MAKE SURE YOU HAVE A PHOTOELECTRIC SENSOR. LOOK FOR A "P" ON THE PACKAGE. Smoke detectors are now recommended in each bedroom, hallway and each floor. They are to be placed not less than 4 nor more than 12 inches from a corner (presumably because smoke adheres to walls as it rises). Combination smoke/CO detectors are now available. Note also continunous radon detectors can be purchased (see previous post on this blog). Now all we need are detectors that will beep right before I'm going to say something stupid to my wife!
Here's a combination Carbon Dioxide and Photoelectric model that also works on Lowes' Iris home management system. Click on image for link.