure or mixed gas, liquid or solid. EXAMPLES: Several kinds of reference materials exist. An internal reference material is an RM developed by a user for its own internal use. An external reference material is an RM provided by someone other than the user. A certified reference material (CRM) is an RM issued and certified by an organisation recognised as competent to do so. Reference measuring system Represents not only a measuring instrument but the set of procedures, operators and environmental conditions associated with that instrument. Reference Standard Standard generally having the highest metrological quality available at a given location or in a given organisation, from which measurements made there are derived [VIM] [45]. Reference toxicant A standard chemical used to measure the sensitivity of the test organisms in order to establish confidence in the toxicity data obtained from the test material. In most instances a toxicity test with