Carbon Monoxide (CO)
Carbon monoxide is a colorless, odorless and toxic gas formed mainly when fuels do not burn completely. It can accumulate in homes, garages, workshops, boiler rooms, vehicles and industrial processes without providing a reliable sensory warning. Understanding where CO comes from, how it affects the body and how it is measured is essential for both household safety and occupational gas monitoring.
What Is Carbon Monoxide?
Carbon monoxide is a simple molecule made of one carbon atom and one oxygen atom. It is usually produced through incomplete combustion: carbon-containing fuel is heated or burned, but the available oxygen, mixing, temperature or combustion time is not sufficient to convert all carbon into carbon dioxide.
Why it is difficult to notice
CO has no useful warning odor or color. A person may be exposed while believing that the room, vehicle or work area is safe.
Why exposure can escalate
Risk depends on both concentration and time. A moderate level over a longer period and a high level over a short period can both be dangerous.
Why context matters
CO may be a residential indoor-air hazard, an occupational toxic gas, a combustion-control indicator or a process gas used intentionally in industry.
CO is not CO2. Carbon monoxide is primarily a toxic combustion gas, while carbon dioxide is a different molecule with different sources, health effects and detection requirements.
Broader chemistry and historical background: Wikipedia — Carbon monoxide. Public-health overview: CDC — Carbon Monoxide Poisoning Basics.
Carbon Monoxide Key Properties
These values are useful when evaluating storage, process safety, ventilation and instrument selection. Actual design decisions should follow the applicable standard, site conditions and equipment documentation.
| Property | Typical value or description | Why it matters |
|---|---|---|
| Appearance | Colorless gas | Visual inspection cannot confirm whether CO is present. |
| Odor | Odorless | Smell is not a dependable warning method. |
| Boiling point | About −191.7°C (−313°F) | CO is normally encountered as a gas under ambient conditions. |
| Relative gas density | About 0.97 compared with air | CO is close to the density of air and can mix through an enclosed space. |
| Lower explosive limit | About 12.5% by volume | CO is flammable, although toxic exposure is usually the earlier concern. |
| Upper explosive limit | About 74% by volume | Process systems handling concentrated CO also require fire and explosion controls. |
| Conversion | 1 ppm ≈ 1.15 mg/m³ | Useful when comparing workplace limits stated in different units. |
Toxicity comes first in most occupied spaces. The flammable range begins at percentage-level concentrations, while health-based monitoring commonly works in parts per million. A combustible-gas alarm alone may therefore be unsuitable for protecting people from CO poisoning.
Property and occupational-hazard reference: CDC/NIOSH Pocket Guide — Carbon Monoxide.
How Is Carbon Monoxide Produced?
Complete combustion ideally converts carbon in a fuel into carbon dioxide. Carbon monoxide forms when combustion is incomplete. This can happen because there is too little oxygen, poor fuel-air mixing, insufficient temperature, short residence time, flame impingement, restricted exhaust flow or equipment that is dirty, damaged or incorrectly adjusted.
The presence of a flame does not prove that combustion is safe. A heater, furnace, engine or burner may continue operating while producing elevated CO because the exhaust pathway or combustion conditions have changed.
Common conditions that increase CO formation
- Insufficient combustion air or blocked air inlets
- Incorrect burner adjustment or fuel pressure
- Blocked, leaking, undersized or disconnected flues
- Poor ventilation in enclosed or semi-enclosed spaces
- Cold starts, idling and inefficient engine operation
- Damaged heat exchangers or poorly maintained appliances
- Recirculation of exhaust into occupied areas
- Improper use of charcoal, generators or fuel-burning tools indoors
Where Does Carbon Monoxide Come From?
Any process that burns carbon-containing fuel can generate CO. The highest risk usually appears when the source operates inside, close to an air intake, or in a space where exhaust cannot disperse safely.
