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

Objectives: Whether the known positive association between blood lead (PbB) levels and urinary δ-aminolevulinic acid (ALAU) also exists at relatively low PbB levels (<40 μg/dL) remains unclear. We aimed to investigate this association at lower PbB levels.

Methods: 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.

Results: The median PbB level across all the analyzed records was 8 μg/dL (range: 1-31 μg/dL). Significant positive associations were observed between PbB and ALAU, with a 1-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 20-24 μg/dL and an 83.1% increase (95% CI, 30.1%-157.7%) for PbB levels ≥25 μg/dL, compared with those <5 μg/dL. The exposure-response curve analysis indicated a plateau followed by an increasing trend.

Conclusions: A positive and nonlinear association between PbB and ALAU levels was observed at relatively low PbB levels.