Symptoms of Carbon Monoxide Exposure From a Furnace

Carbon monoxide (CO) is an odorless, colorless gas produced by incomplete combustion in furnaces and other fuel-burning appliances. Recognizing symptoms early can prevent serious injury or death. This article explains common and severe symptoms, how furnaces generate CO, detection methods, immediate steps to take, prevention tips, and recommended maintenance practices.

Symptom Type	Common Signs	Severe Signs
Neurological	Headache, dizziness, confusion	Seizures, loss of consciousness
Cardiorespiratory	Shortness of breath, rapid breathing, chest pain	Arrhythmia, respiratory failure
Gastrointestinal	Nausea, vomiting	Persistent vomiting, reduced responsiveness
Other	Flu-like symptoms, fatigue	Coma, death

What Is Carbon Monoxide And Why Furnaces Produce It

Carbon monoxide is a byproduct of incomplete combustion when fuels such as natural gas, propane, oil, or wood do not burn fully. Furnaces can produce CO when burners are improperly adjusted, ventilation is blocked, heat exchangers are cracked, or when flue pipes are corroded or disconnected. CO binds to hemoglobin more readily than oxygen, reducing oxygen delivery and causing cellular hypoxia.

How Carbon Monoxide Affects The Body

When inhaled, CO diffuses into the bloodstream and forms carboxyhemoglobin (COHb), which reduces oxygen-carrying capacity. Tissues with high oxygen demand, like the brain and heart, are most vulnerable. Symptoms depend on CO concentration and exposure duration, with acute high-level exposure producing rapid deterioration and low-level chronic exposure causing subtle, cumulative effects.

Common Symptoms Of Carbon Monoxide From A Furnace

Symptoms can be nonspecific and often mimic other illnesses such as influenza. Early or low-level exposure typically causes headaches, dizziness, nausea, fatigue, and shortness of breath. Individuals may report flu-like symptoms that worsen when inside the home and improve when away. Pay attention to multiple household members experiencing similar symptoms.

Neurological Signs

Headache is the most frequently reported symptom and can be persistent or throbbing. Dizziness, lightheadedness, confusion, difficulty concentrating, and memory problems may follow. Visual disturbances and impaired coordination are also possible.

Cardiopulmonary Signs

Shortness of breath, chest pain, and an irregular or rapid heartbeat can occur, especially in people with underlying heart or lung disease. Mild exposures might cause only increased respiratory rate and exertional intolerance.

Gastrointestinal Signs

Nausea, vomiting, and abdominal discomfort are common. These symptoms can lead clinicians to treat for gastroenteritis if CO is not considered, delaying diagnosis.

General Symptoms

Fatigue, malaise, and flu-like symptoms are common early complaints. Some people describe a sense of heaviness, weakness, or generalized aches. Symptoms that improve outside the home are a key clue.

Severe Symptoms And Signs Of High-Level Exposure

High concentrations or prolonged exposure can cause loss of consciousness, seizures, severe metabolic acidosis, arrhythmias, and respiratory failure. Permanent neurological deficits, such as cognitive impairment, movement disorders, and memory loss, may follow severe exposures. Death can occur within minutes at very high CO levels.

How Exposure Patterns Influence Symptoms

Acute high-level exposure typically causes rapid onset of severe symptoms, often with loss of consciousness. Chronic low-level exposure produces subtle, persistent symptoms like chronic headaches, sleep disturbances, and mood changes. Intermittent exposure from an intermittently malfunctioning furnace may cause cyclical symptoms that recur during cold-weather use.

Red Flags That Require Immediate Action

Seek emergency care if anyone has syncope, seizures, severe difficulty breathing, persistent chest pain, sudden confusion, or loss of consciousness. Any unexplained cluster of symptoms among household members warrants immediate evaluation, especially during furnace operation.

How To Detect Carbon Monoxide In The Home

Install CO alarms on each level of the home and outside sleeping areas; alarms should meet UL 2034 standards. Place alarms at head height and away from combustion appliances to avoid false positives. Replace alarms per manufacturer guidance, typically every 5–7 years.

Using CO Monitors

Portable CO monitors provide real-time readings; some display parts per million (ppm). Readings above 50 ppm are concerning over several hours; values above 150–400 ppm can cause severe symptoms within hours to minutes depending on concentration. Follow device instructions for placement and interpretation.