Common sources in homes and buildings
- Gas furnaces and boilers
- Water heaters
- Gas ranges and ovens
- Wood stoves and fireplaces
- Kerosene or gas space heaters
- Portable generators
- Charcoal grills
- Vehicles in attached garages
- Blocked chimneys or flues
- Tobacco and hookah smoke
Common workplace and industrial sources
- Forklifts and internal-combustion engines
- Welding, cutting and hot work
- Foundries and metal heat treatment
- Blast furnaces and coke ovens
- Boiler rooms and power generation
- Parking garages and tunnels
- Firefighting and post-fire overhaul
- Mining and confined spaces
- Warehouses using fuel-powered equipment
- Process gas and synthesis-gas systems
Portable generators are a recurring high-risk source. They should never be operated inside homes, garages, crawlspaces, sheds or similar areas. Opening doors and windows does not guarantee that the exhaust will disperse safely.
Why attached garages matter
Exhaust can move through door gaps, wall penetrations, ductwork or pressure differences, allowing CO to enter living space even when a garage door is open.
Why storms and outages matter
Generators, improvised heating and blocked vents can increase exposure risk during cold weather, severe storms and power interruptions.
Why symptoms may affect several people
When several occupants develop similar headache, dizziness or nausea in the same location, an environmental source such as CO should be considered.
Indoor-source and exposure-control reference: U.S. EPA — Carbon Monoxide's Impact on Indoor Air Quality.
Why Is Carbon Monoxide Dangerous?
After inhalation, CO enters the bloodstream and forms carboxyhemoglobin. This interferes with oxygen transport and use, placing oxygen-sensitive organs—especially the brain and heart—at risk. The severity of poisoning depends on concentration, exposure duration, breathing rate, workload and individual health.
Early or lower-level symptoms
- Headache
- Fatigue or unusual weakness
- Dizziness or light-headedness
- Nausea or vomiting
- Shortness of breath
- Chest discomfort, especially in people with heart disease
More severe symptoms
- Confusion and impaired judgement
- Blurred vision or poor coordination
- Sleepiness and loss of muscle control
- Loss of consciousness
- Abnormal heart effects, seizures or coma
- Permanent injury or death
CO poisoning can resemble influenza but often occurs without fever. Symptoms may improve after leaving the building and return when the person re-enters. Do not use this pattern as a substitute for professional assessment—treat suspected exposure as urgent.
Who may be more vulnerable?
Pregnant people and unborn babies
Fetal blood can take up CO readily, and fetal exposure may persist longer.
Infants, children and older adults
Breathing rate, physiology and existing health conditions can increase risk.
People with heart, lung or blood disorders
Reduced oxygen delivery may worsen chest pain or existing cardiopulmonary stress.
People who are sleeping
They may not recognize symptoms or respond before exposure becomes severe.
People using alcohol or sedating drugs
Sleepiness and impaired judgement may delay escape or recognition.
Workers doing heavy physical activity
Higher breathing rates can increase the amount of CO inhaled.
Delayed effects are possible. Some people develop memory, personality, movement or other neurological problems after the initial event. Anyone with suspected poisoning should receive medical evaluation even if they begin to feel better in fresh air.
Medical overview: Mayo Clinic — Carbon Monoxide Poisoning: Symptoms and Causes.
What Should You Do If CO Is Suspected?
A sounding alarm, multiple people with similar symptoms, sudden headache or confusion near combustion equipment, or symptoms that improve outdoors should be taken seriously.
Immediate actions
- Move to fresh air immediately. Go outside rather than moving to another room.
- Call emergency services. Report a possible carbon monoxide exposure.
- Account for everyone. Include children, visitors, coworkers and pets.
- Do not re-enter. Wait until emergency responders say the area is safe.
- Seek medical evaluation. Tell clinicians that CO exposure is suspected.
What not to do
- Do not ignore or silence an alarm and remain inside.
- Do not search for the source before leaving.
- Do not assume an open window makes the space safe.
- Do not restart a suspected appliance until it has been professionally checked.
- Do not rely on smell, a phone app or symptoms alone to confirm safety.
- Do not assume a normal consumer pulse-oximeter reading rules out CO poisoning.
Emergency and alarm-response reference: U.S. CPSC — Carbon Monoxide Questions and Answers.
How Is Carbon Monoxide Poisoning Diagnosed and Treated?
Medical teams consider the exposure history, symptoms, timing, physical findings and blood testing. Carboxyhemoglobin testing is most useful when performed promptly, but treatment should not be delayed when poisoning is strongly suspected.
