Curvilinear relationship between blood lead level and reaction time - Differential association with blood lead fractions derived from exogenous and endogenous sources

Prior studies demonstrate an inconsistent relationship between occupational inorganic lead exposure and simple reaction time (SRT) performance. In this study, we administered a computerized SRT test to 78 currently enzplo3red lead smelter workers and then investigated the relationship between different measures of blood lead and components of SRT performance. The measures of blood lead included current blood lead (PbB) and mathematically derived blood lead fractions from the environment (PbB-env) and from bone (PbB-bn). Measures of SRT performance, obtained from 44 trials with interstimulus intervals (ISIs) ranging from 1 to 10 seconds, included median SRT (SRT-md), mean SRT for ISIs between 1 and 5 seconds (SRT-1-5), and mean SRT for ISIs between 6 and 10 seconds (SRT-6-10). Multiple regression analysis after adjustment for age and education revealed a curvilinear relationship between PbB and SRT-md. As PbB increased from 0 to 30 mu g dl(-1), SRT-md decreased, and only with PbB level above 30 mu g dl(-1) did SRT-md increase. PbB terms accounted for 13.7% of the variance in this SRT measure (P < 0.01). The longer ISI variable, SRT-G-10, was found to be more strongly related to PbB, to have lesser variability across ISIs, and to be unrelated to age. Additional multiple regression analysis to examine the relationship between components of SRT and the PbB fractions, PbB-env and PbB-bn, showed only PbB-env to account for significant variance in SRT-md, (14.4%, P < 0. 01), SRT-1-5 (9.7%, P < 0. 03), and SRT-1-6 (15%, P < 0.01). We conclude that the relationship between PbB and SRT is U-shaped, that the SRT measure SRT-6-10 has properties that make it the preferred measure of SRT performance in future studies, and finally that only PbB-env, and not PbB-bn, is related to components of SRT.