Professional Inspection

HVAC professionals can perform combustion analysis, flue draft tests, and pressure diagnostics to identify CO sources. If a CO alarm activates, have the furnace inspected before reuse and consider contacting local fire or emergency services when multiple occupants are symptomatic.

Immediate Steps If Carbon Monoxide Is Suspected

If CO poisoning is suspected, move everyone outdoors to fresh air immediately and call 911. Provide first aid: remove the person from exposure, ensure airway and breathing, and administer oxygen if trained personnel and equipment are available. Inform emergency responders about potential CO exposure and any CO alarm readings.

Medical Evaluation And Treatment

Emergency departments evaluate suspected CO exposure with pulse oximetry, blood gas analysis, and a carboxyhemoglobin (COHb) level from blood. Pulse oximetry is unreliable for CO exposure as it cannot distinguish COHb from oxyhemoglobin. Treatment includes high-flow 100% oxygen via nonrebreather mask to accelerate CO elimination.

Hyperbaric Oxygen Therapy

Hyperbaric oxygen (HBO) may be recommended for severe cases, including COHb levels typically above 25%, severe neurological symptoms, cardiovascular instability, or loss of consciousness. HBO reduces CO half-life faster than normobaric oxygen and may lower the risk of delayed neurological sequelae.

Prevention: Furnace Maintenance And Home Safety

Regular professional furnace inspections and annual maintenance are essential. Technicians should clean and adjust burners, check heat exchangers for cracks, test combustion efficiency, inspect flue and venting, and verify proper airflow. Replace filters routinely and ensure vents and chimneys are free of obstructions.

Safe Installation And Ventilation

Ensure proper installation and sizing of furnaces and venting systems. Avoid running auxiliary combustion appliances, such as space heaters or generators, indoors or in attached garages. Maintain adequate ventilation and never block appliance vents, dryer vents, or intake air openings.

What Homeowners Can Do

• Install and maintain CO alarms; test monthly and replace batteries as recommended.
• Schedule annual furnace inspections with licensed HVAC professionals.
• Keep flues, chimneys, and vents clear of debris, nests, and snow.
• Be cautious with home renovations that can affect ventilation or furnace operation.

Legal Standards, Codes, And Recommendations

Building codes and manufacturer recommendations govern furnace installation and venting. Many jurisdictions require CO alarms in new construction and when fuel-burning appliances are present. The U.S. Consumer Product Safety Commission and Centers for Disease Control and Prevention provide guidance on CO prevention and alarm placement.

Common Causes Of Furnace-Related Carbon Monoxide Leaks

Frequent causes include cracked heat exchangers, blocked or corroded vent pipes, improper combustion air supply, backdrafting, and incomplete combustion due to sooting burners. Power outages or poorly adjusted pilots and burners can also increase CO risk.

Recognizing Patterns And Taking Action

Notice if symptoms occur or worsen when the furnace runs and improve when away from home. Check household members and pets for similar complaints. If a CO alarm sounds, evacuate, call emergency services, and do not re-enter until cleared by professionals.

Post-Exposure Considerations And Follow-Up

After medical treatment, follow-up evaluation may include neurological assessment, cardiology consultation for heart symptoms, and repeat COHb testing. Monitor for delayed neuropsychiatric symptoms, which can appear days to weeks later, including mood changes, memory problems, and movement difficulties.

Frequently Asked Questions

Can Carbon Monoxide Smell Like Anything?

No, CO is odorless, colorless, and tasteless. Rely on alarms and symptom recognition rather than scent.

How Quickly Do Symptoms Appear?

Onset depends on concentration and exposure duration. High concentrations can produce symptoms within minutes; lower concentrations may cause symptoms over hours to days.

Are Pets Affected Before Humans?

Pets may show lethargy, vomiting, or collapse and can be affected earlier due to smaller body size. Similar symptoms in pets and humans in the same household are a significant warning sign.

Do CO Alarms Need Replacement?

Yes. Most CO alarms have a finite lifespan, commonly 5–7 years. Replace alarms according to manufacturer guidance and test them regularly.

Resources And Where To Get Help

Contact local emergency services if CO exposure is suspected. For prevention and education, consult the Centers for Disease Control and Prevention (CDC), Consumer Product Safety Commission (CPSC), and local public health or fire departments. HVAC professionals and licensed contractors can perform inspections and repairs.

Key Takeaway: Early recognition of symptoms combined with functioning CO alarms and proactive furnace maintenance significantly reduces the risk of carbon monoxide poisoning from furnaces. When in doubt, evacuate, call 911, and have the heating system inspected before reuse.