Association between relatively low blood lead levels and urinary δ-aminolevulinic acid concentrations among male workers at a Japanese battery factory.

Abstract

Whether the known positive association between blood lead (PbB) levels and urinary δ-aminolevulinic acid (ALAU) also exists at relatively low PbB levels (<40μg/dL) remains unclear. We aimed to investigate this association at lower PbB levels. We analyzed data from biannual medical examinations of workers at a Japanese factory from August 2013 to August 2023. We excluded records from female workers and those with missing data, resulting in a dataset consisting of 1396 records from 155 male workers. We employed mixed-effect linear regression models with a random intercept for workers and additional adjustments for age and smoking status. The median PbB level across all the analyzed records was 8μg/dL (range: 1, 31μg/dL). Significant positive associations were observed between PbB and ALAU, with a one-unit increase in natural logarithm-transformed PbB corresponding to a 10.0% increase in ALAU (95% CI: 2.7, 17.9%). Categorized PbB analyses showed a 23.8% increase in ALAU (95% CI: 2.7, 49.2%) for PbB levels at 20-24μg/dL and an 83.1% increase (95% CI: 30.1, 157.7%) for PbB levels ≥25μg/dL, compared to those <5μg/dL. The exposure-response curve analysis indicated a plateau followed by an increasing trend. A positive and non-linear association between PbB and ALAU levels was observed at relatively low PbB levels.