An air gap is the vertical distance through the atmosphere between the lowest potable water outlet and the highest level of the source of fluid contamination. In other words, it is the point of separation of potable and non-potable piping.
A water treatment system installed with a direct drain connection to a sewer system, a cross connection, has a risk of contamination most anytime. It can happen when a sewer backs up, which is the most common, or when there is a loss of pressure in the system.