35 ILCS 120/1a
"Pollution control facilities" means any system, method, construction, device or appliance appurtenant thereto sold or used or intended for the primary purpose of eliminating, preventing, or reducing air and water pollution as the term "air pollution" or "water pollution" is defined in the "Environmental Protection Act", enacted by the 76th General Assembly, or for the primary purpose of treating, pretreating, modifying or disposing of any potential solid, liquid or gaseous pollutant which if released without such treatment, pretreatment, modification or disposal might be harmful, detrimental or offensive to human, plant or animal life, or to property. Until July 1, 2003, the purchase, employment and transfer of such tangible personal property as pollution control facilities is not a purchase, use or sale of tangible personal property.
35 ILCS 120/1a