addressing the array of adverse health effects, OSHA cited five stages of a disease process—“normal, physiological change of uncertain significance, pathophysiological change, overt symptoms (morbidity), and mortality” (p. 52594) and recognized that there was not a sharp distinction between stages but rather that they are on a continuum. OSHA went on to state that an adopted standard “must prevent pathophysiological changes from exposure to lead”. In questioning whether both “clinical and subclinical effects” of exposure should be considered, OSHA judged that those terms represented “vast over-simplifications of a disease process and, therefore … avoided their use in the final standard” (p. 52963). OSHA regarded subclinical effects to be the “early to middle stages in a continuum of disease development process” (p. 52963).
OSHA summarized key studies on heme-synthesis inhibition (inhibition of the enzymes delta-aminolevulinic acid dehydrogenase [ALAD] and ferrochelatase) and anemia; on neurologic effects (central nervous system symptoms, behavioral symptoms, peripheral nerve effects, and results of nerve-conduction testing); on renal effects; and on reproductive effects. Except for reproductive effects, the evidence demonstrated adverse pathophysiologic effects at BLLs over 40 μg/dL; thus, 40 μg/dL was considered the upper acceptable limit. Concerning reproductive effects and effects on children (hyperactivity at BLLs as low as 25 μg/dL), “OSHA concludes that in order to protect the fetus and newborn from the effects of lead on the nervous system, blood lead levels must be kept below 30 μg/100 g [30 μg/dL]” for workers who wish to plan pregnancies (40 Fed. Reg. 52960 ). OSHA also acknowledged at the time that for many of the adverse health effects there was evidence of a dose-response relationship but that the “no-effect level” remained to be determined.
Air-Blood Relationships: Although industry representatives maintained that BLLs could not be correlated with or predicted from air lead concentration, OSHA judged that despite some data limitations the collective data could be used to make estimates. OSHA described some studies that provided linear models and regression analyses. It relied predominantly on a physiologic model originally developed by S. R. Bernard and adapted by the Center for Policy Alternatives (CPA) that “combines experimentally observed properties of mammalian lead transport and metabolism, including consideration of the dynamics of blood lead response to long term exposure” (40 Fed. Reg. 52961 ). The model also accounted for “the observed physical properties of airborne particulates encountered in the workplace, in order to produce a complete and accurate picture of the response of blood lead levels to particulates” (40 Fed. Reg. 52967 ), and CPA included specific consideration of individual variability in response to air lead. The model was applied to exposures at air concentrations of 50, 100, and 200 μg/m3, and the results were used to predict the percentage of workers that would fall within different BLLs. The results demonstrated the benefit of setting an air concentration of 50 μg/m3 to reduce the number of workers who had predicted BLLs over 40 μg/dL. (See Chapter 3 for the committee’s evaluation of the model and its assumptions.)