Although lead has no physiological function and is present in only negligible amounts in the blood of remote populations, it has become customary to accept the usual blood Pb level(s) (BPb) observed in industrialized society as "normal." Pb interferes with many biochemical systems, among them the heme biosynthetic pathway; this is reflected by an exponential increase in erythrocyte protoporphyrin concentration (EP) as BPb increases. The present study estimated the threshold BPb at which an increase of EP occurs in a population of urban children. In the 2,004 children studied, BPbs ranged from 2 to 98 jug/dl, with
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