Carbon monoxide (CO) is a well-known air pollutant, and Volatile Organic Compounds (VOCs) are another frequently discussed category of airborne substances. This article examines the chemical and regulatory definitions of VOCs to clarify whether carbon monoxide fits this classification. Understanding the precise criteria for VOCs and the unique structure of carbon monoxide is necessary to resolve this question.
Understanding Volatile Organic Compounds
Volatile Organic Compounds are generally defined by two main criteria: volatility and organic composition. Volatility refers to the tendency of a substance to vaporize, or turn into a gas, at normal atmospheric temperature and pressure. For regulatory purposes, this is often linked to a low boiling point, such as having an initial boiling point less than or equal to 250°C.
The “organic” component means the compound must contain carbon atoms. Most organic compounds also feature carbon bonded with hydrogen atoms, forming the basis of organic chemistry. VOCs are prevalent in many common household products, including paints, varnishes, cleaning supplies, and solvents, and they are released as gases from these sources both indoors and outdoors.
In outdoor air quality regulation, the primary concern with VOCs is their role in forming ground-level ozone, a main component of smog. True VOCs react with nitrogen oxides in the presence of sunlight to create this secondary pollutant. This photochemical reactivity is a major factor in how regulatory bodies determine which compounds to control under the VOC umbrella.
The Chemical Structure of Carbon Monoxide
Carbon monoxide has a simple chemical formula, CO, meaning each molecule consists of one carbon atom and one oxygen atom. These two atoms are connected by a powerful triple bond, making it the simplest carbon oxide. It is a colorless, odorless, and tasteless gas, which is why it poses such a significant hazard indoors.
The primary source of carbon monoxide is the incomplete combustion of carbon-containing fuels, such as wood, gasoline, natural gas, or coal. This occurs when there is insufficient oxygen present to produce carbon dioxide (CO2). Carbon monoxide is highly toxic because it binds to the hemoglobin in the blood, displacing oxygen and preventing its transport to organs like the brain and heart.
Why Carbon Monoxide Is Not Classified as a VOC
Despite containing carbon and being highly volatile—its boiling point is approximately -191.5°C—carbon monoxide is not classified as a Volatile Organic Compound for regulatory purposes. The most definitive reason for its exclusion is that it lacks the hydrogen atoms typically present in organic compounds. Carbon monoxide is a simple oxide of carbon, and simple carbon oxides like CO and CO2 are generally classified as inorganic compounds, not organic ones.
Specifically, the United States Environmental Protection Agency (EPA) regulatory definition of VOC explicitly excludes carbon monoxide, along with carbon dioxide and metallic carbonates. This exclusion is based on the compound’s negligible contribution to the atmospheric photochemical reactions that form ground-level ozone, which is the main environmental concern driving VOC regulation.
While carbon monoxide participates in atmospheric chemistry, it does not function as a precursor to smog in the same manner as true VOCs. The EPA focuses its VOC control efforts on compounds that significantly increase ozone concentrations. Carbon monoxide is instead regulated as a separate criteria air pollutant due to its direct toxicity to human health.