For many years there has been an increasing rate of complaints on the air quality in nonindustrial buildings. The occupants suffer from many diseases especially problems of breathing. Besides other reasons the air-conditioning systems themselves pollute the indoor air because dust, aerosols and other substances deposit inside the system. Therefore an airconditioning system, which has been operating 26 years was investigated. The aim was to find the main pollution sources of the system and possibilities to eliminate them. Some conclusions to problems of odour, cleaning and design of ventilation and air-conditioning systems are made.