PRINCIPLES OF TOXICOLOGY

19.3 LEAD EXPOSURE AND WOMEN OF CHILD-BEARING AGE 481
is used to illustrate the toxicity assessment step of the risk assessment process and emphasizes the need
for incorporation of new, mechanistic data regarding the carcinogenic effects of chemicals.
In each example, conservative default risk assessment procedures are used to familiarize the reader
with these assumptions and methods. Use of these procedures is not necessarily intended to be an
endorsement of their use and no systematic critique of the technical or scientific validity of each
assumption or method was performed. Rather, the reader is encouraged to critically evaluate the
scientific basis for the procedures and, where applicable, adopt alternate assumptions or methods when
justified by site-specific information or other data.
19.3 LEAD EXPOSURE AND WOMEN OF CHILD-BEARING AGE
The human health effects of lead are better known than nearly all industrial or environmentally
important chemicals. The extensive database of human information regarding the toxicity of lead
allows exposure and risks to be characterized with greater certainty than other chemicals. Human lead
exposure is most often evaluated by measuring the blood lead concentration. The blood lead concentration
provides information regarding the absorbed dose of lead from environmental sources. In
contrast, risk assessments for nearly all other chemicals calculate exposures rather than absorbed doses
and provide no information regarding the absorbed dose of the chemical, the time course of the
chemical in the body, or the concentration of the chemical in the target organ or tissue.
Lead is different in that good information exists to predict human blood lead concentrations
resulting from inhaled or ingested lead. Studies of the absorption, distribution, metabolism, and
excretion of lead in humans allow the determination of constants that relate ingested or inhaled blood
lead to the amount of lead in blood. For example, the USEPA uses a kinetic factor of 0.4 µg/dL per
µg/day to relate the amount of lead absorbed from the gastrointestinal tract to the amount of lead in
the blood in adults. Thus, for every microgram of lead absorbed from the gastrointestinal the blood
lead concentration will increase 0.4 µg/dL.
In addition, there exists a large human toxicological database that allows the blood lead concentration
to be related to lead’s toxic effects. At blood lead concentrations less than 20 µg/dL, lead may
cause neurobehavioral and developmental effects in children and affect vitamin D metabolism. There
is an obvious difference in the sensitivity of children and adults to the effects of lead. For example,
young children absorb more lead from the gastrointestinal tract. The immature nervous system of young
children is also more sensitive to the adverse effects of lead. In the case of a pregnant worker exposed
to lead, elements of adult lead exposure and a child’s greater sensitivity to lead must be considered. In
a pregnant worker, lead absorption, distribution, and rate of excretion from the body is that of an adult.
In this way, the lead exposure of the fetus is nearly the same as that of the adult.
The Occupational Safety and Health Administration requires medical monitoring of workers with
blood lead levels of 40 µg/dL and higher. However, for a pregnant worker, the rapidly growing fetus
is the sensitive individual of greatest concern. OSHA regulations are not specifically designed to protect
the fetus. The Centers for Disease Control (CDC) in Atlanta has established a blood lead level of
concern for children of 10 µg/dL. Thus, blood lead levels tolerated under OSHA regulations may be
potentially harmful to the fetus. It is particularly important to assess the risks posed by lead exposure
for female workers of child-bearing age.
Given information concerning the kinetic behavior of lead in the human body and the relationship
of blood lead concentration to lead toxicity, the toxic effects of lead exposure can be assessed with a
greater degree of certainty than nearly every other environmental chemical. The USEPA interim lead
exposure model may be used to evaluate fetal lead exposure that may result from the mother’s exposure
in the workplace. A brief description of the equations used to predict fetal blood lead concentrations
resulting from maternal ingestion of lead in soil or dust is presented below.
The equation for adult lead exposure is