Abstract: An air conditioning system for a semiconductor clean room includes a chemical filter between an air conditioner including a humidifier and a ULPA filter of the clean room, for ionizing chemical impurities using moisture supplied from the humidifier and then adsorbing the ionized chemical impurities by using the chemical filter. The chemical filter is installed downstream of the humidifier, which applies phosphoric acid for the prevention of scale-formation. This downstream location allows the chemical filter to prevent the phosphoric acid from being included in the fresh air as a new chemical impurity, which makes regulation of temperature and humidity of the air possible without having to use a special and expensive pure steam system.