Propane isn't toxic per se, but it displaces oxygen. In a room wiht a constant propane flow, you can suffocate. But from shooting a GBB, I wouldn't be concerned. Propane is heavier than air, so it basically drops to the ground. And it dissipates rather quickly. The smell will linger for a while, so if you can open a window to let fresh air in, that's probably a good idea. It'll help dissipate the smell faster.