I'm sure anyone reading this blog has both smoke detectors and carbon monoxide (CO) detectors in their home. If not, do the right thing for safety and get them installed ASAP.
While fires and smoke are easy to see and smell with your own senses, CO is invisible, tasteless, and odorless. In short, it is dangerous because it combines with the hemoglobin in your blood and interferes with oxygen transport to body tissues. More info here.
My house is a split level, so I have two of each. One of each type is installed in the downstairs hallway outside the closet in which our furnace is located. The others are located in the hallway upstairs outside our bedrooms. My house was built in 1957, so I'm using battery powered units.
More modern units may be hard wired for electrical power. You can also get detectors which have a battery backup. That would be a good idea, since if fire or a CO problem strikes your home while the power is out you need to be alerted.
Tonight when I walked in after work I was greeted with the chirp from the downstairs CO detector which indicated that the batteries were low. While I was replacing the batteries in that one I also got did the upstairs unit. CO detectors have limited lifespans. When first introduced they lasted as short as two years, but more recent units may be good for up to seven. I checked my CO detectors and they are supposed to be good for seven years, but the manufacture date on them is 2005, so I'll be replacing them shortly. I just ordered two Kidde KN-COPP-B Front Load Battery-Operated Carbon Monoxide Alarm with Digital Display units from Amazon.com (yes that's a referral link) for delivery Thursday.