CO levels

Hi Everyone,
When you are checking the hot water heater and the furnace, what are considered high CO levels that should be reported?

There are no acceptable levels of CO to be found in a home inspection. All should be reported. A properly vented unit will exhaust all gases outside of the building. Recommend that the unit be serviced and allow the HVAC tech to make the necessary adjustments.

OSHA has “acceptable” limits for people to be exposed to over an 8-hour day, but let’s think about this for a moment. Let’s say that your client works in an automotive garage and is already exposed to the maximum ppm by the time he gets home, at which time his wife fries him a huge dinner on the gas stove that does not exhaust to the outdoors, but blows the CO back into the kitchen…and after dinner, he smokes a pipe next to his unvented, “ventless” fireplace that also puts CO in the room. What would be a “safe” level to have him breathe from the water heater?

Report it all…and recommend CO detectors in every home that uses fossil fuels.

Thanks James!

CO Levels in Homes

Average levels in homes without gas stoves vary from 0.5 to 5 parts per million (ppm). Levels near properly adjusted gas stoves are often 5 to 15 ppm and those near poorly adjusted stoves may be 30 ppm or higher.

Standards or Guidelines

No standards for CO have been agreed upon for indoor air. The U.S. National Ambient Air Quality Standards for outdoor air are 9 ppm (40,000 micrograms per meter cubed) for 8 hours, and 35 ppm for 1 hour


Here are some CO numbers for you:
9ppm ASHRAE standard for allowable spillage from vented appliances, indoors for 8 hours exposure daily.

15-20ppm World Health Organization lists as causing impaired performance

25ppm Max allowable in a Parking Garage (International Mechanical Code)

35ppm US NIOSH recommended 8 hr max workplace exposure

50ppm US OSHA recommended 8 hour max workplace exposure

BPI Numbers: (Not ambient air numbers)(ambient air 35 or great leave the house)

              Oven less than 100ppm
              Combustion Safety Test (Worst case CAZ) - water heater 0-25ppm if it passes spillage and draft tests

Normal level in human tissues produced by body (non-smoker) 1-4ppm