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MODELING ENERGY EXPENDITURE AND OXYGEN CONSUMPTION IN HUMAN EXPOSURE MODELS: ACCOUNTING FOR FATIGUE AND EPOC
Citation:
ISAACS, K. K., G. GLEN, T. R. MCCURDY, AND L. SMITH. MODELING ENERGY EXPENDITURE AND OXYGEN CONSUMPTION IN HUMAN EXPOSURE MODELS: ACCOUNTING FOR FATIGUE AND EPOC. Journal of Exposure Science and Environmental Epidemiology . Nature Publishing Group, London, Uk, 18(3):289-298, (2007).
Impact/Purpose:
The overall goal of this work is to develop information to assess potential environmental health risks and susceptibility in the aging population. Initial work will be directed toward developing information that can be used to identify and characterize what is known about activity, exposure, and dose for key life stages in the aging population and to identify key data gaps to be addressed through further research. Specific research objectives have been identified to address four discrete elements of the environmental paradigm for an aging population.
1) Identify key chemical and biological stressors in the older adult population, compile extant information on exposures to these agents and the extent to which they may be different in aging and other populations, and identify key gaps in our knowledge of exposure to important stressors.
2) Identify the key life stages in the older adult population with regard to exposures and susceptibilities to chemical and biological stressors. Compile activity pattern and physiological information for older Americans in key life stages, including information on physical activity, dietary intakes, pharmaceutical use, and other possible stressors that may impact exposures and/or susceptibility. Identify key gaps in our knowledge of activities in subpopulations of the aging by life stages.
3) Incorporate changes in physiological parameters that result from aging in physiologically-based pharmacokinetic models. Assess the relative sensitivity of aging-related changes in parameters for generation of dose estimates. Determine which parameters may require additional knowledge to improve the models.
4) Provide information developed for exposure, activity, and pharmacokinetics to extend existing exposure and PBPK models to aging populations and susceptible subpopulations at different life stages for use in risk assessment. Also, appropriate information will be provided for incorporation into an Older Adults Exposure Factors Handbook being prepared by NCEA.
Description:
Human exposure and dose models often require a quantification of oxygen consumption for a simulated individual. Oxygen consumption is dependent on the modeled Individual's physical activity level as described in an activity diary. Activity level is quantified via standardized values of metabolic equivalents of work (METS) for the activity being performed and converted into activity-specific oxygen consumption estimates. However, oxygen consumption remains elevated after a moderate- or high-intensity activity is completed. This effect, which is termed excess post-exercise oxygen consumption (EPOC), requires upward adjustment of the METS estimates that follow high energy expenditure events, in order to model subsequent increased ventilation and intake dose rates. In addition, since an individual's capacity for work decreases during extended activity, methods are also required to adjust downward those METS estimates that exceed physiologically-realistic limits over time. A unified method for simultaneously performing these adjustments is developed. The method simulates a cumulative oxygen deficit for each individual and uses it to impose appropriate timedependent reductions in the METS time series and additions for EPOC. The relationships between the oxygen deficit and METS limits are non-linear and are derived from published data on work capacity and oxygen consumption. These modifications result in improved modeling of ventilation patterns, and should improve intake dose estimates associated with exposure to airborne environmental contaminants.