Carbon monoxide (CO) is a common indoor air contaminant because it is easy to generate during everyday combustion. Understanding why it is dangerous, how exposure occurs, and how to prevent it is essential for environmental health.

What carbon monoxide is and why it is dangerous

Carbon monoxide (CO) is a colorless, odorless, non-irritating gas produced when carbon-based fuels burn incompletely. Because people cannot reliably detect it with their senses, hazardous exposures can occur without warning.

CO is classified as an asphyxiant because it disrupts the body’s ability to transport oxygen, even when the surrounding air still contains normal oxygen levels.

This mechanism makes CO especially dangerous for the brain and heart, which are highly sensitive to reduced oxygen.

How CO exposure harms the body (asphyxiation mechanism)

CO enters the body through inhalation and quickly diffuses into the bloodstream. It binds to haemoglobin (the oxygen-carrying protein in red blood cells) more strongly than oxygen does, forming carboxyhaemoglobin.

Oxyhemoglobin dissociation curves comparing normal blood with blood containing 50% carboxyhemoglobin (COHb) and with 50% anemia. The COHb curve shows a lowered oxygen content (reduced carrying capacity) and an altered curve shape/left shift that impairs oxygen unloading to tissues at typical tissue $P_{O_2}$. This helps explain why CO exposure can cause severe tissue hypoxia even when environmental oxygen levels are normal. Source

Key outcomes include:

• Reduced oxygen transport: less oxygen can be carried in the blood.

• Impaired oxygen release: oxygen that is carried is released less effectively to tissues.

• Cellular hypoxia: body cells cannot produce enough energy aerobically, stressing organs.

Because CO is an asphyxiant, symptoms can escalate rapidly, particularly during sleep or in enclosed indoor spaces.

Indoor sources of carbon monoxide

CO becomes an indoor air pollutant when combustion happens in or near living spaces, especially with poor ventilation. Common indoor-related sources include:

• Faulty or poorly vented fuel-burning appliances

  • furnaces/boilers

  • gas stoves/ovens

  • water heaters

  • fireplaces and wood stoves

• Portable combustion devices used indoors

  • generators

  • charcoal grills

  • camping stoves/heaters

• Vehicle exhaust in attached or enclosed areas

  • idling cars in garages

  • exhaust infiltration from nearby traffic

• Backdrafting (exhaust pulled back indoors) due to pressure differences or blocked flues

High-risk settings include tightly sealed buildings, small rooms, and homes with inadequate exhaust systems.

Exposure conditions that increase risk

The severity of indoor CO exposure depends on both emissions and building conditions. Factors that increase risk include:

• Poor ventilation (closed windows, sealed building envelope)

• Malfunctioning burners (improper air-to-fuel ratio increasing incomplete combustion)

• Blocked chimneys/flues preventing exhaust from leaving

• Use of combustion devices in enclosed spaces during power outages or cold weather

• Time of exposure (longer exposure increases dose)

• Sleeping occupants who cannot notice early symptoms

Health effects and vulnerable populations

CO poisoning can range from mild to fatal. Typical health impacts include:

• Early symptoms: headache, dizziness, nausea, fatigue, shortness of breath

• Worsening exposure: confusion, impaired coordination, chest pain

• Severe exposure: loss of consciousness, organ damage, death

Vulnerable groups include:

• Infants and children (higher breathing rates)

• Pregnant people and fetuses (oxygen demand and sensitivity)

• Older adults and those with cardiovascular disease

• People at high exertion (greater oxygen demand)

Prevention and control (indoor environmental management)

Reducing indoor CO risk relies on source control and safe building operation:

• Maintain and inspect fuel-burning appliances and venting systems

• Ensure proper ventilation and functioning exhaust pathways

• Avoid operating generators, grills, or unvented heaters indoors

• Prevent vehicle idling in garages, even with the door open

• Use CO detectors as an engineering/monitoring control, especially near sleeping areas

Cutaway home diagram illustrating recommended carbon monoxide alarm placement on every level and near sleeping areas. The visual reinforces that CO risk is a whole-building issue (it can accumulate and spread through indoor air), so monitoring should not be limited to a single room. This supports source-control and building-operation strategies by emphasizing early detection where exposure is most dangerous (e.g., during sleep). Source