A simple example is the space between a wall mounted faucet and the sink rim (this space is the air gap). Water can easily flow from the faucet into the sink, but there is no way that water can flow from the sink into the faucet without modifying the system. This arrangement will prevent any contaminants in the sink from flowing into the potable water system by siphonage and is the least expensive form of backflow prevention.
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 will be siphoned into the water pipes through the hose and faucet. The dirty water then will be dispersed throughout the drinking water system.
All plumbing codes require backflow prevention in several ways. The plumbing fixture manufacturers build the fixtures to meet these codes. A plumber must not build cross-connections in his daily work practices, and Plumbing Inspectors look for improper designs or connections of piping and plumbing fixtures.
An air gap must meet the requirements of being two times the inner diameter of the pipe (2*D) in order to be sufficient. However, an air gap MUST NOT be less than one inch, and NEED NOT be more than twelve inches. Air gap is measured from the end of the faucet to the upper rim of the reservoir (sink, drain, etc.).