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WHAT ARE THE ADVERSE EFFECTS OF LEAD? In man, lead exerts its effects in the renal, hematopoietic and nervous systems. The severity of effects is related to both the degree of illness and the frequency of recurring illness as well as the dosage and duration of exposure. There are basically three stages in childhood lead poisoning: 1) asymptomatic lead poisoning, in which no clinical symptoms are apparent, but in which measurable metabolic changes occur, 2) symptomatic lead poisoning, in which clinical symptoms such as anorexia, vomiting, apathy, ataxia, drowsiness or irritability occur, and 3) lead encephalopathy with cerebral edema, in which coma or convulsions occur (see Appendix D). The sequelae of lead encephalopathy include seizure disorders, severe mental retardation and death. The sequelae of symptomatic but less severe lead poisoning includes seizure disorders as well as various behavioral and functional disorders, usually grouped under the heading of minimal brain dysfunction. Clinical studies suggest that the latter syndrome may include hyperactivity, impulsive behavior, prolonged reaction time, perceptual disorders and slowed learning ability. Recent evidence suggests that minimal brain dysfunction might also follow asymptomatic lead poisoning. The sequelae associated with each diagnostic category of lead poisoning do not necessarily occur in every child with a par- ticular diagnosis. Each individual is unique in his response. The effects of lead in the hematopoietic system are reversible and therefore do not constitute sequelae. Lead interferes with the formation of hemoglobin at several stages. In addition, lead reduces the life span of the red blood cells and this results in lead induced anemia. In cases of encephalopathy, acute renal injury (Fanconi syn- drome) may also occur, and in children this is reversible. The "critical effect" concept, provides a framework for examining the effects of lead. The term "critical effect" is used to mean first effect, rather than most serious effect. Since effects in the kidney do not appear in the early stages of lead poisoning, the kidney cannot be considered the site of the critical or first effect. It is not presently known whether the first effects occur in the neurologic or hematopoietic systems. Subtle neurologic effects are difficult to measure. There are currently no simple neurochemical tests for measuring early metabolic changes in the nervous system. However, several labora- tory tests are currently available for measuring early effects in the hematopoietic systems. At the present time, the hematopoietic system is considered the site where the "critical effect" occurs (see Appendix A). If this is correct, then environmental limits, set to prevent reversible effects in the hematopoietic system, should serve to prevent potentially irreversible effects in the nervous system. WHAT DOSE OF LEAD IS REQUIRED TO PRODUCE ADVERSE EFFECTS? In relation to lead, the general term "dose" may be variously interpreted to mean: 1) the quantity of lead administered, 2) the
