Is it legal to discharge a fire extinguisher outside?

I came home today and the apt Managers at my complex were discharging a bunch (about 40 or 50) of Fire extinguishers into the air next to a cornfield. I think they were emptying them to have them recharged or disposed of. anyways as far as I know they are all ABC Dry Chem fire extinguishers. I was wondering if it is legal for them to do so. and if their are any health risks since they are letting a huge cloud of that stuff right next to the apartments and there are kids outside playing.