Workshop on the scientific evaluation of the safety factor for the acceptable daily intake (ADI): editorial summary

Advancing human health risk assessment: integrating recent advisory committee recommendations

Over the last dozen years, many national and international expert groups have considered specific improvements to risk assessment. Many of their stated recommendations are mutually supportive, but others appear conflicting, at least in an initial assessment. This review identifies areas of consensus and difference and recommends a practical, biology-centric course forward, which includes: (1) incorporating a clear problem formulation at the outset of the assessment with a level of complexity that is appropriate for informing the relevant risk management decision; (2) using toxicokinetics and toxicodynamic information to develop Chemical Specific Adjustment Factors (CSAF); (3) using mode of action (MOA) information and an understanding of the relevant biology as the key, central organizing principle for the risk assessment; (4) integrating MOA information into dose-response assessments using existing guidelines for non-cancer and cancer assessments; (5) using a tiered, iterative approach developed by the World Health Organization/International Programme on Chemical Safety (WHO/IPCS) as a scientifically robust, fit-for-purpose approach for risk assessment of combined exposures (chemical mixtures); and (6) applying all of this knowledge to enable interpretation of human biomonitoring data in a risk context. While scientifically based defaults will remain important and useful when data on CSAF or MOA to refine an assessment are absent or insufficient, assessments should always strive to use these data. The use of available 21st century knowledge of biological processes, clinical findings, chemical interactions, and dose-response at the molecular, cellular, organ and organism levels will minimize the need for extrapolation and reliance on default approaches.

Dispelling urban myths about default uncertainty factors in chemical risk assessment--sufficient protection against mixture effects?

Assessing the detrimental health effects of chemicals requires the extrapolation of experimental data in animals to human populations. This is achieved by applying a default uncertainty factor of 100 to doses not found to be associated with observable effects in laboratory animals. It is commonly assumed that the toxicokinetic and toxicodynamic sub-components of this default uncertainty factor represent worst-case scenarios and that the multiplication of those components yields conservative estimates of safe levels for humans. It is sometimes claimed that this conservatism also offers adequate protection from mixture effects. By analysing the evolution of uncertainty factors from a historical perspective, we expose that the default factor and its sub-components are intended to represent adequate rather than worst-case scenarios. The intention of using assessment factors for mixture effects was abandoned thirty years ago. It is also often ignored that the conservatism (or otherwise) of uncertainty factors can only be considered in relation to a defined level of protection. A protection equivalent to an effect magnitude of 0.001-0.0001% over background incidence is generally considered acceptable. However, it is impossible to say whether this level of protection is in fact realised with the tolerable doses that are derived by employing uncertainty factors. Accordingly, it is difficult to assess whether uncertainty factors overestimate or underestimate the sensitivity differences in human populations. It is also often not appreciated that the outcome of probabilistic approaches to the multiplication of sub-factors is dependent on the choice of probability distributions. Therefore, the idea that default uncertainty factors are overly conservative worst-case scenarios which can account both for the lack of statistical power in animal experiments and protect against potential mixture effects is ill-founded. We contend that precautionary regulation should provide an incentive to generate better data and recommend adopting a pragmatic, but scientifically better founded approach to mixture risk assessment.

Mathematical modelling and quantitative methods

The present review reports on the mathematical methods and statistical techniques presently available for hazard characterisation. The state of the art of mathematical modelling and quantitative methods used currently for regulatory decision-making in Europe and additional potential methods for risk assessment of chemicals in food and diet are described. Existing practices of JECFA, FDA, EPA, etc., are examined for their similarities and differences. A framework is established for the development of new and improved quantitative methodologies. Areas for refinement, improvement and increase of efficiency of each method are identified in a gap analysis. Based on this critical evaluation, needs for future research are defined. It is concluded from our work that mathematical modelling of the dose-response relationship would improve the risk assessment process. An adequate characterisation of the dose-response relationship by mathematical modelling clearly requires the use of a sufficient number of dose groups to achieve a range of different response levels. This need not necessarily lead to an increase in the total number of animals in the study if an appropriate design is used. Chemical-specific data relating to the mode or mechanism of action and/or the toxicokinetics of the chemical should be used for dose-response characterisation whenever possible. It is concluded that a single method of hazard characterisation would not be suitable for all kinds of risk assessments, and that a range of different approaches is necessary so that the method used is the most appropriate for the data available and for the risk characterisation issue. Future refinements to dose-response characterisation should incorporate more clearly the extent of uncertainty and variability in the resulting output.

