In industrialized countries most of the time is spent indoors. The basic ambient parameters for a thermally comfortable indoor climate are air temperature, air velocity, humidity and radiation pattern. Besides the thermal component, the concentrations of air pollutants in the indoor air are also of importance for wellbeing and health. Their levels are influenced both by the outdoor concentrations and the indoor emissions. The increasing use of air conditioning systems in many cases has not resulted in improving the indoor climate but causes a wide range of irritations and health problems summarized as 'sick building syndrome'.