The impact of increased ventilation on polychlorinated biphenyl (PCB) air concentration by installation of mechanical balanced ventilation units was studied. The intervention was carried out in three PCB-contaminated rooms; one classroom in an elementary school and two small bedrooms in an apartment in a residential building. In the classroom, the air exchange rate (ACH) was raised from 0.2 (without mechanical ventilation) to 5.5 /h during the intervention. In the two bedrooms, the highest ACH was 6.6 /h and 0.5 /h without mechanical ventilation. The corresponding concentration decrease achieved from the intervention was 30% and in one of the bedrooms 45%. Emissions of PCB rose dramatically during periods of increased ventilation as the drop in concentration did not match the increased ACH. When ventilation in the bedrooms was increased incrementally from 0.5 to 2.2, 4.5, and 6.6 /h, emissions were found to increase linearly with increasing ACH. Compared with the sparse literature regarding estimated reductions due to ventilation, the measured effect on concentrations was less than expected.