CARBON MONOXIDE

• CO is an odorless colorless gas that is harmful in high levels or prolonged low levels. It is 
  produced by the incomplete burning of fuels. If you have an appliance flame that is burning 
  yellow instead of blue, or giving off soot, that is a warning. It is the same weight as air. 
  You cannot smell it, you cannot see it.

• It bonds to the hemoglobin at the oxygen receptor. The bond is 200X stronger then oxygen.
  It can kill you, but also do brain damage. It effects all organs. Pets might be affected first since 
  they are smaller.

• CO can be detected by sensors. Most common and popular is the electromechanical type.
  Sensors can fail at any time. They have a limited and indeterminable life span.
  Most units last 3-6 years. The test button only tests the battery and circuitry, not the sensor,
  although the newer ones might be better. Just because you have a sensor with good batteries
  does not mean you are safe.

• The sensors can be tested with CO. You get that in a spray can for about $15. on line.
  It is not carried by local hardware stores.
  You put the unit in a zip lock bag and spray in some CO. Make sure the batteries are working.
  A sensor should be tested when installed and then at least every year.
  So, there are two tests that need to be preformed on the alarm: battery/circuitry and sensor.

  https://en.wikipedia.org/wiki/Carbon_monoxide_detector