Carbon Monoxide in Air Quality

What is Carbon Monoxide?

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas produced by the incomplete combustion of carbon-containing fuels. Its invisible nature makes it particularly dangerous because humans cannot detect it without specialized equipment. Often called the "silent killer," CO is responsible for thousands of poisoning cases each year.

In the context of air quality monitoring, carbon monoxide is one of the six criteria pollutants regulated by the EPA under the Clean Air Act. While outdoor CO levels have declined significantly in recent decades due to cleaner vehicle technology, it remains an important component of overall air quality assessment.

Outdoor Sources of Carbon Monoxide

The primary outdoor sources of carbon monoxide are related to fuel combustion:

• Motor vehicles: Cars, trucks, buses, and motorcycles are the largest source of outdoor CO, especially in urban areas with heavy traffic
• Industrial processes: Factories and manufacturing facilities that burn fossil fuels
• Non-road engines: Construction equipment, boats, lawn mowers, and recreational vehicles
• Wildfires: Forest fires and prescribed burns release significant amounts of CO
• Power generation: Fossil fuel power plants contribute to ambient CO levels
• Wood burning: Residential wood stoves and fireplaces

Outdoor CO concentrations are typically highest near busy roadways and intersections, especially during rush hour traffic. Cold winter mornings often see elevated levels because cold engines produce more CO and cold air traps pollutants close to the ground.

Indoor Carbon Monoxide Dangers

While outdoor CO levels are generally low in most areas thanks to emissions controls, indoor CO poisoning remains a serious health threat. Indoor sources can create dangerously high concentrations in enclosed spaces.

Common Indoor Sources

• Gas furnaces and boilers: Malfunctioning heating systems are a leading cause of CO poisoning
• Gas water heaters: Improperly vented units can release CO into living spaces
• Gas stoves and ovens: Can produce CO, especially when used for heating
• Fireplaces and wood stoves: Blocked chimneys or poor ventilation create hazards
• Generators: Extremely dangerous when used indoors or in enclosed spaces
• Grills and camp stoves: Should never be used indoors
• Vehicles: Running cars in attached garages allows CO to enter homes
• Blocked vents: Snow, debris, or bird nests blocking exhaust vents

Health Effects of Carbon Monoxide

Carbon monoxide affects health by binding to hemoglobin in the blood, forming carboxyhemoglobin (COHb). This reduces the blood's ability to carry oxygen to vital organs and tissues. The health effects depend on the concentration and duration of exposure.

Mild to Moderate Exposure

• Headache (most common early symptom)
• Dizziness and lightheadedness
• Fatigue and weakness
• Nausea and vomiting
• Shortness of breath during exertion
• Confusion and impaired judgment
• Blurred vision

Severe Exposure

• Loss of consciousness
• Chest pain and irregular heartbeat
• Seizures
• Coma
• Death

Vulnerable Populations

Some individuals are more susceptible to CO poisoning:

• Unborn babies (fetal hemoglobin binds CO more readily)
• Infants and young children
• Elderly individuals
• People with anemia
• People with heart disease or respiratory conditions

Detection and Monitoring

Indoor CO Detection

• CO detectors: Install battery-operated or plug-in CO detectors on every floor of your home, especially near sleeping areas
• Test regularly: Check detectors monthly and replace batteries at least annually
• Replace units: CO detectors have a limited lifespan (typically 5-7 years)
• Know the alarm: Familiarize yourself with what the CO alarm sounds like

Outdoor CO Monitoring

Outdoor carbon monoxide is monitored as part of the AQI system. However, due to significant improvements in vehicle emissions technology, outdoor CO rarely reaches levels that affect the overall AQI in most U.S. cities. The EPA maintains a network of monitoring stations that track CO levels continuously.

Carbon Monoxide and the AQI

The AQI includes carbon monoxide as one of its tracked pollutants. The EPA calculates CO AQI values based on 8-hour average concentrations:

• Good (0-50): 0-4.4 ppm (8-hour average)
• Moderate (51-100): 4.5-9.4 ppm
• Unhealthy for Sensitive Groups (101-150): 9.5-12.4 ppm
• Unhealthy (151-200): 12.5-15.4 ppm
• Very Unhealthy (201-300): 15.5-30.4 ppm
• Hazardous (301-500): 30.5+ ppm

In most urban areas today, CO rarely drives the overall AQI because levels have decreased substantially since the 1970s. Other pollutants like PM2.5 and ozone are more commonly the dominant pollutants in AQI calculations.

Protection Strategies

Indoor Protection

• Install CO detectors on every floor and near sleeping areas
• Have fuel-burning appliances inspected annually by qualified technicians
• Never use generators, grills, or camp stoves indoors or in garages
• Never run a vehicle in an attached garage, even with the door open
• Keep vents and chimneys clear of debris and blockages
• Never use a gas oven or stovetop to heat your home
• Open the garage door before starting your car

If Your CO Detector Alarms

• Immediately move to fresh air outdoors
• Call 911 or your local fire department
• Account for all household members
• Do not re-enter the home until emergency responders say it is safe
• Seek medical attention if anyone has symptoms of CO poisoning

Outdoor Protection

• Avoid prolonged exposure near heavy traffic or idling vehicles
• Check AQI forecasts before extended outdoor activities
• Be aware of elevated CO levels during winter inversions
• People with heart disease should be especially cautious during high CO days