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02 · Environmental hazard

Carbon
monoxide
exposure.

Carbon monoxide is a colorless, odorless gas that can come from fuel-burning appliances and exhaust. This guide covers what to look for, how to test, and how to move forward.

Know what to look for ↓Find your next step ↗
Understand the signsExplore your options
Simple powder-blue illustration of a carbon monoxide alarm

An invisible hazard. A practical next step.
Know your alarms. Know your home.

CombustionYour homePatternsDetectionFresh airYour next step

A place to begin

Understand the space.
Notice the patterns.

A guide to the building, the symptoms, and the questions worth asking.

01

Understand CO

Combustion, household sources, and why CO can be missed.

↗02

Notice the signs

What to look for in your home and your daily patterns.

↗03

Explore testing

What different methods measure—and their limits.

↗04

Take the next step

Protect, investigate, and get the right support.

↗

What It Is

Carbon monoxide and chronic low-level exposure

Carbon monoxide is produced when fuels do not burn completely. Faulty appliances, blocked vents, and exhaust entering a home can create a serious exposure risk. Repeated or lower-level exposure may be harder to recognize than an acute incident.

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Carbon monoxide (CO) is a colorless, odorless gas produced by the incomplete combustion of fuels — natural gas, propane, oil, wood, and gasoline.

CO is generated by a wide range of indoor and adjacent sources:

  • Furnaces, water heaters, and boilers
  • Gas stoves, ovens, and fireplaces
  • Clothes dryers and attached garages
  • Outdoor generators and vehicle exhaust that infiltrate through foundation gaps, utility penetrations, and air intake systems

CO binds to hemoglobin in the bloodstream with an affinity 200 to 250 times greater than oxygen, reducing the blood’s ability to carry oxygen to organs and tissue.

High concentrations can be life-threatening. Repeated exposure can also cause symptoms that are easily mistaken for other conditions. A symptom pattern alone cannot establish CO as the cause.

Why It's Missed

Standard detectors, flu-like symptoms, delayed recognition

CO has no color or smell, and symptoms can resemble the flu or other illnesses. Residential alarms are essential for safety, but the absence of an alarm does not prove there is no exposure.

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Headache, fatigue, dizziness, nausea, shortness of breath, confusion, and sleep disruption can have many causes. Notice whether symptoms occur together, in particular places, or around appliance use.

A low-level display monitor can provide additional information about readings over time. It does not replace properly installed, functioning CO alarms or a professional assessment.

If you suspect poisoning, leave and seek urgent help. Do not wait for a reading or a routine appointment.

Commonly reported symptoms

What people notice.

Patterns reported with this exposure type. Symptoms vary and can have other causes; this list does not establish a diagnosis.

Persistent headachesFatigue and weaknessDizziness / lightheadednessNauseaShortness of breathConfusion / brain fogSleep disturbanceMemory problemsIrritability / mood changesChest tightnessVisual disturbancesFlu-like symptoms without fever

Signs to watch for

The source may
be invisible.

The symptoms often aren’t. These patterns can help guide a professional assessment.

01

Flu-like symptoms without fever

Persistent headache, fatigue, nausea, or dizziness that does not include a fever and does not fully resolve between episodes — particularly during heating season.

02

Household-wide patterns

Multiple people in the home—or pets—experiencing similar symptoms at the same time can be a reason to consider a shared environmental source. This pattern is not specific to CO.

03

Symptoms that improve away from home

Symptoms that improve away and recur at home are useful observations to share with a clinician and qualified building professional. They do not identify the source on their own.

04

Seasonal correlation

Symptoms that worsen in fall and winter when combustion appliances run more frequently and homes are sealed tight with reduced natural ventilation.

05

Appliance age or recent service issues

An aging furnace, recently serviced gas system, blocked flue, or history of appliance repairs are all factors that increase combustion-related risk.

06

Yellow or flickering flame on gas burners

An unusual flame or a change in appliance operation warrants professional inspection. Flame appearance alone cannot confirm or exclude CO. Do not use it as a safety test.

How to test

Different tools.
A fuller picture.

Detection and inspection work together. Combine working CO alarms, appropriate monitoring, appliance history, and a qualified inspection. Explore how the methods fit into a full assessment.

01

Low-level CO monitors

A continuous readout that can add context to your CO alarms.

Read details +

Low-level CO monitors display concentrations and may help identify patterns that an alarm-only device does not show. Detection limits, accuracy, and alert settings vary by model.

Use them in addition to working CO alarms. Follow manufacturer placement and maintenance instructions, and have concerning readings evaluated professionally.

02

Professional CO inspection

Combustion analysis and checks of fuel-burning appliances.

Read details +

A licensed HVAC technician can perform a combustion analysis on all gas-burning appliances. The inspection typically includes:

  • Measuring CO in the flue gas
  • Checking heat exchanger integrity
  • Testing for proper draft and venting

03

Flue and venting inspection

An assessment of flues, vents, and exhaust pathways.

Read details +

A chimney sweep or HVAC professional can inspect vent pipes, flues, and exhaust pathways.

They are looking for blockages, deterioration, backdrafting, or improper installation that allows CO to enter the living space rather than exhaust outside.

04

Indoor air quality assessment

CO measurement alongside ventilation and other combustion byproducts.

Read details +

A certified industrial hygienist or indoor air quality professional can conduct a full assessment that includes CO measurement alongside other combustion byproducts and ventilation evaluation.

What to do next

Start small.
Take the next step.

For routine prevention and follow-up after immediate safety concerns are addressed. Mark what you’ve discussed or arranged.

  • Start with essential protection. Install and maintain alarms near sleeping areas, following manufacturer instructions.
  • Add information when useful. A low-level monitor can supplement alarms; it should not replace them.
  • Use a qualified technician. Include furnaces, boilers, water heaters, fireplaces, and fuel-burning dryers.
  • Ask for written findings. Request combustion analysis and a record of repairs or concerns.
  • Inspect the full route. Have a professional check vents and flues for blockage, damage, or disconnection.
  • Ask about backdrafting. Include draft and pressure conditions that could bring exhaust indoors.
  • Never idle a vehicle inside. An open garage door does not make this safe.
  • Review shared pathways. Ask a qualified professional about doors, walls, penetrations, and shared ductwork.
  • Act urgently when poisoning is possible. Move outside and get emergency help; do not wait for confirmed readings.
  • Share the exposure history. Tell the clinician about symptom timing, appliance use, readings, and time spent away.
  • Confirm the source is addressed. Ask the technician how repair effectiveness and safe operation will be verified.
  • Keep the follow-up together. Save service reports and monitor information, and maintain alarms according to their instructions.

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A little more clarity.

CHRONICALLY EXPOSED · COMING SOON

Full article on carbon monoxide exposure

An in-depth guide on this topic is in progress.

Coming soon

CHRONICALLY EXPOSED · RESOURCE

Download the carbon monoxide checklist

Check combustion appliances, detector placement, and symptom timing.

Download checklist ↓

Safety guidance: CDC · EPA

Explore other exposure & health topics
Mold / biotoxin ↗Natural gas leak ↗Sewer gas ↗Plumbing / ventilation ↗Chemicals / VOCs ↗Water quality ↗Air pathways ↗MCAS ↗POTS / dysautonomia ↗Chronic fatigue ↗Immune dysregulation ↗Neurological ↗Respiratory ↗Musculoskeletal ↗Detox pathways ↗

A practical first step

The absence of an alarm
is not the absence of exposure.

Keep working alarms in place, maintain fuel-burning appliances, and bring concerns to qualified professionals. If an alarm sounds or poisoning is suspected, leave and seek urgent help.

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