The Dutch Standard NEN 1087 "Ventilation of dwellings": Requirements (1) is at this moment under review. A requirement can be found about the quality of air. In fact a statement is made that outside air is required as fresh air for bedrooms. Bathroom, kitchen, W.C. and living-room are allowed to be ventilated with air from other rooms. During the last years air heating systems became more popular. These systems have,in its most simple form,recirculation of air from the living-room to the bedrooms. During the reviewing process of the standard, TNO has carried out some studies to investigate the differences in concentrations of contaminants in dwellings due to different ventilation and heating systems. The aim of the studies was to evaluate several heating/ventilation systems on pollutant levels in houses.