effects. The committee also found compelling evidence of developmental effects in offspring exposed to lead in utero and during breastfeeding, and this raises additional concerns about exposures of women of childbearing age.
BLLs are generally considered to represent recent exposure to lead (on the basis of the lifespan of the erythrocyte and BLL’s representing the integrated dose over the prior 4 months or so). Because lead in blood is in equilibrium with lead stored in bone, where lead can reside for decades, some BLLs can also reflect past higher or cumulative exposures. Therefore, BLLs measured later in life can reflect both current and past cumulative exposure, so interpretation is difficult. For example, studies that used data from the National Health and Nutrition Examination Survey to relate BLLs to risk of chronic kidney disease have reported a striking rise in risk of the disease in the highest quintile of BLL compared with the lowest quintile even though the mean BLL in the highest quintile is only 3-4 μg/dL. Those in the highest quintile may have had higher BLLs earlier in life that resulted in a greater cumulative lifetime exposure to lead than may be inferred from the current BLL, which is in equilibrium with possibly higher bone stores. In that case, cumulative exposure is more likely to be associated with the observed chronic effect on renal function, so the BLL of 3-4 μg/dL might not represent a “threshold level”.
Despite those shortcomings of the BLL, the committee decided to present the range of BLLs that have been associated with various acute and chronic health outcomes:
• Adverse renal effects are manifested by increases in serum creatinine at BLLs of 8-12 μg/dL, decreases in creatinine clearance and glomerular filtration rate at BLLs of 20-30 μg/dL, and effects on renal endocrine functioning at BLLs of 30-40 μg/dL. The latter might be responsible, in part, for the increases in blood pressure observed with high BLLs.
• Adverse cardiovascular effects of concern include increased blood pressure at BLLs under 10 μg/dL and increased cardiovascular-disease mortality at BLLs of 8 μg/dL or higher. A relationship between BLLs under 40 μg/dL and cardiovascular mortality and some subclinical cardiovascular outcomes has also been observed in older and other susceptible subpopulations.
• Adverse nervous system effects include dose-related changes in cognitive and psychomotor performance at a BLL of about 18 μg/dL and such neurophysiologic changes as hearing loss at BLLs under 10 μg/dL, changes in balance at BLLs of about 14 μg/dL, changes in visual function at BLLs of 17-20 μg/dL, slowed auditory evoked potentials at BLLs of 26-30 μg/dL, changes in autonomic function at BLLs over 20 μg/dL, and changes in peripheral sensory nerve function at BLLs around 30 μg/dL.
• Adverse hematologic effects include impaired formation and impaired survival of erythrocytes at BLLs of about 20-30 μg/dL.
• Adverse developmental effects were found in infants and children at maternal BLLs under 10 μg/dL, and reduced fetal growth and low birth weight