Principles of risk assessment for determining the safety of chemicals: recent assessment of residual solvents in drugs and di(2-ethylhexyl) phthalate

Risk assessment of chemicals is essential for the estimation of chemical safety, and animal toxicity data are typically used in the evaluation process, which consists of hazard identification, dose-response assessment, exposure assessment, and risk characterization. Hazard identification entails the collection of all available toxicity data and assessment of toxicity endpoints based on findings for repeated dose toxicity, carcinogenicity or genotoxicity and species-specificity. Once a review is compiled, the allowable lifetime exposure level of a chemical is estimated from a dose-response assessment based on several measures. For non-carcinogens and non-genotoxic carcinogens, the no-observed-adverse-effect-level (NOAEL) is divided by uncertainty factors (e.g. with environmental pollutants) or safety factors (e.g. with food additives) to derive a tolerable daily intake (TDI) or acceptable daily intake (ADI), respectively. These factors include interspecies and individual differences, duration of exposure, quality of data, and nature of toxicity such as carcinogenicity or neurotoxicity. For genotoxic carcinogens, low dose extrapolation is accomplished with mathematical modeling (e.g. linearized multistage model) from the point of departure to obtain exposure levels that will be associated with an excess lifetime cancer risk of a certain level. Data for levels of chemicals in food, water and air, are routinely used for exposure assessment. Finally, risk characterization is performed to ensure that the established 'safe' level of exposure exceeds the estimated level of actual exposure. These principles have led to the evaluation of several existing chemicals. To establish a guideline for residual solvents in medicine, the permitted daily exposure (PDE), equivalent to TDI, of N,N-dimethylformamide was derived on the basis of developmental toxicity (malformation) and of N-methylpyrrolidone on the basis of the developmental neurotoxicity. A TDI for di(2-ethylhexyl)phthalate was derived from assessment of testicular toxicity.