Frequently, the dominant pollutant is ‘heat’ itself. Particularly in large commercial office buildings, high heat loads are developed through lighting, computing and other electrical sources. Further heat gains are derived from occupants, solar radiation and high outdoor temperatures. These factors make cooling of the indoor air essential. The choice is either to introduce refrigerative cooling or to introduce ventilation cooling. In either case heat gains should be minimised by good building design and reduced power consumption. Refrigerative cooling is energy intensive and contributes to peak power loads. Often, however, climate conditions dictate no other choice especially when the humidity level must be controlled. When refrigerative cooling is needed, ventilation must be minimised to prevent the unnecessary loss of conditioned air. Cooling is sometimes possible by introducing cooler, outdoor air (cooling by ventilation). This may be through window opening or by mechanical means. Ventilation rates for cooling will normally be well in excess of that needed to meet the basic fresh air requirements of occupants but may, nevertheless, accomplish dramatic energy savings over refrigerative cooling. The choice between ventilation cooling and refrigerative cooling is a function of heat gains, humidity loads and outdoor climate. Reducing heat gains by good building design ( e.g. minimising solar gains and introducing thermal mass) and by introducing low energy lighting and night cooling can often bring the threshold in favour of cooling by ventilation or reduce the periods in which refrigerative cooling is necessary.