Hospital treatment commonly includes high-concentration oxygen. Hyperbaric oxygen therapy may be considered for severe cases and for selected patients, including some pregnant patients. Treatment decisions depend on symptoms, examination, exposure history, laboratory findings and clinical judgement.
A standard fingertip pulse oximeter is not a CO detector. It may display an apparently reassuring oxygen saturation even when carboxyhemoglobin is present. Medical CO-oximetry and clinical evaluation are different from ordinary home pulse-oximetry.
Information to share with clinicians: suspected source, location, exposure duration, symptoms, loss of consciousness, pregnancy, smoking history and whether other people or pets were affected.
Diagnosis and treatment reference: Mayo Clinic — Carbon Monoxide Poisoning: Diagnosis and Treatment.
Carbon Monoxide Levels and Exposure Limits
There is no single concentration that produces the same effect in every person. Health impact depends on concentration, time, activity level and individual susceptibility. Residential alarm behavior, workplace exposure limits and emergency-response thresholds serve different purposes and should not be treated as interchangeable.
| Reference | Value | Meaning |
|---|---|---|
| NIOSH REL | 35 ppm as an 8-hour TWA | Recommended occupational exposure limit for a work shift. |
| NIOSH ceiling | 200 ppm | Concentration not intended to be exceeded. |
| OSHA PEL | 50 ppm as an 8-hour TWA | U.S. federal permissible exposure limit for general industry. |
| NIOSH IDLH | 1,200 ppm | Immediately dangerous to life or health; specialized emergency controls are required. |
| Typical homes without gas stoves | Approximately 0.5–5 ppm | EPA-reported average range; actual values vary by source and ventilation. |
| Near properly adjusted gas stoves | Often 5–15 ppm | Localized levels may rise during use. |
| Near poorly adjusted gas stoves | May reach 30 ppm or higher | Indicates a need to investigate combustion and ventilation. |
Do not copy workplace limits directly into residential alarm settings. A household alarm, an occupational exposure monitor and an industrial process detector may use different standards, averaging periods, alarm delays and response logic.
Limits and properties: NIOSH Pocket Guide. Indoor level context: U.S. EPA.
How Is Carbon Monoxide Detected?
CO cannot be managed by human senses. Detection systems convert a chemical or optical interaction into an electrical signal that can be displayed, logged or used to trigger alarms, ventilation and safety actions.
Electrochemical detection
CO reacts at an electrode and produces a current related to concentration. This method is widely used for ppm-level personal monitors, portable instruments, fixed toxic-gas transmitters and residential alarms.
Key considerations: sensitivity, cross-interference, temperature, humidity, lifetime and calibration.
Infrared absorption
Infrared systems estimate CO by measuring absorption at selected wavelengths. They can suit process, emissions and higher-range applications, but optical path design and interference control are important.
Key considerations: optical path, water vapor, range, pressure, contamination and maintenance.
Semiconductor sensing
A heated sensing material changes resistance when exposed to reducing gases. Semiconductor devices can be compact and economical, but selectivity, drift and environmental compensation require careful evaluation.
Key considerations: warm-up, cross-sensitivity, baseline drift, humidity and power use.
Other measurement approaches
- Colorimetric detector tubes for spot checks
- Laboratory gas chromatography
- Combustion analyzers for appliance and flue testing
- Open-path or extractive systems for selected industrial installations
- CO-oximetry for carboxyhemoglobin in medical evaluation
Questions to answer before choosing a method
- Is the goal life safety, occupational exposure, combustion tuning or process control?
- What concentration range and response time are required?
- Will other gases cause cross-interference?
- Is the location humid, dusty, hot, cold or vibration-prone?
- Is periodic calibration possible?
- What standard, certification or local code applies?
Detection range must match the hazard. A ppm-level toxic-gas monitor is designed for a different purpose than a percentage-level process analyzer or LEL detector. The expected concentration and safety objective should be defined before selecting instrumentation.
General selection background: Gas Detection Guide. NIOSH measurement-method references are listed in the NIOSH Pocket Guide.
