Emergency crews reopened the Physical Sciences Building at Eighth and Carter on the ISU campus after a student reported the scent of gas in the building.
Firemen said one student complained of feeling ill and was treated and released at the scene shortly after 1 pm Tuesday.
Equipment used to detect gas leaks could find no sign of any trouble, but a Hazardous Materials incident team was called in to investigate the scene. Classes in the building were dismissed for the day.
University officials reopened the building at about 5 p.m.