Negligence risk assessment

The cancer risk equation described below estimates tlie incremental individual lifetime cancer risk for simultaneous exposure to several carcinogens and is based on EPA s risk assessment guidelines. Tliis equation represents an approximation of the precise equation for combining risks wliich accounts for tlie joint probabilities of tlie same individual developing cancer as a consequence of exposure to two or more carcinogens. The difference between tlie precise equation and tlie approximation described is negligible for total cancer risks less tlian 0.1. Thus, tlie simple additive equation is appropriate for most risk assessments. The cancer risk equation for multiple substances is given by ... 

Develop models for scenarios and acquire data. The release and exposure scenarios described above are evaluated through modeling. The models embody the mathematical interrelationships of the possible steps in each scenario. Simplifications and approximations usually are introduced to reflect limitations in knowledge and data or the results of previous risk assessments that show certain scenarios and pathways to be negligible. The result often is a series of models describing (1) degradation of... 

In many respects, the foundations and framework of the proposed risk-based hazardous waste classification system and the recommended approaches to implementation are intended to be neutral in regard to the degree of conservatism in protecting public health. With respect to calculations of risk or dose in the numerator of the risk index, important examples include (1) the recommendation that best estimates (MLEs) of probability coefficients for stochastic responses should be used for all substances that cause stochastic responses in classifying waste, rather than upper bounds (UCLs) as normally used in risk assessments for chemicals that induce stochastic effects, and (2) the recommended approach to estimating threshold doses of substances that induce deterministic effects in humans based on lower confidence limits of benchmark doses obtained from studies in humans or animals. Similarly, NCRP believes that the allowable (negligible or acceptable) risks or doses in the denominator of the risk index should be consistent with values used in health protection of the public in other routine exposure situations. NCRP does not believe that the allowable risks or doses assumed for purposes of waste classification should include margins of safety that are not applied in other situations. 

When risks are very low, the subtracted term is so small that its impact on Rm is negligible (e.g., for r, = 0.01 and r2= 0.02, Rm = 0.01 + 0.02 - 0.0002 = 0.0298, or -0.03) thus, low risks such as those typically found in environmental risk assessments can be approximated by simple summation. Effects addition, a concept rarely applied, is a special case of RA where the biological measurements are summed across the mixture components and then a judgment is made regarding potential adverse effects based on the total measurement. Finally, when a mixture contains components with more than 1 MOA that cause the same health outcome, CA and RA methods can be integrated to assess risk (USEPA 2003b). See also Chapter 4. 

Integral to the tiered approach is the concept of negligible exposure . Many jurisdictions will waive the requirement for a quantitative exposure and risk assessment, on a case-by-case basis, citing negligible exposure. Clear guidance on what constitutes negligible exposure is not available and a harmonized set of criteria for making this determination is required. 

The PCAPP transportation risk assessment for hydrolysate (PMACWA, 2003) quantified the risks of heavy truck accidents independent of the hydrolysate cargo. The report concluded that the risks from a hydrolysate spill would be negligible because of the low volatility. Either that assessment was a qualitative one, or the quantitative supporting analysis was not provided. It is important to provide quantitative data to calm the anxiety that can be triggered by the prospect of offsite transportation. 

In certain cases, the FDA has applied a negligible risk concept for food additives. This is demonstrated in the case of dimethyl dicarbamate, a yeast inhibitor for use in beverages (FDA 2000). The additive evenmaUy decomposes to methanol and carbon dioxide, but in the presence of ammonium ions (not uncommon in certain beverages) a carcinogenic chemical may also be formed in small amounts. The FDA used formal quantitative risk assessment procedures to estimate the upper-bound limit of carcinogenic risk to humans posed by urethane generated by decomposition of the additive. It was concluded that the potential risk was sufficiently low that the additive would be safe for the requested use, and the FDA s final rule approved its use (56 FR 40502 1988). 

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