315 ILCS 20/3-11
"Slum and Blight Areas" means those urban districts in which the major portion of the housing is detrimental to the health, safety, morality or welfare of the occupants by reason of age, dilapidation, overcrowding, faulty arrangement, lack of ventilation, light or sanitation facilities, or any combination of these factors. In St. Clair County, "slum and blighted area" also means any area of not less in the aggregate than 2 acres located within the territorial limits of a municipality where buildings or improvements, by reason of dilapidation, obsolescence, overcrowding, faulty arrangement or design, lack of ventilation, light and sanitary facilities, excessive land coverage, deleterious land use or layout or any combination of these factors, are detrimental to the public safety, health, morals, or welfare.
315 ILCS 20/3-11