We have all heard about incidents with carbon monoxide (CO). Many home inspectors are not familiar with the sources or the “acceptable” exposure levels. The sources are straightforward: improperly vented combustion appliances and motor vehicles. Any home with combustion appliances, stoves, furnaces, water heaters, dryers, or an attached garage should have at least one CO detector.
So what are the acceptable levels? It depends on whom you ask. According to OSHA, you can be exposed to 50 parts per million as an average during an 8-hour workday. NIOSH says 35 ppm and has a 200 ppm ceiling (maximum) limit. The ACGIH says 25 ppm. But these are all workplace requirements.
In the home, the Environmental Protection Agency is the one that sets the “recommended” limits. This limit is set at 9 ppm for occupants in a home. Other entities, WHO and ASHRAE, have the same 9 ppm limit. There are studies that indicate that even low-level exposures can have long-term health effects, especially if the occupants already have pre-existing pulmonary (lung) or circulatory (heart) conditions.
Studies indicate that even low-level CO exposures in the home can have long-term health effects, especially if the occupants already have pre-existing lung or heart conditions.
At low-level concentrations, the symptoms are fatigue in healthy people and chest pain in people with heart disease. At higher concentrations, symptoms include impaired vision and coordination, headaches, dizziness, confusion, and nausea. Exposures can cause flu-like symptoms that clear up after leaving the home.
Acute effects are due to the formation of carboxyhemoglobin in the blood, which inhibits oxygen intake. At moderate concentrations, angina, impaired vision, and reduced brain function may result. At higher concentrations, CO exposure can be fatal.
Different from smoke alarms
CO detectors do not function like smoke detectors. They have a certain concentration of CO that must be attained over a certain time period. One manufacturer, for example, has the following requirements for the alarm to sound (threshold):
Parts Per Million
Detector Response Time, Minutes
30 +/- 3ppm
No alarm within 30 days
70 +/- 5ppm
150 +/- 5ppm
400 +/- 10ppm
CO detectors should be placed much like smoke detectors: in bedrooms, in hallways, one per floor minimum, etc. Consult manufacturer’s installation instructions for proper placement.
New construction: For new construction, CO detectors must be provided in dwelling units where either or both of the following conditions exist. (1) The dwelling unit contains a fuel-fired appliance. (2) The dwelling unit has an attached garage with an opening that communicates with the dwelling unit.
CO alarms in dwelling units must be installed outside of each separate sleeping area in the immediate vicinity of the bedrooms.
Alterations, repairs and additions: Where alterations, repairs or additions requiring a permit occur, or where one or more sleeping rooms are added or created in existing dwellings, the individual dwelling unit must be equipped with CO alarms located as required for new dwellings. Exceptions are: (1) work involving the exterior surfaces of dwellings, such as the replacement of windows or doors, or the addition of a porch or deck and (2) installation, alteration, or repairs of plumbing or mechanical systems.
Location: CO alarms in dwelling units must be installed outside of each separate sleeping area in the immediate vicinity of the bedrooms. Where a fuel-burning appliance is located within a bedroom or its attached bathroom, a CO alarm must be installed within the bedroom.
Steven O’Donnell is the Co-founder of Newcomer’s Inspection Services, which began in 1994. He is a professional home inspector in the State of Arizona and a Graduate of the University of Michigan with a Bachelor of Science degree. As a senior trainer for AHIT, O’Donnell revised the 700-plus page go-to industry guide, “A Practical Guide to Home Inspection,” and AHIT’s “Professional Home Inspection Online Course.”
Kristin is the Marketing Director at AHIT. She has authored content for numerous real estate brands, and managed corporate communications for a public real estate company. She is passionate about the home inspection and real estate industries, and loves digging into research to provide insights that empower home inspectors and real estate agents in their businesses.