Carbon Monoxide Alarms, Monitors and Detectors
These terms are often used interchangeably, but the intended function matters. A household CO alarm is designed to warn occupants under the applicable product standard. A workplace monitor may display ppm and calculate exposure. A fixed detector may control ventilation or report to a building or safety system.
Household CO alarm
Intended primarily to provide an audible warning before conditions become life-threatening. Alarm timing may intentionally depend on both concentration and duration to reduce nuisance alarms.
Personal or portable monitor
Used by workers, technicians or emergency responders to view concentration, track exposure and receive alarms while moving through different areas.
Fixed monitoring system
Permanently installed to provide continuous measurement, remote alarms, ventilation control, event logs or integration with a building-management or safety system.
Where should household CO alarms be installed?
- Follow the alarm manufacturer's installation instructions.
- Use alarms on every level of the home and outside sleeping areas.
- Battery backup is important for hardwired or plug-in units.
- Interconnected alarms can improve warning throughout the home.
- Avoid locations where furniture, drapes or airflow may obstruct the unit.
- CPSC does not recommend placing alarms in kitchens or directly above fuel-burning appliances.
Testing, maintenance and end of life
- Use the test button at the frequency stated by the manufacturer.
- Understand that the test button checks circuitry, not necessarily sensor accuracy.
- Replace batteries or the complete alarm as instructed.
- Do not paint, cover or modify the alarm.
- Replace the unit at its specified end-of-life date.
- An alarm does not replace appliance maintenance, ventilation or safe generator use.
Installation and alarm-response reference: U.S. CPSC — Carbon Monoxide Questions and Answers.
How Can Carbon Monoxide Exposure Be Prevented?
The strongest prevention strategy combines source control, correct equipment use, ventilation, maintenance, monitoring and a clear response plan. An alarm is an important layer, but it should not be the only layer.
For homes and buildings
- Have fuel-burning appliances and heating systems professionally inspected.
- Keep chimneys, flues, vents and exhaust outlets clear and connected.
- Never use a gas oven or range as a space heater.
- Never burn charcoal inside a home, garage, vehicle or tent.
- Operate portable generators outdoors, far from doors, windows and vents.
- Never leave a vehicle running in an attached garage.
- Use vented appliances correctly and provide required combustion air.
- Install and maintain CO alarms according to instructions and local requirements.
For workplaces and industrial sites
- Identify combustion and process sources in the risk assessment.
- Use engineering controls such as exhaust ventilation and source isolation.
- Restrict engine-powered equipment in enclosed and semi-enclosed areas.
- Monitor during startup, maintenance, shutdown and abnormal operation.
- Use personal monitoring where worker movement or changing conditions create risk.
- Define alarm actions, evacuation routes and emergency communication.
- Calibrate and function-test instruments according to the monitoring program.
- Review local exposure limits, confined-space rules and respiratory requirements.
Look for patterns, not only single readings. Repeated small increases during burner startup, vehicle entry, ventilation failure or a specific work task can reveal a developing source before a major event occurs.
Related application pages: Indoor Air Quality and Industrial Safety.
Is Carbon Monoxide Always an Unwanted Gas?
CO is hazardous when people are exposed unintentionally, but it is also an important industrial feedstock and process gas. Facilities that use CO intentionally must control both its toxicity and flammability.
Chemical manufacturing
CO is used in synthesis gas and carbonylation or hydroformylation processes to manufacture methanol, acetic acid, aldehydes and other chemical intermediates.
Metallurgy
Carbon monoxide acts as a reducing gas in processes such as ironmaking. Blast-furnace and coke-oven gases may contain significant CO and require strict containment and monitoring.
Controlled atmospheres
CO may be present in heat treatment, research systems and specialized process atmospheres. Gas purity, leak control and ventilation become part of the process-safety design.
CO in outdoor air
Road traffic, combustion, fires and industrial activity contribute to outdoor CO. In the atmosphere, CO also participates in chemical reactions that influence the lifetime of methane and the formation of ground-level ozone.
CO as a combustion indicator
Rising CO can indicate incomplete combustion, poor air-fuel control, restricted exhaust or equipment deterioration. Trend data can therefore support both safety and maintenance decisions.
Carbon Monoxide Myths and Practical Answers
“CO always rises to the ceiling.”
