Update... I read the manual and called the manufacturer. The units that are showing 10ppm need to be replaced. The tech said to believe the new unit which reads zero. I've moved the new unit around the house and next to the others and it always stays at "0" even next to a unit reading "10."
According to OSHA a CO concentration of up to 50ppm for an 8-hour period is okay for a healthy individual. A concentration of 200ppm will cause nausea, headache, fatigue. A concentration of 800ppm will cause death in 2-3 hours. I know that when CO2 is present there is also a (much) smaller amount of CO present, both are colorless/odorless gases.
FYI: Common sources of CO include: Gas appliances not properly ventilated Using an oven for heating the house Using a propane or charcoal grill indoors Running a gasoline engine in an enclosed or partially-enclosed area Leaving a house door open to a garage that has a vehicle running
I appreciate all your responses and encourage everyone to have a CO detector in addition to a smoke detector on all levels of your home.