Conceptual and theoretical aspects of human exposure and dose assessment

A formal, conceptual and mathematical framework for refining the characterization of human exposure and dose, resulting from contact with a toxicant, is introduced. The conceptual approach to the exposure characterization process leads to a mathematical description necessary for formalizing this process. Twelve steps are identified for guiding the information collection, management, and analysis needs for characterization and estimation of exposure and dose. These steps are discussed in terms of ten general principles for exposure and dose assessment that are related to: (1) The complicated character of exposure systems and models, (2) the need for using appropriate metrics associated with the exposure dynamics of individuals, populations and locations, (3) the need to consider exposure as a sequence of coupled events and systems, (4) the need to treat exposure assessment as a dual prognostic and diagnostic problem and (5) as a dual phenomenological and mechanistic problem, (6) the fact that uncertainty in exposure characterization includes both irreducible and reducible components that have to be quantified, (7) the need to specify probability distributions of exposures for individuals and populations, (8) the need for implementing state-of-the-art computational data management methods for exposure related information, (9) the need to formally and thoroughly evaluate exposure assessment modeling studies, and (10) the limitations in the exposure characterization process that affect the current practice of risk assessment. The theoretical approach described here represents an 'ideal' characterization model that will rarely, if ever, be fully implemented in practice. However, this approach offers a consistent and general framework that can help identify specific needs for improvement in current measurements and estimation practices, and move exposure assessments closer to 'real' exposure systems.