Environmental standard limit fluoride exposure prioritizes neurotoxicity over osteotoxicity in larval zebrafish: A benchmark dose analysis.

Fluoride is a widespread environmental contaminant, but current water standards, based on skeletal toxicity, may underestimate neurodevelopmental risks. This study directly compares the sensitivity of the central nervous system (CNS) and skeletal system to fluoride at concentrations (0.5-20.0 mg/L) spanning international regulatory limits, using larval zebrafish. An integrated assessment combining behavior, transcriptomics, and benchmark dose modeling was conducted. Fluoride accumulated in the brain dose dependently and induced hyperlocomotion at 0.5 mg/L, indicating neuroexcitation. In contrast, skeletal changes were biphasic, with reduced mineralization only at 20.0 mg/L. Transcriptomics showed broader enrichment of CNS related pathways versus skeletal pathways. Consistently, key neural markers (neun, tmem119a) were upregulated at all concentrations, while skeletal markers (bglap, acp5b) responded inconsistently. Bayesian model averaging revealed that the average benchmark concentration lower limit for CNS markers was 0.17 mg/L-an order of magnitude lower than for skeletal markers (1.83 mg/L). These results quantitatively establish the developing CNS as a more sensitive target than the skeleton at regulatory level exposures. The significant sensitivity disparity suggests that standards based primarily on osteotoxicity may not adequately protect against neurodevelopmental effects. Further research is needed to translate these biomarker based thresholds into functional outcomes for comprehensive health risk assessment.