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Lead poisoning is a particularly insidious public health threat because people exposed to harmful levels of lead usually do not show immediate or clear symptoms of such exposure. Consequently, standards to control absolute toxicity cannot be set as there is no reference dose (RfD). In addition, since lead is pervasive and exposure to lead very common, no true "control" group exists for sensitive populations such as young children, and an exposure threshold for health effects cannot be identified at present. It is probable that no safe exposure level exists for lead since even limited exposure levels have been linked to some health threats.
Although lead has no biological function in humans it is readily absorbed through the gut and persists in both blood and bone. The amount of lead absorbed from the gastro-intestinal tract of adults is typically 10-15% of the ingested quantity; for pregnant women and children, the amount absorbed can increase to as much as 50%. The quantity absorbed increases significantly under fasting conditions and in people suffering from iron or calcium deficiency.
Once in the blood, lead is distributed primarily among three compartments -- blood, soft tissue (kidney, bone marrow, liver and brain), and mineralizing tissue (bones and teeth). Mineralizing tissue contains about 95% of the total body burden of lead in adults. The rate of lead uptake decreases as the dose increases, and a healthy diet can help reduce the absorption of ingested lead.
For additional information on the risks which lead can pose to human health, consult:
* EPA/600/D-86/185 Lead Exposures in the Human Environment (1986)-- Overview on lead exposure in humans; describes how exposure sources of environmental lead are determined; discusses how an exposure baseline is estimated and how other factors may then be added to complete an estimate for lead exposure. This publication may be ordered from EPA's National Center for Environmental Publications and Information (NCEPI).
* EPA/440/4-85/010 "Exposure and Risk Assessment for Lead" (1985)-- This document contains a risk assessment for lead based on U.S. data. Areas studied include identification of lead releases to the environment during production, use or disposal of lead-containing substances; the fate of lead upon entering the environment; and ambient levels to which different human populations and aquatic life are exposed. Exposure levels are estimated and toxicity data available are presented and interpreted. Information on these different areas is then combined to assess the risks of lead exposure to various sub-populations. This publication may be ordered from EPA's National Center for Environmental Publications and Information (NCEPI).
For on-line information from EPA on the health effects of lead and lead compounds, refer to the Health Effects Notebook for Hazardous Air Pollutants:
Also see the EPA Office of Emergency and Remedial Response's site entitled "Lead and Human Health":