Vents release air and are a dynamic and integral part of tank operation. The air pressure inside of a tank is always trying to equalize with the air pressure outside as the water level rises and falls in the tank. When the tank is filling with water, displaced air has less space and puts pressure on the tanks. The air is forced out of the tank through the vent and overflow as well, if it is not overflowing with water. When water is drawn out of the tank, the air has more space and creates a vacuum. Outside air is pulled into the tank through the vent and overflow. Thin walled metal tanks can be protected against excessive pressure and vacuum with a pressure/vacuum relief mechanism. Also, storage tank vents cannot serve as the overflow; tanks must have a vent separate from the overflow. Protection from contamination being inhaled through the vent Vents present a pathway for contamination to enter the tank. Having the vent opening at least 24 inches above the nearest horizontal surface protects against the inhalation of contamination (dried feces, dust, etc.). A bird dropping can contain thousands of salmonella. Bird spikes can be added to any intermediate horizontal surfaces. In some cases, the height of the vent should be raised higher than 24 inches to address severe problems with birds or other animals. Vents not accessible for inspection can trigger a significant deficiency.