The Lexington Fire Department said a couple fell ill from carbon monoxide poisoning in their home on Clinton Road.
The original call for help to the Fire Department was for a man suffering a stroke.
Fire crews said when they entered the home, their carbon monoxide detectors immediately went off, indicating high levels of the colorless, odorless gas.
The Lexington Fire Department equips each of its bags with the CO2 monitors.
"The monitor not only helped save their life, but it let the crew know not to leave anyone because if they didn't have a CO2 detector, we could have left a family member there and suffer the same fate," said Battalion Chief Ed Davis.
Davis urged everyone to get a carbon monoxide detector, which usually cost anywhere from $10-to-$50.