In our lab we have three forms of zinc. We were initially experimenting with these three forms to optimize our application. We have "zinc shot", "mossy zinc", and "zinc granules". The granules are ~20 mesh. I am tasked with creating chemical standard operating procedures (SOPs) for all reagents that are considered "particularly hazardous". Zinc granules are considered particularly hazardous by our University and is an "acute toxin to aquatic life with lasting effects" in a recent safety data sheet (SDS) while zinc shot and mossy zinc are not considered hazardous by our University or on comparable recent SDSs. I see zinc powder is also considered toxic. To me this suggests particle size dictates toxicity, but why? I understand surface area influences the rate of reactions but if the effects are long lasting, then why are all forms of zinc not considered to be an aquatic life toxin?
The key to toxicity is concentration, which is related to how rapidly the material disperses (or concentrates) in the environment. Which is also related to water type (fresh or salt) and temperature. MSDS sheets are a bit of a joke. They are worded in a way that fends off lawyers rather than gives detailed useful toxicity data. Consider this MSDS for Sodium Chloride
Bear in mind this useful advice when using it at home: "Eye Protection Wear safety glasses with side shields (or goggles) (European standard - EN 166). Hand Protection Protective gloves"