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Health Effects of Lead

Lead is one of the most intensively studied hazardous agents of the twentieth century. The more toxicologists and other researchers investigated the health effects of lead, the more they realized that even very low levels of lead exposure were hazardous (Gilbert and Weiss, 2006). The most common biomarker of lead exposure is the blood lead level, usually measured in micrograms (μg) per one tenth of a liter of blood (dL) or μg/dL. For example, many regulatory agencies set 40 μg/dL as a level of concern for adult male workers. Typically, at this level workers would be removed from the environment responsible for the exposure and ideally some determination would be made as to the cause of the exposure.

The blood level of concern for children has dropped steadily as shown in the graph below, and some believe that there is sufficient data on the health effects below 10 μg/dL that the CDC should significantly lower the blood lead action level (Gilbert and Weiss, 2006).

The decline in acceptable childhood blood levels was a function of research and of improved control of lead contamination, such as the removal of lead from gasoline. A blood lead level of 10 μg/dL does not represent a "safe" level, only one where it is prudent to take action to reduce exposure. But it must be noted that a level of 10 μg/dL is considered an action level and does not provide any margin of safety for a child's developing nervous system. Currently, there appears to be no safe level of lead exposure for the developing child.

The nervous system is the most sensitive target of lead poisoning. Fetuses and young children are especially vulnerable to the neurological effects of lead because their brains and nervous systems are still developing. At high levels of lead exposure, the brain will swell (encephalopathy), possibly resulting in death. At one time it was thought that children who survived high levels of exposure would recover and have no adverse effects. However, in the 1940s, persistent learning and developmental effects were demonstrated in children exposed to high levels of lead. In 1979, a study by Needleman showed that even low levels of lead exposure would reduce the school performance of children. This study was one of the first to use tooth lead as marker of childhood exposure, which correctly classified early childhood exposure even if current blood lead levels were normal.

Numerous studies found similar results and it is now generally accepted that in every 10 μg/dL increase in blood lead levels, within the range of 5 to 35 μg/dL, there is a 2- to 4-point IQ deficit. Subsequent long-term studies of infants and young children exposed to lead showed that as they became older, there was an increased likelihood that they would experience decreased attention span, reading and learning disabilities, and failure to graduate from high school.

Adult nervous system effects are also apparent following lead exposure. In the past, painters using lead-based paint developed damage to the peripheral nervous system, which caused a wrist or foot drop. Nerve damage in the forearm could be evaluated by using an instrument to measure how fast the nerves conduct an electrical signal from one point to the next. But, as was the case with children, when more subtle effects were looked for, they were found. There is evidence of decreased cognitive performance in adults with blood levels greater that 25 μg/dL.

Lead exposure can produce a number of other effects. One of the most common effects is on the red blood cells: they become fragile and hemoglobin synthesis is impaired, which results in anemia. Similar to other metals, lead adversely affects kidney function, but this is now rare with reductions in occupational exposure. Several studies have demonstrated that elevated lead exposure is related to elevated blood pressure levels, particularly in men. There also appears to be a weak association between lead exposure and increased incidence of lung and brain cancer. Lead exposure is a reproductive hazard for both males and females. In males, lead affects sperm count and sperm motility, resulting in decreased offspring.

Figure 8.2 Effects of Blood Lead - Children vs. Adults

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