Asphyxiation is the biggest potential issue I guess. 20 lb of CO2 will fill 162 cu ft of space at STP, or reduce the oxygen to <15% (~danger level) in 650 cu ft, if that space is mostly sealed. If there's any air change, then the leak rate is more important, and the size of the space required to be dangerous is much smaller.
Basically, keep your tank in a largish room (bigger than 10' x 10' x 6.5'), with some air flow in it. Don't keep it in the keezer, then stick your head into the keezer to check it for leaks and don't keep 20lb or 50lb tanks in small walk-in coolers.