The European AIRLESS programme was started to identify the pollution sources in air-handling units, to characterise these sources using measurement protocols developed on purpose, to propose measures to avoid or eliminate this pollution, and to assess the effect of such measures on the energy use. This paper addresses the energy use in air-handling units and proposes several energy efficient ways to provide good indoor air and even efficient indoor environment conditioning. Cooling energy may be important in office buildings, even in temperate climate. An efficient and cheap cooling strategy is to combine a mechanical ventilation system designed for the minimum hygienic airflow rate with night passive cooling using natural ventilation. (C) 2004 Published by Elsevier B.V.