Carbon Monoxide Guide: Symptoms, Where CO Comes From & What Firefighters Do on CO Calls
Last updated: · 9 min read
Carbon monoxide is the most common cause of poisoning death in the United States. It is invisible, odorless, and tasteless — and it produces symptoms that most people mistake for the flu, food poisoning, or exhaustion. Fire companies respond to CO alarm activations daily, and the difference between a detector malfunction and an actual life-threatening CO level is measured on a meter, not by smell or symptoms. This guide covers what CO is, where it comes from, how to recognize exposure, and what fire companies do on CO calls.
Jump to:What carbon monoxide is · Where CO comes from · Symptoms and exposure levels · CO detector basics · Fire company CO response · CO meters · Treatment basics for EMS · Prevention for homeowners · FAQ
What Carbon Monoxide Is
Carbon monoxide (CO) is a colorless, odorless, tasteless gas produced by the incomplete combustion of carbon-containing fuels. Any fuel-burning appliance or process that does not have adequate oxygen supply for complete combustion will produce CO. The chemical formula is CO — one carbon atom bonded to one oxygen atom, one atom short of the two-oxygen-atom carbon dioxide (CO⊂2;) that is the product of complete combustion.
Why CO is so dangerous
CO binds to hemoglobin in red blood cells to form carboxyhemoglobin (COHb). The bond between CO and hemoglobin is approximately 200–250 times stronger than the bond between hemoglobin and oxygen. When CO binds to a hemoglobin molecule, that molecule can no longer carry oxygen. The result is cellular hypoxia — the body's cells are starved of oxygen even though there is oxygen in the lungs and the heart is pumping normally. This is why symptoms of CO poisoning appear similar to altitude sickness or anemia.
The blood's affinity for CO means that even relatively low concentrations of CO in the air — measured in parts per million (PPM) — can produce significant COHb saturation with prolonged exposure. A concentration of 200 PPM will produce headache and dizziness in 2–3 hours. Higher concentrations accelerate this dramatically.
Where Carbon Monoxide Comes From
Any combustion process can produce CO when oxygen is insufficient. The most common residential and commercial sources:
| Source | Mechanism | Common failure mode |
|---|---|---|
| Gas furnace | Incomplete combustion of natural gas or propane | Cracked heat exchanger leaks combustion gases into supply air; blocked flue; improper air-fuel mixture |
| Gas water heater | Incomplete combustion | Blocked flue or vent; insufficient combustion air in enclosed utility room |
| Attached garage | Vehicle exhaust | Running vehicle in attached garage; inadequate air seal between garage and living space |
| Gas range/oven | Burner combustion | Improper burner adjustment; using oven for space heating; unvented combustion in enclosed kitchen |
| Fireplace/wood stove | Incomplete combustion of wood | Blocked or downdrafting chimney; wet wood; restricted combustion air |
| Portable generator | Engine exhaust | Running generator inside garage or near window; CO enters living space through air infiltration |
| Charcoal grill/hibachi | Charcoal combustion | Using charcoal indoors or in garage; significant CO production even from small quantities of charcoal |
| Gas-powered tools | Engine exhaust | Operating pressure washers, concrete saws, or similar equipment indoors or in enclosed spaces |
Generators kill. Portable generator-related CO deaths spike after every major storm when people run generators indoors or in garages during power outages. A generator running in a garage with the door open can produce fatal CO concentrations inside an attached home within minutes. Generators must operate outdoors, at minimum 20 feet from any window, door, or vent.
Symptoms and Exposure Levels
CO poisoning symptoms progress with increasing concentration and duration of exposure. The most dangerous aspect: many victims attribute early symptoms to other causes and do not leave the building or seek help.
| CO concentration (PPM) | Duration | Symptoms |
|---|---|---|
| 35 PPM | 8 hours | Headache and dizziness (OSHA PEL for workplace exposure) |
| 200 PPM | 2–3 hours | Headache, dizziness, disorientation |
| 400 PPM | 2–3 hours | Life-threatening headache, dizziness, nausea within 45 minutes |
| 800 PPM | 45 minutes | Dizziness, nausea, convulsions; death within 2–3 hours |
| 1,600 PPM | 20 minutes | Headache, dizziness, nausea within 20 minutes; death within 1 hour |
| 3,200 PPM | 5–10 minutes | Headache, dizziness, nausea within 5–10 minutes; death within 25–30 minutes |
| 6,400 PPM | 1–2 minutes | Convulsions; death within 10–15 minutes |
| 12,800 PPM | Immediate | Immediate physiologic effects; death within 1–3 minutes |
The "flu" misidentification
Early CO poisoning symptoms — headache, nausea, fatigue, dizziness — are nearly identical to influenza, food poisoning, and labyrinthitis (inner ear disturbance). Multiple household members developing the same symptoms simultaneously is a key clinical indicator of CO poisoning rather than illness. CO poisoning does not cause fever. If multiple people in a household have flu-like symptoms without fever, CO should be considered immediately.
CO Detector Basics
CO detectors are required by code in most U.S. states in any residence with gas appliances or an attached garage. What firefighters need to know about how they work:
Alarm threshold
Residential CO detectors per UL 2034 are designed to alarm at:
- 70 PPM for 60–240 minutes
- 150 PPM for 10–50 minutes
- 400 PPM for 4–15 minutes
These thresholds are intentionally set higher than OSHA occupational limits to avoid nuisance alarms. This means a CO detector that has just activated may have been accumulating CO at lower levels for a significant period before alarming. "The detector just went off" does not mean CO levels are low.
Detector placement
CO detectors should be installed on every floor of the residence and within 15 feet of sleeping areas. CO is slightly lighter than air but disperses fairly uniformly throughout a space; mid-wall height (5 feet) is effective but ceiling or floor mounting also work for most residential detectors.
Detector lifespan
Most residential CO detectors have a 5–7 year lifespan. The electrochemical sensing cell degrades over time. A detector at the end of its life may fail to alarm at dangerous CO levels, may nuisance alarm, or may display end-of-life warnings. Check the manufacture date on the back of the unit — firefighters conducting CO investigations should note the detector age as part of their assessment.
