Carbon monoxide (CO) occurs as a result of combustion taking place in an atmosphere where there is a shortage of oxygen. Emissions of CO are low from Diesel engines because, overall, the charge is plentiful in oxygen. Nevertheless, some CO is still generated in localised regions of the combustion chamber which are oxygen deficient. There is a bigger problem with spark-ignition engines, especially during cold starts or transient conditions.
The main reason that CO is considered a problem is because of its effects on people rather than the environment. At relatively low concentrations (e.g. 100 ppm) it causes headaches, fatigue and nausea. At higher concentrations, or after prolonged exposure, unconsciousness and even death can be the result.
