The average human's maximum lung pressure is a little above 1.4 psi ("Orders of magnitude (pressure)"). There is a little wiggle room between different people. Gabe C. and I demonstrate this fact by placing water in a straw and blowing on each end. The loser gets a mouthful of water and air.
The same limitations that apply to how
much a human can suck also apply the how much pressure
can be generated by blowing. After all, we are only
human.