Originally used for industrial buildings for ventilation and to remove airborne contaminants from the occupied zone by natural convection, displacement ventilation is now applied to office buildings. Compared to conventional air conditioning, it reduces the concentration of polluting particles and gases generated by structural materials, furnishings and the occupants themselves. Conventional air conditioning relies on mixing the room air with the cold supply air, typically from ceiling diffusers (giving between 5 and 15 air changes per hour). Displacement ventilation uses about 2.5 to 3 air changes per hour  and is designed to minimize mixing in the occupied zone. Arguably this lower mixing can reduce sick building syndrome , but local discomfort due to draft and the vertical temperature gradient may be critical .