We recently ordered this compound, not knowing it is apparently very toxic (acute toxicity with deadly outcome). Does anyone know the target and/or mechanism of toxicity?
Toxicity to fish (Golden orfe; LC50 [96 hrs]) is around 220 to 320 mg/L and within ranges of other chemicals of concern. Toxicity to mammals: LD50 for rats, 3.4 mg/kg if ingested; inhalation LC50 1.5 mg/m3 in 4 hrs. This compound is toxic if certain thresholds are exceeded as you mentioned. However, ranges of toxicities are similar to other elements. Data is limited.
You may want to look at the many types of information available for this compound in the (U.S.) National Library of Medicine's ChemIDplus. The attached link will take you to this information.