Carbon Monoxide
About This Health Driver
Carbon monoxide is a colorless, odorless gas produced by incomplete combustion of carbon-based fuels. Indoor concentrations in homes without combustion appliances are typically near outdoor background (under 1 to 2 ppm in non-urban locations, somewhat higher in urban areas). Concentrations rise with combustion appliance operation, including gas stove use, fireplace burns, and atmospherically vented water heater or furnace cycles, and rise further when combustion gases backdraft from intended exhaust paths into occupied space. Backdrafting becomes likely when negative pressure within the building exceeds the draft strength of an atmospherically vented appliance, a common consequence of envelope tightening, large exhaust fan operation, or duct leakage in unconditioned space. Standard UL 2034 residential alarms do not respond below 70 ppm sustained for at least an hour; sub-alarm concentrations between 5 and 50 ppm are documented in homes with poorly maintained or poorly drafted combustion equipment and can persist for years without detection.
How It Affects Bodies
Acute high-dose CO exposure produces hypoxic injury through hemoglobin binding (CO binds hemoglobin with approximately 250 times the affinity of oxygen) and through inhibition of cellular oxygen utilization at cytochrome c oxidase. Chronic low-dose exposure has been documented to produce persistent neurological symptoms (headache, fatigue, cognitive impairment, mood disturbance, autonomic dysregulation) in occupants of homes with problem combustion installations, with dose-response patterns reported even where alarm thresholds are never crossed. Mechanistically, sub-poisoning CO concentrations modify myelin basic protein through lipid peroxidation adduct formation, demonstrate selective cytotoxicity to oligodendrocytes, drive systemic inflammatory responses through neutrophil activation, and disrupt cellular respiration through cytochrome inhibition. The full body of evidence on chronic low-dose effects remains incomplete; existing research weighs heavily toward acute poisoning sequelae.
Where It Comes From
- Gas stoves and ranges: incomplete combustion during normal operation, particularly without effective range hood capture. - Gas stoves and ranges: incomplete combustion during normal operation, particularly without effective range hood capture.
- Atmospherically vented gas water heaters: combustion gas spillage when room pressure exceeds flue draft. - Atmospherically vented gas water heaters: combustion gas spillage when room pressure exceeds flue draft.
- Atmospherically vented gas furnaces and boilers: similar backdraft mechanism. - Atmospherically vented gas furnaces and boilers: similar backdraft mechanism.
- Wood-burning stoves and fireplaces: intermittent downdrafting, particularly during cold-flue startup or under negative house pressure. - Wood-burning stoves and fireplaces: intermittent downdrafting, particularly during cold-flue startup or under negative house pressure.
- Vented gas fireplaces: backdraft under common depressurization scenarios. - Vented gas fireplaces: backdraft under common depressurization scenarios.
- Ventless gas fireplaces and decorative log sets: combustion products released into occupied space by design. (Chronic low-dose effects from intended in-room combustion release have not been characterized for sensitive populations.) - Ventless gas fireplaces and decorative log sets: combustion products released into occupied space by design. (Chronic low-dose effects from intended in-room combustion release have not been characterized for sensitive populations.)
- Attached garages with idling or recently operated vehicles: CO infiltration through shared envelope, particularly when bedrooms are above garages. - Attached garages with idling or recently operated vehicles: CO infiltration through shared envelope, particularly when bedrooms are above garages.
- Aging propane systems in RVs, mobile homes, and off-grid installations: corroded or improperly installed combustion equipment. - Aging propane systems in RVs, mobile homes, and off-grid installations: corroded or improperly installed combustion equipment.
- Outdoor air infiltration in urban or traffic-adjacent settings: vehicle exhaust as ambient source. - Outdoor air infiltration in urban or traffic-adjacent settings: vehicle exhaust as ambient source.
How to Address It
- All-electric specification [Mechanical] : Eliminate combustion at the appliance level across all building systems. Highest-leverage single decision available. - All-electric specification [Mechanical] : Eliminate combustion at the appliance level across all building systems. Highest-leverage single decision available.Operations
- Induction or electric cooking [Mechanical] : Preferred over gas regardless of range hood specification. - Induction or electric cooking [Mechanical] : Preferred over gas regardless of range hood specification.Operations
- Heat pump water heater [Plumbing] : Replaces atmospherically vented gas water heater. - Heat pump water heater [Plumbing] : Replaces atmospherically vented gas water heater.Operations
- Heat pump space heating [Mechanical] : Replaces gas furnace or boiler. - Heat pump space heating [Mechanical] : Replaces gas furnace or boiler.Operations
- Sealed-combustion equipment [Mechanical] : Direct outdoor combustion air for builds where combustion is retained. Never specify atmospherically vented equipment for sensitive occupants. - Sealed-combustion equipment [Mechanical] : Direct outdoor combustion air for builds where combustion is retained. Never specify atmospherically vented equipment for sensitive occupants.Operations
- Combustion appliance zone (CAZ) testing [Mechanical] : Worst-case depressurization testing at commissioning and after envelope changes. - Combustion appliance zone (CAZ) testing [Mechanical] : Worst-case depressurization testing at commissioning and after envelope changes.Operations
- Low-level CO monitor [Operations] : 10 to 30 ppm sensitivity, continuous readout (not alarm-only), bedroom and primary daytime zone placement. - Low-level CO monitor [Operations] : 10 to 30 ppm sensitivity, continuous readout (not alarm-only), bedroom and primary daytime zone placement.Operations
- Garage isolation [Envelope] : Gasketed weather-stripped door, no return-air ducting from the garage, exhaust fan vented to outdoors. - Garage isolation [Envelope] : Gasketed weather-stripped door, no return-air ducting from the garage, exhaust fan vented to outdoors.Operations
- Bedroom siting [Site] : No bedrooms above attached garages or adjacent to combustion equipment locations. - Bedroom siting [Site] : No bedrooms above attached garages or adjacent to combustion equipment locations.Operations
- Range hood with verified capture efficiency [Mechanical] : Make-up air provision for hoods over 400 cfm. - Range hood with verified capture efficiency [Mechanical] : Make-up air provision for hoods over 400 cfm.Operations
- Annual combustion appliance servicing [Operations] : Operating-manual specification for any retained combustion equipment. - Annual combustion appliance servicing [Operations] : Operating-manual specification for any retained combustion equipment.Operations