Threshold for lead damage to heme synthesis in urban children.

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Abstract
Although Pb has no physiological function and is present in only negligible amounts in the blood of remote populations, it is accepted that the usual blood Pb level(s) (BPb) observed in industralized 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 threshold BPb at which an increase of EP occurred in a population of urban children was estimated. In the 2004 children studied, BPb ranged from 2-98 .mu.g/dl, with 1852 having a BPb of .ltoreq. 30 .mu.g/dl, a value considered normal. Preliminary analysis suggested that an exponential increase in the concentration of EP occurred after a threshold BPb (apparently between 12-20 .mu.g/dl) was reached. Precise definition of the threshold BPb for an increase of EP was determined by 2 approaches: segmented line techniques and probit analysis. Whether the entire population was analyzed or only the subset of samples with normal BPb (.ltoreq. 30 .mu.g/dl), both methods yielded a threshold BPb of 15-18 .mu.g/dl (average value, 16.5). The heme synthetic pathway was affected by Pb at a level of exposure commonly observed in urban children, which was below the limit presently too easily accepted as normal.