Simplicity saves lives
Simple and comprehensible indicators will allow anyone to immediately assess the threat.
SafeCarbon monoxide was not detected or its concentration is extremely low, you can relax.
ModerateThe CO concentration could affect how you are feeling, make sure you air the room, check ventilation and find the source.
CO AlarmCO levels are high enough to be a threat to life - follow the rules applicable for critical levels of CO concentration.