I bought my boat twenty years ago and it had been laid up on the hard for four years before I got it. There was a dry chemical fire extinguisher on board when I cleaned it out. I hung the extinguisher in the shop because the guage was still in the green. This morning I took it outside to see if it would still work. It did perfectly. But I was astonished by the cloud of dust that filled the air. I stood with my back to the wind but the dust was dense and as airborne as smoke. I can imagine that it would present a breathing hazard in a confined space.