The numerical example provided in Slide 15 is actually too simplistic, not representative of most exposure scenarios in real life. A typical case is likely to involve repeated exposures, such as with agricultural applicators who have been handling a pesticide for several days immediately prior to the monitoring period. If the pesticide has a long half-life of several days in the human body, then the urine collected over a 24- or 48-hour period would contain the chemical absorbed mostly from exposures received prior to the sampling day. In other words, further efforts must be made to discount the portion of the chemical recovered from exposures occurring on days prior to the sampling period.

To complicate the issue further, for repeated exposures lasting several times longer than the chemical's relatively long half-life in the human body, its contents in the plasma (and hence later in the urine as well) will eventually reach a steady state. At that time, the daily dose absorbed should be more or less equal to the daily amount eliminated by all routes. In that case, no effort is needed to determine the portion of the urinary recovery that is (or is not) due to repeated exposures occurring prior to the biomonitoring period.

In short, the exposure history of the test subject and the chemical's half-life in humans are some additional factors critical to the interpretation and hence the use of human biomonitoring data.