Quantitative Risk Noncarcinogens

The measure used to describe the potential for noncarcinogenic toxicity to occur in an individual is not expressed as tlie probability of an individual suffering an adverse effect. The EPA does not at tlie present time use a probabilistic approach to estimate tlie potential for noncarcinogenic healtli effects. Instead, tlie potential for non carcinogenic effects is evaluated by comparing an exposure level over a specified time period (e.g., lifetime) witli a reference dose derived for a similar exposure period. Tliis ratio of exposure to toxicity is called a liazard quotient and is described below. (The reader is referred to Chapter 11 for additional details on tlie material tliat follows). The noncancer liazard quotient assumes tliat tliere is a level of exposure (i.e., RfD) below which it is unlikely for even sensitive populations to experience adverse healtli effects. 

RfD = reference dose E and Rfd are expressed in tlie same units 

If the exposure level (E) exceeds tliis tlireshold (i.e., E/RfD exceeds unity), tliere may be concern for potential noncancer effects. As a rule, tlie greater tlie value of E/RfD above unity, tlie greater tlie level of concern. However, one should not interpret ratios of E/RfD as statistical probabilities a ratio of 0.001 does not mean tliat tliere is a one in one tliousand cliance of the effect occurring. Furtlier, it is important to empliasize tliat tlie level of concern does not increase linearly as tlie RfD is approached or exceeded because RfDs do not have equal accuracy or precision and are not based on tlie same severity of toxic effects. Thus, tlie slopes of the dose-response curv e in excess of the RfD can range widely depending on tlie substance. 

To assess tlie overall potential for noncarcinogenic effects posed by more dian one chemical, a liazard index (HI) approach has been developed based on EPA s Guidelines for Healdi Risk Assessment of Chemical Mixtures. This approach assumes that simultaneous subtlu eshold exposures to several chemicals could result in an adverse healtli effect. It also assumes tliat tlie magnitude of the adverse effect will be proportional to tlie sum of the ratios of the subtlireshold exposures to acceptable exposures. The non cancer hazard index is equal to tlie sum of the hazard quotients, as described below, where E and tlie RfD represent the same exposure period (e.g., subclironic, clironic, or shorter-term). 

It should be noted diat E and RfD are expressed in die same units and represent die same exposure period (i.e., clironic. subclironic, or shorter term) 

When tlie hazard index exceeds miity, tliere may be concern for potential health effects. While any single chemical with an exposure level greater than the toxicity value will cause tlie hazard index to e.xceed unity, tlie reader should note tliat for multiple chemical exposures, tlie hazard index can also exceed unity even if no single chemical exposure exceeds its RfD. 

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Overview of Oral Health Effects. Studies in humans and animals indicate that ingestion of asbestos causes little or no risk of noncarcinogenic injury. However, there is some evidence that acute oral exposure may induce precursor lesions of colon cancer, and that chronic oral exposure may lead to an increased incidence risk of gastrointestinal tumors. Studies that provide quantitative data on the effects of ingested asbestos are summarized in Table 3-3 and Figure 3-3, and the data are discussed below. 

The value for risk is the quantitative end point determined in risk assessment calculations it is commonly used in regulatory and management decisions regarding hazardous wastes. As with noncarcinogens, the risk term is calculated for each contaminant, each route of exposure, and for all sets of receptor populations each element of risk is then summed to provide the value of cumulative risk. 

Carlson-Lynch H, Price PS, Swartout JC, Dourson ML, and Keenan RE (1999) Application of quantitative information on the uncertainty in the R to noncarcinogenic risk assessments. HERA 5(3) 527-547. 

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