Venting of potable water tanks must be carefully considered to prevent the ingress of all of the following EXCEPT:

Venting of potable water tanks is crucial to maintain water quality and safety. The primary purpose of venting is to allow air to escape as the tank fills or drains, which helps to prevent a vacuum that could damage the tank or impede proper water flow. Adequate air circulation is necessary to avoid pressure build-up inside the tank.

The concern with venting systems is to prevent contaminants from entering the potable water supply. Sea water, mosquitos, and gray water all represent potential threats to the integrity of drinking water. Sea water could cause salinity issues, mosquitos can introduce harmful pathogens, and gray water contains organic material and germs that could compromise water safety.

In contrast, the necessity to allow air to enter and exit the tank serves the functional aspect of the system rather than posing a contaminant risk. Thus, while it may seem counterintuitive, allowing air into the tank is acceptable and necessary to maintain the tank's operation and function.