Uncertainty in health risk assessments

Human Exposure Assessment II: Quantifying and Reducing the Uncertainties

Alternative methods of human exposure assessment that reduce and/or allow quantification of the uncertainties associated with exposure estimates are surveyed and illustrated. These alternative approaches include (1) use of more appropriate exposure parameter default values rather than values that result in extreme exposure estimates; (2) incorporation of time-activity data to better define appropriate exposure duration values; (3) the use of reasonable exposure scenarios rather than the traditional Maximally Exposed Individual (MEI) approach; (4) the use of stochastic approaches such as Monte Carlo-based and information analysis-based methods; (5) use of bivariate analysis to identify the extent to which interdependencies between different exposure parameters affect the distribution of exposure estimates; (6) use of less-than-lifetime exposure and risk assessment; and (7) incorporation of physiological considerations relevant to absorbed dose estimation, including route-specific impacts, use of improved absorption factors, and application of pharmacokinetic models. Other ways to improve the exposure assessment process, including assuring statistical equivalency in comparing different exposure estimates and incorporation of sensitive subpopulation considerations are also discussed, as are key research needs.

Risk assessment in regulatory policy making for human and veterinary public health

Risk assessment is the method of systematically identifying and assessing factors that influence the probability and consequences of a negative event occurring. One responsibility of veterinary medicine is to protect animal and human health. Food animal production uses antibiotics to enhance production. Regulators evaluate new production technology to ensure animal safety and safe, edible products and to make public policy decisions by assessing risks/benefits. The U.S. Food and Drug Administration, Center for Veterinary Medicine's (CVM's) first risk assessment addressed the potential human health impact of campylobacter effects associated with the use of fluoroquinolines in food-producing animals. CVM used the Monte Carlo method to estimate risk byprobability distributions that reflect the uncertainty and variability in the data used for the assessment. Enterococci faecium is a species more likely to be resistant to antibiotics of last resort. Effective control of multidrug-resistant enterococci will requirea better understanding of the transfer of E. faeciumfrom animals to humans and the interaction between E. faecium, the hospital environment, and humans; prudent antibiotic use; better contact isolation in hospitals; and better surveillance. CVM will model these factors in a second, more complex risk assessment designed to examine the indirect transfer of resistance from animals to humans. Use of risk assessments allows researchers, the industry, regulatory authorities, and educators to make better policy decisions regarding antimicrobial use in food animals and humans and the development of resistance. Today the question of whether the use of antimicrobials for growth enhancement infood animals should or should not be terminated for the benefit of human health remains unresolved.

Risk assessment for chemical substances: the link between toxicology and public health

Virtually all chemical substances may cause adverse health effects, depending on the dose and conditions under which individuals are exposed to them. Toxicology - the study of harmful effects of chemicals on living organisms - provides the scientific data base on which risk assessment of adverse health effects stands. Risk assessment (RA) is the process of estimating the probability that a chemical compound will produce adverse effects on a given population, under particular conditions of exposure. Risk assessment process consists of four stages: Hazard Identification (HI), Exposure Assessment (EA), Dose-Response Assessment (DRA), and Risk Characterization (RC). The risk assessment process as a whole makes it possible to carry out cost(risk)/benefit analysis, and thus risk management, on a rational basis. A capacity to undertake risk assessment is thus sine qua non for making decisions that are concerned with achieving a balance between economic development and adequate protection of public health and the environment.

Cancer risk--does anyone really care?

Congress has enacted numerous laws in an attempt to protect people from carcinogens. But neither regulators nor oncologists can hope to accomplish this protection via studies in epidemiology. Nor can they hope to accomplish this using unverified models in carcinogenesis bioassays. The only viable hope is to test and utilize models of carcinogenicity that will tell us what we really need to know; i.e., does this substance pose a reasonable risk of carcinogenesis in man? And if so, what is a quantitative estimate of that risk? There is a substantial number of known human carcinogens. Yet, the most elaborate cancer-testing facility in the world continues to operate without the benefit of validating its results using positive controls (known human carcinogens). Without such controls to gauge the potency of the response in the test animal, Congress can pass 10,000 more laws and still the public will remain unprotected.

A critique of the use of the maximum tolerated dose in bioassays to assess cancer risks from chemicals

The maximum tolerated dose (MTD) regimen for testing substances for their ability to induce cancer and other chronic diseases in laboratory rodents has been required by governmental authorities for several decades. Cancer researchers originally suggested the MTD approach and it was then adopted by the FDA and EPA. The intention was to detect the ability of any substance under any circumstances, including the most extreme, to induce cancer in laboratory rodents. We question the validity of using the MTD in animal bioassays to evaluate risk for human cancer. The paradox is that the safer the chemical, the higher the MTD, but the higher the MTD, the more likely that biochemical distortions will result and cause cellular injury, abnormal cell replication, toxic hyperplasia, and toxicity-induced cancer. All chemicals are toxic at some dose, whether relevant to anticipated human exposure or vastly exceeding it. New approaches to cancer-testing lifetime bioassays are needed. A minimally toxic dose is defined and suggested to avoid specific tissue toxicity detected by clinical or pathology examination in animals subchronically exposed to the test compound for 90 days. The highest subtoxic dose that can be tolerated by test animals over a long period of time is suggested as being more appropriate for carcinogenicity bioassays.

Standard setting processes and regulations for environmental contaminants in drinking water: state versus federal needs and viewpoints

The primary objective of a standard setting process is to arrive at a drinking water concentration at which exposure to a contaminant would result in no known or potential adverse health effect on human health. The drinking water standards also serve as guidelines to prevent pollution of water sources and may be applicable in some cases as regulatory remediation levels. The risk assessment methods along with various decision making parameters are used to establish drinking water standards. For carcinogens classified in Groups A and B by the United States Environmental Protection Agency (USEPA) the standards are set by using nonthreshold cancer risk models. The linearized multistage model is commonly used for computation of potency factors for carcinogenic contaminants. The acceptable excess risk level may vary from 10(-6) to 10(-4). For noncarcinogens, a threshold model approach based on application of an uncertainty factor is used to arrive at a reference dose (RfD). The RfD approach may also be used for carcinogens classified in Group C by the USEPA. The RfD approach with an additional uncertainty factory of 10 for carcinogenicity has been applied in the formulation of risk assessment for Group C carcinogens. The assumptions commonly used in arriving at drinking water standards are human life expectancy, 70 years; average human body weight, 70 kg; human daily drinking water consumption, 2 liters; and contribution of exposure to the contaminant from drinking water (expressed as a part of the total environmental exposure), 20%. Currently, there are over 80 USEPA existing or proposed primary standards for organic and inorganic contaminants in drinking water. Some of the state versus federal needs and viewpoints are discussed.