Acute toxicity of sulfur mustard and its degradation products to Danio rerio embryos.

Sulfur mustard (HD) is an organosulfur chemical warfare agent that is present in marine environments due to historical dumping of munitions. Although its degradation products are frequently detected in contaminated sediments and waters, their toxicity to aquatic vertebrates remains poorly characterized. This study evaluated the 96-hour acute toxicity of HD and seven major degradation products: thiodiglycol (TDG), thiodiglycolic acid (TDGA), thiodiglycol sulfoxide (TDGO), 1,4-oxathiane (1,4-OT), 1,4-dithiane (1,4-DT), 1,4,5-oxadithiepane (1,4,5-ODT), and 1,2,5-trithiepane (1,2,5-TTP), using the zebrafish (Danio rerio) embryo model in accordance with OECD Test No. 236 guidelines. Mortality and developmental endpoints, including hatching success, hypopigmentation, and morphological defects, were assessed. HD exhibited the highest toxicity, with a 96-h LC50 of 0.48 mg/L, classifying it as very toxic to aquatic organisms. Among degradation products, 1,2,5-TTP (LC40 = 7.14 mg/L, toxic) and 1,4,5-ODT (LC50 = 13.98 mg/L, harmful) induced significant mortality, while TDGA caused effects only under unbuffered acidic conditions. TDG, TDGO, and 1,4-OT showed no acute lethality or developmental toxicity. Notably, 1,4-DT, despite not affecting mortality, caused pronounced developmental abnormalities, including complete hatching inhibition and hypopigmentation. Experimental toxicity thresholds were substantially lower than QSAR-based predictions, indicating that current models may underestimate ecological risks associated with marine-dumped chemical munitions. Although predicted environmental concentrations of the tested organosulfur compounds are below acute effect thresholds, localized sediment contamination may pose risks to fish, and particularly their early life stages. These findings provide critical data for refining environmental risk assessments and establishing environmental quality standards for marine areas affected by historical chemical weapons disposal.