Probably because they’re cutting corners. Document, as others have said. Take video of the cloud of dust (if it’s recurring). Record conversations with others corroborating what you witnessed. Don’t be intimidated. If something's wrong you’re not the only one who knows it. Look for comrades who have a sense of morality/doing the right thing.

They don't want to deal with the fallout if the dust was actually harmful. Write everything down with dates and time and as much detail you can provide, like you've done here. Keep it for your records. Continue to write anything you notice and any future conversations with management or if anyone else mentions potentially unsafe conditions. I don't know the relevant laws for what you're protected in discussing with your Coworkers, but it's absolutely worth looking into: this might be something they're not allowed to fire you or discipline over.

You should talk to your coworkers about it, but you might have to be sneaky about it and be sure they won't tell on you.