Effective January 1, 2007, Illinois State law requires an approved carbon monoxide detector be installed in residences. 

Approved carbon monoxide detectors can be obtained an local hardware and retail stores.  Various models are available.  The law does  require the units "be either battery powered, plug-in with battery back-up, or wired into the structure's AC power line with secondary battery back-up."

The law requires one detector within a defined measurement of every sleeping area in the structure. Detectors are to be installed by the owner of the property, and maintained by the occupant of the property if different. The law does exempt some properties from having a CO detector if the structure  "(i) does not rely on combustion of fossil fuel for heat, ventilation, or hot water; (ii) is not connected in any way to a garage; and (iii) is not sufficiently close to any ventilated source of carbon monoxide, as determined by the local building commissioner, to receive carbon monoxide from that source."

For a complete reading of the law as enacted, click here.