Which of the following is a common characteristic of evaporative coolers?

Evaporative coolers, also known as swamp coolers, work by utilizing the principle of evaporation to cool the air. As warm air passes through the water-saturated pads, the water evaporates, absorbing heat from the air and thereby lowering its temperature. This process inherently increases the humidity level of the air because moisture is added to it during cooling.

High humidity levels can be a common result of using evaporative cooling systems, which distinguishes them from traditional air conditioning units that remove humidity from the air as part of their refrigerant cycle.

The other characteristics mentioned, such as using refrigerant cycles, would pertain more to traditional air conditioning systems rather than evaporative coolers. Operating under low-pressure air or requiring electrical power can also apply to various cooling systems, but those are not defining features unique to evaporative coolers. Therefore, the correct characteristic that is consistently true for evaporative coolers is that they increase humidity.