A simple example is the vertical space between a wall-mounted faucet and the
sink rim (this space is the air gap). Water can easily fall from the faucet into the sink, but there is no way that water can be drawn up from the sink into the faucet. This arrangement prevents any contaminants in the sink from entering into the
potable water system by
siphonage; this is the simplest form of
backflow prevention. A common use of the term "air gap" in domestic plumbing refers to a specialized fixture that provides backflow prevention for an installed
dishwasher. This "air gap" is visible above the sink as a small cylindrical fixture mounted near the faucet. In the base cabinet under the sink, the drain hose from the dishwasher feeds the "top" of the air gap, and the "bottom" of the air gap is plumbed into the sink drain below the basket, or into a
garbage disposal unit. When installed and maintained properly, the air gap works as described above, and prevents drain water from the sink from backing up into the dishwasher, possibly contaminating dishes. To further illustrate the air gap, consider what could happen if the air gap were eliminated by attaching a
hose to the faucet and lowering the hose into a sink full of contaminated water. Under the right conditions (if the water supply loses pressure and the sink is higher than the point at which the water supply enters the house, for instance), the dirty water in the sink could be siphoned backwards into the water pipes through the hose and faucet. The dirty water could then be dispersed throughout the
drinking water system. ==Standards and codes==