ABSTRACT
If direct measurements are not available, it is sometimes possible to make inferences from analyses which can be performed. For example, current workplace levels can be extrapolated backward in time if conditions are known to have been essentially identical for some period in the past. In this case, current levels are probably a good surrogate for previous levels. Of course, if industrial processes were changed, or new work rules went into effect, this type of inference would be much less valuable. In an environmental situation, it may be possible to trace the chemical of concern from its present location to a particular source and, from knowledge about this source and environmental movement of the chemical, again make some estimate of previous exposures.
