Carbon Monoxide Guide: Symptoms, Where CO Comes From & What Firefighters Do on CO Calls

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.

Fire Company CO Response: What Happens

A CO alarm activation is a fire service call, not a nuisance alarm by default. Every CO activation is treated as a potential life safety event until the meter says otherwise. The response sequence:

1. Don SCBA before entry. If CO levels are unknown, crews enter with SCBA. CO is immediately dangerous to life and health (IDLH) at 1,200 PPM. Readings that high are possible with significant appliance failure.
2. Evacuate occupants immediately. Before meters are read, all occupants are moved to fresh air outside the structure.
3. Assess for symptoms. Any occupant with headache, nausea, dizziness, or altered mental status is treated as a potential CO poisoning patient and EMS is notified or upgraded.
4. Read the detector. Note the CO level displayed on the detector (many modern detectors display current and peak PPM). Does it match what the meter finds?
5. Meter the structure. Use a calibrated CO meter to measure CO levels in all rooms, particularly near potential sources. Identify the highest reading and the likely source.
6. Identify the source. Check all gas appliances, the garage, and any combustion equipment. A furnace with a cracked heat exchanger, a vehicle left running, or a malfunctioning water heater must be identified before the structure is cleared.
7. Ventilate. Open windows and doors to ventilate the structure and allow CO levels to drop to safe levels before reoccupancy.
8. No reoccupancy until source is identified and mitigated. A structure should not be cleared for reoccupancy until the CO source has been identified and repaired or shut down by a qualified technician. A cleared CO level with an unidentified source means the CO will return when the source runs again.

CO Meters: What Firefighters Use

Most engine companies and hazmat units carry handheld CO meters that provide real-time PPM readings. Key points for effective meter use:

• Calibrate regularly. CO meters require periodic calibration with certified CO reference gas. An uncalibrated meter may read significantly higher or lower than actual CO levels.
• Allow warmup time. Many CO meters require 30–60 seconds of warmup before readings are stable. Taking a reading immediately upon powering on may not be accurate.
• Check for cross-sensitivity. Some CO meters are sensitive to hydrogen gas (produced by batteries) and certain other gases. Know your meter's cross-sensitivity limitations.
• Note peak and current readings. Some meters log the highest reading since being turned on. This peak reading may be more relevant than the current reading if CO sources have been disrupted before meter use.
• Document readings by room. A structured survey with readings documented by location helps identify the source and provides a defensible record for reoccupancy decisions.

Treatment Basics for EMS: CO Poisoning

First-line treatment for CO poisoning:

• Remove from exposure immediately. The half-life of carboxyhemoglobin (the CO-hemoglobin bond) in room air is approximately 4–5 hours. In high-flow oxygen, the half-life drops to approximately 60–90 minutes. Every minute in fresh air (or on oxygen) improves the prognosis.
• High-flow 100% oxygen via non-rebreather mask. For any symptomatic patient or any patient with significant potential exposure, administer 15 LPM via NRB. Do not use a nasal cannula — it does not deliver adequate FiO⊂2; to displace CO from hemoglobin at the rate required.
• Transport to the ED. Even asymptomatic patients with significant exposure should be evaluated in the emergency department. Serum COHb levels can be measured by blood test — this provides the definitive exposure assessment. Patients with neurologic symptoms, loss of consciousness, or COHb >25% may require hyperbaric oxygen treatment.
• Hyperbaric oxygen (HBO) therapy. HBO reduces the half-life of COHb further and is indicated for severe poisoning, pregnancy (fetal hemoglobin has higher CO affinity), and patients with cardiac complications. Not all facilities have hyperbaric chambers — know your nearest HBO-capable facility.

Prevention for Homeowners

• Install CO detectors on every floor and within 15 feet of every sleeping area
• Replace CO detectors every 5–7 years (check manufacture date on the back of the unit)
• Have gas appliances (furnace, water heater, range) professionally inspected annually
• Never run a generator, charcoal grill, or any combustion equipment indoors or in an attached garage
• Never idle a vehicle in an attached garage, even with the garage door open
• Ensure fireplaces and wood stoves have clean, clear chimneys before each heating season
• Never use a gas oven for space heating
• If the CO detector activates: get everyone out immediately, call 911, and do not re-enter until cleared by the fire department

Frequently Asked Questions

What are the symptoms of carbon monoxide poisoning?

Early symptoms: headache, dizziness, fatigue, shortness of breath, nausea, and confusion. These mirror flu symptoms, which is why many victims delay seeking help. Key difference from flu: multiple household members develop symptoms simultaneously, and there is no fever with CO poisoning. Severe poisoning causes loss of consciousness, seizures, and death without treatment.

What level of CO is dangerous?

35 PPM is the OSHA permissible exposure limit for 8 hours of workplace exposure. 200 PPM causes headache and dizziness in 2–3 hours. 400 PPM is life-threatening with sustained exposure. 1,200 PPM is the NIOSH IDLH (immediately dangerous to life and health). Residential CO detectors alarm at 70 PPM sustained exposure, meaning the detector has lagged behind actual CO accumulation by design.

What should I do if my CO detector goes off?

Get everyone (including pets) out of the building immediately. Call 911 from outside. Do not re-enter the building. Wait for the fire department to measure CO levels, identify the source, and clear the building for reoccupancy. Do not assume the alarm was a malfunction — treat every CO alarm as a real event until the fire department confirms otherwise.

Can you smell carbon monoxide?

No. Carbon monoxide is completely odorless and colorless. There are no sensory warning signs. The only reliable way to detect CO is with a working CO detector or an electronic meter. This is why CO detectors are required by code — there is no other way for occupants to know CO is present before symptoms develop.

How long does it take to recover from CO poisoning?

Minor exposure with brief symptoms typically resolves within hours of breathing fresh air or receiving supplemental oxygen. Significant poisoning with high COHb levels can cause neurological symptoms that persist for days to weeks (delayed neurological sequelae). The prognosis improves with rapid treatment and high-flow oxygen. Patients with loss of consciousness or very high COHb levels may require hyperbaric oxygen therapy.