The air in contact with open cooling water systems often contains many of the same suspended materials found in surface water makeup sources such as rivers and lakes. Because the cooling tower uses fans to draw in air to maximize the efficiency of evaporative cooling, the tower in effect functions to scrub the air of any sand, silt, clay, dirt or bacteria that it may contain. These contaminants add to the overall suspended solids loading in the cooling system and, for reasons discussed in the previous section, will increase the potential for fouling in the system.

Airborne contamination by gases can also increase deposition. Oxygen and carbon dioxide accelerate corrosion, leading to deposition and further corrosion by the underdeposit mechanism. Since pickup of both gases occurs continuously, near saturation levels of these dissolved gases are present in the water. Gaseous contaminants such as sulfur dioxide, hydrogen sulfide and ammonia may also be scrubbed from the air. The first two reduce oxidizing corrosion inhibitors to insoluble foulants. Furthermore, hydrogen sulfide is very corrosive and quickly forms iron sulfide deposits which lead to further corrosion. Ammonia selectively corrodes copper and its alloys, leading to the deposition of copper corrosion products.