Biokinetic models changes

Concentrations of lead in the various environmental media described in this section are presented for extended periods. The available data that meet minimal statistical and measurement criteria generally only extend from the late 1960s/early 1970s to the present. The purposes of a wide temporal look at environmental lead concentrations are several. First, the nature of lead as an accumulating contaminant in the bodies of human populations requires an appreciation of the amounts of environmental lead that existed in past decades. As noted earlier, lead levels in media have been changing, mainly downward, so that current human body lead burdens are only partially quantifiable from current lead intakes into body compartments. Secondly, the use of predictive, biokinetic models of human lead exposures for simulating Ufe-time lead exposures requires knowledge of lead intakes from the earliest periods of life. 

Single-shot, i.e., isolated blood lead, surveys have been employed in a number of lead-impacted U.S. communities. Some of these efforts have been carried out in areas where the U.S. EPA has done biokinetic modeling of lead exposure via the EPA lEUBK model. One needs to be careful about calibrating EPA s model or any other biokinetic model on haphazard, i.e., unstructured, PbB surveys. The merits and limits of such surveys have been critiqued elsewhere (Mushak, 1998). The problems are several (1) such measures may be affected by various artifacts that are less influential in more structured sampling and (2) such measures relate to bioavaUability at some specific point in time and with a certain current land-use practice and population distribution. Demographic or socioeconomic characteristics of affected populations could change over time with movements in and out of areas of... 

According to the biokinetic model, the finction of inhaled retained in the thyroid only changes by about a factor of three over the whole monitoring period. In the absence of better evidence from a review of the sources of possible worlqrlace expostrre, this refined assumption provides a more rehable basis for dose assessment. The committed effective dose for this example would then be 270 pSv. A second mine sample obtained after a few mote days should be used to verify this conclusioa... 

A sensitivity-analysis has to show finally how the model reacts to changing values of parameters. An efficient biokinetic experimental design has been demonstrated by Johnson and Berthouex (1975a), and multiresponse data are used for parameter estimation by the same authors (1975b). 

A conclusion from these considerations on mathematical modeling of biokinetics is that in both interaction cases (transport and metabolic changes). 

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