The use of safety or uncertainty factors in the setting of acute reference doses

A 100-fold safety or uncertainty factor has been used for about 40 years to derive safe daily intakes for humans based on animal studies; the 100-fold factor comprises separate 10-fold factors to allow for species differences and inter-individual variability. Each factor has to allow for toxicokinetic and toxicodynamic differences. Sub-dividing the 10-fold factors into kinetic and dynamic defaults, which when multiplied give a product of 10, offers a number of advantages. The main rationale for this sub-division is so that chemical-specific data can be introduced to replace one or more of the default sub-factors, hence contributing to a chemical-related overall factor. However, sub-division of the 10-fold factors has allowed analysis of the appropriateness of the overall 10-fold defaults, and analysis of special situations, such as infants and children. The establishment of an acute reference dose based on animal studies has to allow for both species differences and inter-individual variability; comparison with the factors used for chronic effects suggests that modification of the usual defaults may be appropriate under certain specific circumstances, but that the usual default of 100 remains appropriate for most cases.

Duration of intake above the ADI/TDI in relation to toxicodynamics and toxicokinetics

The duration of intake necessary for the production of a toxic response depends on the mechanism of toxicity and the accumulation of the chemical to reach a toxic body load. There is a paucity of data on the cellular processes and changes associated with non-cancer effects detected in subchronic and chronic studies, and which are usually the basis for calculation of health-based exposure limits, such as the ADI. Data on the time course for the critical mechanistic process are normally unknown at the cellular level. Consequently, the duration of an excess intake in humans must be compared with the time course for the generation of the overall toxic effects in animals, which will include both toxicodynamic and toxicokinetic components. The extent of accumulation of a chemical during chronic intake is directly proportional to the half-life. The duration of intake at dosages above the ADI, necessary to produce body loads greater than those resulting from intake at the ADI, depends on the magnitude of the excess intake, the elimination half-life of the chemical, and the initial intake and body load. The effect on the body load of a short period of intake above the ADI is inversely proportional to the half-life of the chemical.

Incidence and severity in relation to magnitude of intake above the ADI or TDI: use of critical effect data

Noncancer effects are considered to show thresholds, such that no effect would be produced when the intake is below the threshold. Application of a 100-fold uncertainty factor to the no-observed-adverse-effect level (NOAEL) for the critical effect in animal studies provides an estimate of an intake for sensitive humans (the acceptable daily intake or ADI) without significant adverse health effects. The risks of intakes by humans above the ADI theoretically move the most sensitive subjects from negligible risk to possible risk. An increase in intake above the ADI would move the population distribution of internal dose toward the dose-response curve for sensitive subjects. The proportion of a population affected depends on the magnitude of the excess intake, the relationship of the NOAEL and the biological threshold in animals, and the coefficient of variation for the kinetic parameters (e.g., clearance and bioavailability) which determine the internal dose in humans. The severity of any effect in sensitive and high-intake individuals depends on the magnitude of the excess intake, the nature of the critical effect, and the slope of the dose-response relationship.

Veterinary drugs no longer need testing for carcinogenicity in rodent bioassays

The putative carcinogenic risk to humans from ingestion of edible tissues containing traces of nongenotoxic veterinary drugs is so slight that the routine application of rodent cancer bioassays cannot be justified. This argument is based, first, on the pharmacological similarity of veterinary and human drugs: many of the latter that are carcinogenic to rodents have been deemed on mechanistic and/or potency grounds not to pose a cancer risk to humans. Second, the distribution of a veterinary drug through the target animal body before ingestion of a portion of edible tissue by humans days or weeks later means that the human dose from a residue is several orders of magnitude lower than the normal dose of human drugs. The dose of residue is also much lower than the exposure of humans to the most potent carcinogens.

Human variability and noncancer risk assessment- An analysis of the default uncertainty factor

A 10-fold uncertainty factor is used for noncancer risk assessments to allow for possible interindividual differences between humans in the fate of the chemical in the body (kinetics) and target organ sensitivity (dynamics). Analysis of a database on the variability in each of these aspects is consistent with an even subdivision of the 10-fold factor into 10(0.5) (3.16) for kinetics and 10(0.5) (3.16) for dynamics. Analysis of the number of subjects in a normally and log-normally distributed population which would not be covered by factors of 3.16 supports this subdivision and also the use of a 10-fold factor to allow for both aspects. Analysis of kinetic data for subgroups of the population indicates that the standard default value of 3.16 for kinetics will not be adequate for all routes of elimination and all groups of the population. A scheme is proposed which would allow the selection of appropriate default uncertainty factors based on knowledge of the biological fate and effects of the chemical under review. Copyright 1998 Academic Press.