Reports say the power went off in the house and one resident hauled a gas-powered generator into the laundry room and fired it up.
A door and a window were left open, but within a half-hour, the fumes became overpowering.
One resident called 911 about 11 PM and when firefighters arrived, they found one person unconscious in a doorway.
That woman, her two roommates and a 15-month-old child were rushed to University Hospitals in Iowa City for carbon monoxide poisoning.
All four are expected to survive.