Carbon monoxide (CO) gas is a colorless, odorless, and toxic gas principally generated by the incomplete combustion of carbon-based fuels. The mechanism of its toxicity to people and animals is the preferential binding of CO gas molecules to the hemoglobin in the blood. At significant concentrations of carbon monoxide gas in air, the hemoglobin in your blood latches on to CO molecules instead of oxygen (O2) molecules, and remains bound to the hemoglobin, preventing it from transporting oxygen. The result is that your blood rapidly loses its oxygen-carrying capacity, and your body asphyxiates from within. Like hydrogen sulfide, carbon monoxide is also flammable (LEL = 4%), but its toxic properties are generally the larger concern when released into the atmosphere.
Carbon monoxide is not to be confused with carbon dioxide (CO2) gas, which is almost completely inert to the human body. Carbon dioxide is principally produced by the complete combustion of carbon-based fuels. Its only safety hazard potential is the capacity to displace breathable air in an enclosed area if rapidly released in large volumes.
Combustion burners operating on carbon-based fuels may produce excess carbon monoxide if operating at too rich an air/fuel mixture. Even when adjusted optimally, there will always be some carbon monoxide present in the exhaust. This makes high CO concentrations possible where burners operate in enclosed areas.
Some industrial processes such as catalytic cracking in the petroleum refining industry generate huge amounts of carbon monoxide, but these extremely high concentrations are normally present only within the process piping and vessels, not released into the atmosphere. Nevertheless, personnel working near such processes must wear portable CO gas safety monitors at all times to warn of leaks.
Carbon monoxide may be sensed by an electrochemical cell, using iodine pentoxide as the reacting compound. The balanced chemical reaction is as follows:
5CO + I2O5 → 5CO2 + I2
The strength of the electric current produced by the cell indicates the concentration of carbon monoxide gas.
Instrumentation Basics 