CO is only slightly less dense than air and usually mixes with room air. Installation should follow the detector manufacturer's instructions rather than a universal “high” or “low” rule.
“Opening the garage door makes idling safe.”
Exhaust can still accumulate or enter the building. A vehicle should not be left running in an attached garage, even with the door open.
“A smoke alarm also detects CO.”
Only a listed combination unit specifically designed for both hazards detects both. A standard smoke alarm does not measure carbon monoxide.
“If I feel better outside, I do not need medical care.”
Improvement in fresh air does not rule out significant exposure or delayed effects. Suspected poisoning requires urgent medical evaluation.
“The test button confirms the sensor is accurate.”
The test button usually verifies alarm circuitry and sounder operation. Follow the manufacturer's guidance for replacement, maintenance and any permitted functional testing.
“A phone can detect CO by itself.”
A phone without a compatible external sensing device does not contain a certified CO sensor. Safety should rely on suitable, maintained detection equipment.
Carbon Monoxide FAQ
Concise answers to common questions about CO sources, symptoms, alarms and measurement.
What does carbon monoxide smell like?
Carbon monoxide has no odor. A smell near a faulty appliance may come from fuel, combustion by-products or overheating materials, but the smell itself is not CO.
Can you see or taste carbon monoxide?
No. CO is colorless, odorless and tasteless, which is why suitable monitoring and alarms are necessary.
Is carbon monoxide heavier or lighter than air?
CO is slightly lighter than air, with a relative gas density of about 0.97. In real rooms it generally mixes with air because of heat, ventilation and air movement.
What is the difference between CO and CO2?
CO contains one oxygen atom and is a highly toxic product of incomplete combustion. CO2 contains two oxygen atoms and is mainly an asphyxiation and indoor-air-quality concern at elevated concentrations. The gases require different detection approaches.
Can a gas stove produce carbon monoxide?
Yes. Any fuel-burning appliance can produce CO, especially if it is poorly adjusted, damaged, inadequately ventilated or operated with restricted combustion air.
Can an electric heater produce CO?
An electric resistance heater does not burn fuel and therefore does not normally generate CO. However, a building may still have other combustion sources such as a furnace, fireplace, generator or attached garage.
Can carbon monoxide poisoning happen while sleeping?
Yes. Sleeping people may not notice early symptoms and may be unable to respond before exposure becomes severe. This is one reason alarms are recommended outside sleeping areas.
Do pets react to carbon monoxide before people?
Pets can also be poisoned and may show weakness, vomiting, breathing difficulty or collapse. Do not use animal behavior as a detection method; evacuate people and pets when CO is suspected.
Does opening a window remove carbon monoxide?
Ventilation may reduce concentration, but opening a window does not make an active source safe and should not delay evacuation, emergency response or repair.
Why did my CO alarm sound and then stop?
The concentration may have changed, the alarm may use time-weighted logic, or the unit may be signaling a fault or end of life. Treat a CO alarm as a real warning, move outside and follow the manufacturer's instructions and emergency guidance.
How often should a CO alarm be replaced?
Service life varies by product. Check the label and manual for the replacement date or end-of-life signal. Replace the complete alarm when its stated life has expired.
Can a standard combustible-gas detector protect against CO poisoning?
Not necessarily. Many combustible-gas detectors are designed for percentage-of-LEL concentrations, while CO poisoning protection requires ppm-level toxic-gas measurement. Verify the target gas, range and certification.
Can carbon monoxide exposure cause long-term problems?
Yes. Severe exposure may cause lasting brain or heart injury, and delayed neurological symptoms can occur after initial recovery. Medical follow-up is important.
What should a workplace CO monitoring plan include?
It should define sources, expected ranges, sensor locations, personal-monitoring needs, alarm levels, calibration, bump testing, ventilation actions, recordkeeping, emergency response and the applicable exposure limits or standards.
Sources and Further Reading
Gas Nose uses external references to support technical, health and safety information. Requirements may vary by country, jurisdiction, industry and product standard; always verify the rules that apply to your project.
Health and emergency guidance
Educational content only: This page does not replace emergency services, medical advice, workplace exposure assessment, local building or fire codes, appliance instructions, or a qualified gas-safety professional.