It depends to some degree on your technique. It's certainly less than a spray can. I've seen videos where the brush is held very close to the model and it looked like there was hardly any overspray. I tend to cover a larger area and I get some, but not too much. I work in a small room in my garage next to an open door, and I don't have or have room for a spray booth. So I just rig up a newspaper drop cloth and I don't see any problem. However, I wear an organic vapors mask to protect my lungs. Airbrush spray is so fine that it can drift quite a ways, and I don't want to breathe it. But whatever drifts around and lands in my little shop is indistinguishable from the rest of the dust.
Don