The condition and cleanliness of ventilation systems were studied at four buildings with a visual inspection method and three quantitative measuring methods. The amount of dust was low in the supply air systems which were cleaned within 5 years or system was installed within 10 years. The supply air system which was installed 20 years ago and system was not cleaned after installing the amount of accumulated dust was high in the system. Studied extract air systems were cleaned 5 years ago and some exhaust air terminal devices and air flow controllers were so highly contaminated that it may effect on air flow rates and even function of the systems. It was also found some correlation between Finnish vacuum test method and optical tape method. For example the dust accumulation 0.7g/m² measured with the Finnish vacuum test method correspond approximately to the dust cover percent of 13% measured with optical tape method.