Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas produced by the incomplete burning of carbon-containing fuels. It forms when there is insufficient oxygen or low temperature during combustion, leading to only partial burning of the fuel source. Understanding CO’s classification in environmental science is important for regulating its impact on air quality.
How Pollutants Are Classified
Environmental agencies categorize air pollutants based on their origin. A primary pollutant is any substance emitted directly into the air from a specific source. These pollutants enter the atmosphere in the same chemical form in which they were produced, such as soot or sulfur dioxide.
A secondary pollutant is not released directly but forms in the atmosphere through chemical reactions involving primary pollutants. Ground-level ozone is a common example, created when nitrogen oxides and volatile organic compounds react in sunlight. Carbon monoxide fits the definition of a primary pollutant because it is emitted directly from combustion sources.
Regulators worldwide monitor CO closely due to its abundance and health risks. In the United States, the Environmental Protection Agency (EPA) designates CO as one of six Criteria Air Pollutants. This designation mandates its monitoring and regulation under the federal Clean Air Act to protect public health.
Direct Emission Sources of Carbon Monoxide
CO is classified as a primary pollutant because it is generated directly from incomplete combustion sources. Combustion aims to convert carbon fuels into carbon dioxide and water. However, a lack of available oxygen or a drop in temperature prevents this full conversion, resulting in carbon atoms partially oxidizing to form CO instead of carbon dioxide.
The single largest source of CO emissions is mobile sources, specifically the exhaust from cars, trucks, and non-road engines. These vehicles contribute the majority of outdoor CO emissions, especially in urban areas. Other human-made sources include industrial processes, such as metal processing and chemical manufacturing, and the burning of wood or biomass.
Household appliances like gas stoves, furnaces, and water heaters are also major sources, particularly in poorly ventilated indoor spaces. Natural sources, including wildfires and volcanic activity, contribute to the atmospheric load. The direct link between the combustion source and the CO emission confirms its primary pollutant classification.
Effects on Human Health and the Atmosphere
The regulation of CO is driven by its immediate and severe effects on human health. When inhaled, CO is readily absorbed through the lungs and enters the bloodstream. It binds to the hemoglobin in red blood cells with an affinity 200 to 250 times greater than that of oxygen.
This binding forms carboxyhemoglobin, which displaces oxygen and prevents its delivery to the body’s tissues and organs. Initial exposure causes flu-like symptoms, such as headache, dizziness, and fatigue, due to the resulting lack of oxygen (hypoxia). Higher concentrations rapidly lead to impaired judgment, loss of consciousness, and death from asphyxiation.
CO also plays a role in complex air chemistry. While not a direct greenhouse gas, it contributes indirectly to global warming by influencing the concentrations of other gases. CO reacts with hydroxyl radicals, which are atmospheric cleansing agents that break down gases like methane and ozone precursors. By consuming these radicals, CO allows the atmospheric concentration of these other potent gases to increase.