Why is it important to maintain positive air pressure in an controlled environment?

If there is a positive air pressure in a controlled environment any breach will allow air to flow outwards, rather than allow contaminated air to flow inwards. Most controlled environments will have a double air lock for entry and exit.Why is it important to maintain positive air pressure in an controlled environment?
When a balloon is let loose, until the positive pressure inside the balloon has gone, the air outside the balloon has no chance of getting inside the balloon.





Therefore, set up a positive pressure in a controlled environnment and you can assure the purity of the air within that environment