Can I safely brush down old spray foam that is very dirty?
The dust by itself isn't dangerous since the foam's not breaking up but large quantities of regular dirt and dust are not good to breath under any circumstance. You would be better off getting a good shop vac and vacuuming the foam with the attachment brush. If you could get one with a long hose so the pump wouldn't be exhausting into the same room it would be better. You should also wear a mask to keep from inhaling the dust.