Outdoor and indoor design conditions are used to calculate the design space loads. In energy use calculations, hour-by-hour outdoor climate data of a design day should be adopted instead of summer and winter design values. Indoor design criteria, such as space temperature, humidity and air cleanliness, specify the requirements for the indoor environment as well as the quality of an air-conditioning or heating, ventilating, air-conditioning, and refrigerating project. There are three basic means of reducing the concentration of indoor air contaminants and improving indoor air quality: eliminate or reduce the source of air pollution, enhance the efficiency of air filtration and increase the ventilation air intake. Dilution of the concentrations of indoor contaminants by outdoor ventilation air is often the simple and cheapest way to improve indoor air quality. Most air-conditioning systems are designed to maintain a slightly higher pressure than the surroundings, a positive pressure, to prevent or reduce infiltration and untreated air entering the space directly.