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important uncertainty with human exposure assessment is (long-term) exposure duration or
frequency. It is not easy to give a fair account of all the exposure events for a day. It
is even harder to determine exactly how many times a person would be exposed in a year or
a work season.
Applicators are likely to work for more than one grower in a season. Workers also could harvest celery for another grower when they are not harvesting broccoli; yet both crops could have been treated with the same pesticide.
In terms of long-term inhalation exposure, one cannot be too sure what the pattern of population or occupational mobility is. That is, how likely is a person living within the vicinity for years? Even if residents can be assumed to live in a community for years, there is still an uncertainty about how much time they stay indoors versus outdoors. Often times, the outdoor and indoor air levels are not the same. A six-year-old child can have very different oral exposure to a contaminant compared to a two-year-old. A three-year-old child may still stay home or in a day care center where meals are provided frequently. Yet a six-year-old would spend more time learning in school.
Perhaps the impact of exposure frequency or duration can best be illustrated with the following analogies: Would the effect of the dosage be the same for a person taking 1 vitamin tablet a day for a year, as for the person taking 5 tablets a day continuously for 73 days (note that 1 year = 73 x 5)? Or would the effect be the same for a person taking 5 tablets a day for 73 consecutive days, as for the person taking 5 tablets a day intermittently for 73 days?