Underground excavations in which an airflow is present are affected by both natural and technological sources of heat. As a result, the air flowing through the mine excavations is characterised by high temperatures, which—combined with high humidity—causes microclimatic conditions to deteriorate. Bad microclimatic conditions affect miners underground, causing a reduction of perception, concentration, attention, and perceptiveness. This negative influence of temperature and humidity on the human body is referred to as the climatic hazard. For this reason, the design of ventilation and air conditioning systems to be implemented in underground mining plants is an issue of increasing significance. This article investigates the performance of cooling systems in the Polish hard-coal mining industry. Based on the conducted research on air-cooling installations in six hard-coal mines, the actual efficiency of air-cooling is presented. The calculated values of cooling power were compared with the design intent. The obtained results have allowed identifying with greater precision the factors that diminish the efficiency of cooling systems in the mining plants under analysis. These factors may be treated on equal terms with key performance indicators (KPIs), which can be used to monitor the performance of cooling system components and to provide managers with high-level indicators on which decisions can be based.