Shown here are four representations chemists use for carbon monoxide. In the colored models, carbon is light gray and oxygen is red. Click on image for full size (19 Kb GIF) Windows to the Universe original artwork by Randy Russell.

Carbon monoxide is a colorless, odorless, tasteless gas. It is also flammable and is quite toxic to humans and other oxygen-breathing organisms. A molecule of carbon monoxide (CO) contains one carbon atom and one oxygen atom.

Carbon monoxide is present in Earth's atmosphere at very low concentrations. A typical concentration of CO in Earth's troposphere is around 100 ppb (parts per billion; meaning one hundred out of every billion air molecules is carbon monoxide), although especially clean air can have concentrations as low as 50 ppb. Carbon monoxide has a typical "lifespan" of several months in Earth's atmosphere. The gas eventually reacts with oxygen (O₂) to form carbon dioxide (CO₂).

Natural sources of carbon monoxide in Earth's atmosphere include volcanoes and bushfires. Volcanic gases contain between 0.01 and 2% carbon monoxide. Humans contribute vast quantities of CO to our atmosphere, mostly as a result of automobile emissions. Carbon monoxide is the most abundant, by mass, pollutant gas. Atmospheric carbon monoxide levels in typical urban areas are around 10 ppm (parts per million), about 100 times higher than in Earth's atmosphere overall. In areas with heavy traffic, CO levels can rise to as high as 50 ppm. Certain industrial processes, along with fossil fuel and biomass burning, are major human-produced sources of CO. In the United States, about 2/3rds of carbon monoxide emissions come from automobiles. Scientists estimate than the total annual production of carbon monoxide, from human and natural sources, is between two and five gigatons.

Carbon monoxide is a relatively unreactive compound, so it doesn't pose much threat to plants or exposed materials. However, the gas is quite poisonous to humans and other air-breathing creatures that need oxygen. Hemoglobin, the protein in blood that carries oxygen from the lungs to cells throughout the body, is more than 200 times more prone to bind with carbon monoxide than it is with oxygen. This means that someone who breathes too much CO can have their hemoglobin become saturated with it, making it impossible for the blood to deliver oxygen to their cells.

Carbon monoxide is often a product of incomplete combustion. If there is too little oxygen, or too much carbon, present when something burns, the burning produces carbon monoxide (CO) instead of (or as well as) carbon dioxide (CO₂). Improperly ventilated kerosene lanterns and stoves can generate dangerously high carbon monoxide levels in enclosed spaces.

Since CO takes longer to mix completely throughout the troposphere than a typical CO molecules survives (before being oxidized to CO₂), carbon monoxide exhibits locally elevated or depleted concentrations in the vicinity of sources (places where it is created or released into the atmosphere) or sinks (places where it is destroyed or removed from the atmosphere) respectively. Worldwide measurements of CO concentrations can, therefore, help to locate sources and sinks of this gas.

Carbon monoxide indirectly contributes to the buildup of some greenhouse gases in the troposphere. It reacts with certain chemicals that would otherwise destroy methane and ozone, thus helping to elevate the concentrations of methane and ozone.

Although the CO molecule is shown in the figure on this page as having a double bond, the bond in CO is somewhat unusual. The bond between the oxygen and the carbon atom is a bit of a hybrid that can be considered, at times, a single, double, or triple bond.

Carbon monoxide is found beyond Earth as well. It has been detected in gaseous nebulae, in the atmospheres of other planets, and in the ices of comets.

Here are four different ways chemists use to show a molecule of carbon monoxide. In the colored molecule models, carbon is light gray and oxygen is red. Click on image for full size (19 Kb GIF) Windows to the Universe original artwork by Randy Russell.

Carbon monoxide is kind of gas. It is poisonous. A molecule of carbon monoxide (CO) has one carbon atom and one oxygen atom. Earth's atmosphere has a small amount of carbon monoxide in it.

The carbon monoxide in air comes from nature and from things people do. Volcanoes and forest fires give off carbon monoxide. Most of the carbon monoxide people make comes from the engines of cars and trucks. Carbon monoxide is an important kind of air pollution.

When fuel burns, it usually makes carbon dioxide (CO₂). Sometimes when fuel burns there isn't enough oxygen, or there is too much carbon. When this happens, the burning creates carbon monoxide, too.

Earth is not the only place we find carbon monoxide. It has been found in gaseous nebulae, in the atmospheres of other planets, and in the ices of comets.

Here are four different ways chemists use to show a molecule of carbon monoxide. In the colored molecule models, carbon is light gray and oxygen is red. Click on image for full size (19 Kb GIF) Windows to the Universe original artwork by Randy Russell.

Carbon monoxide is kind of gas. It is poisonous. A molecule of carbon monoxide (CO) has one carbon atom and one oxygen atom. Earth's atmosphere has a small amount of carbon monoxide in it.

The carbon monoxide in air comes from nature and from things people do. Volcanoes and forest fires give off carbon monoxide. Most of the carbon monoxide people make comes from the engines of cars and trucks. Carbon monoxide is an important kind of air pollution.

When fuel burns, it usually makes carbon dioxide (CO₂). Sometimes when fuel burns there isn't enough oxygen, or there is too much carbon. When this happens, the burning creates carbon monoxide, too.

Earth is not the only place we find carbon monoxide. It has been found in gaseous nebulae, in the atmospheres of other planets, and in the ices of comets.