When CO is produced
Complete propane combustion produces CO2 and water — both harmless in normal residential concentrations. Incomplete combustion produces CO. Incomplete combustion happens when:
- Vent is blocked — animal nest, snow accumulation, leaves, debris
- Burner is malfunctioning — clogged orifice, wrong air-fuel ratio, worn components
- Combustion air supply is restricted — sealed-up modern home with no makeup air, blocked combustion-air opening
- Appliance is poorly maintained — buildup, drift from design tolerances over time
Why CO is so dangerous
Three factors combine to make CO lethal:
- Colorless and odorless — no warning
- Hemoglobin binding is ~240× stronger than oxygen — small concentrations displace blood oxygen
- Cumulative — sustained low-level exposure builds up over hours, causing increasing impairment without obvious symptoms
Symptoms of CO poisoning
Early: headache, fatigue, nausea, dizziness, mild confusion. Later: drowsiness, severe confusion, loss of consciousness. Death follows sustained exposure to elevated concentrations. Pets often show symptoms before humans because of smaller body mass and higher metabolic rate — a sick pet near a combustion appliance is a CO red flag.
Detection — CO detectors are essential
Every US home with combustion appliances should have working UL-listed CO detectors on every occupied floor. See CO detectors for selection and placement. Battery-only detectors fail when batteries die; combination smoke-and-CO units with hardwired backup are the strongest option.
Prevention
- Annual professional inspection of every combustion appliance (furnace, water heater, range, dryer, fireplace)
- Keep vents clear — check exterior vent caps for blockage seasonally, especially after snow and storm events
- Adequate combustion air — modern airtight homes may need dedicated makeup-air systems
- CO detectors on every floor, replaced per manufacturer schedule (typically 7–10 years)
- Never use outdoor propane equipment indoors — grills, patio heaters, camp stoves
FAQ
How much CO is dangerous?
OSHA workplace exposure limit: 50 ppm over 8 hours. UL-listed home CO detectors typically alarm at 70 ppm over 60 minutes (or higher concentrations over shorter periods). At 800 ppm, unconsciousness in 2 hours, death in 3.
If my detector alarms, what do I do?
Leave the building, call 911, do not re-enter until ventilated and cleared. Even if symptoms seem mild, CO can cause delayed cardiovascular and neurological effects — medical evaluation is appropriate after any positive alarm.