Sources of Carbon Monoxide

3. Carbon monoxide is produced by the incomplete combustion of organic substances or those that are essentially just carbon, such as coke. Complete combustion, in the presence of sufficient oxygen, leads to production of carbon dioxide, whereas if there is a slight deficiency of oxygen some carbon monoxide is formed. Thus most combustion processes produce some carbon monoxide, depending on the efficiency of the process and the availability of oxygen. From the point of view of human health, some forms of exposure to carbon monoxide are particularly important. For example, in some industrial processes and in coal mines, pockets of high concentrations of carbon monoxide may occur and cause serious health effects. In the home, gas cookers, paraffin heaters and bottled gas heaters operating in poorly ventilated rooms, or badly installed or maintained flued gas, oil or solid fuel heaters may cause high levels of carbon monoxide in the air, leading to lassitude, unconsciousness or even death. In suicide bids the role of the carbon monoxide contained either in town gas, prior to the introduction of natural gas, or in motor car exhaust gases is well known; as are the dangers of carbon monoxide to those, including fire fighters, involved in fires in buildings or other confined spaces.
4. Apart from the major accidental and domestic sources of comparatively high levels of carbon monoxide, the most important general exposures of individuals to the gas come from cigarettes and vehicles. Cigarettes are mainly responsible for exposure of smokers, although some passive exposure of others will occur from inhalation of other people's smoke indoors. The amount of carbon monoxide in the blood of a regular smoker is considerably greater than that which can be obtained by breathing air in even heavily polluted streets (see Table 1), and therefore, the recommended Air Quality Standard for carbon monoxide aims to limit the exposure of non-smokers. For non-smokers, the main source of outdoor exposure to carbon monoxide is general pollution of the atmosphere by vehicle exhausts. As far as total exposure is concerned, again for non-smokers, indoor sources can account for a larger proportion than traffic.

Table 1 Average blood carboxyhaemoglobin* (COHb) levels in persons exposed to carbon monoxide

	COHb%
	Non-smokers	Smokers
Calculated values:
Rural background exposure	0.5**	-
Urban exposure to 10 ppm***	1.0	-
Urban exposure to 25 ppm***	2.7	-
Measured values****
Urban background exposure	0.8	3.5
Point-duty police (after 3 hours in busy street)	1.9	3.6
Others on foot in busy streets	1.2	-
Cyclists (city streets)	1.7	-
Motorists	1.8	-
Staff in parking garages	2.4	5.0
Staff in customs sheds, ferries	1.3	4.2

*	The absorption of carbon monoxide results in the formation of carboxyhaemoglobin in the blood.
**	This represents the background value due to the natural formation of carbon monoxide in the body.
***	These values are estimated from Figure 4 (see page 22), on the assumption that the individual is undertaking light activity whilst continuously exposed to the stated concentration of carbon monoxide for a period of 8 hours. In reality, such exposures are unlikely to occur in ambient air since the measured value would vary depending both on the individual and the circumstances of exposure. The concentration of 25 parts per million (ppm) is not exceeded in ambient air nowadays in the United Kingdom.
****	Adapted from Lawther and Commins, 1970. Values of blood COHb measured in the 1960s in groups of between 5-165 persons. Levels of carbon monoxide in urban areas in the United Kingdom have decreased significantly since then and levels of COHb in blood are expected to be lower nowadays.

 

5. Combustion systems in vehicles are normally designed to operate most efficiently when there is just enough air to oxidise the carbon in the fuel. When the engine is cold or badly tuned, or when the engine is idling or moving slowly, it will depart from this optimal condition and produce more carbon monoxide.Thus it might be anticipated that levels of the gas in the ambient air would be highest close to busy roads in cities, especially when traffic flow is reduced as in rush hours. Atmospheric conditions, such as the periods of cold still air occurring at times in winter, may of course prevent the dispersal of this and other gases, leading to higher concentrations. Carbon monoxide emitted by motor vehicles in urban areas is normally rapidly dispersed away from roads and then is destroyed by photochemical reaction over a period of months.
6. In the United Kingdom, it has been estimated that some 6.7 million tonnes of carbon monoxide are emitted into the atmosphere each year, and that 87% of this comes from petrol-engined vehicles. The relative contributions of different sources are shown in Table 2. In line with the increase in traffic, there has been an increase in these emissions of approximately 50% from the 1970 values (see Figure 1). Recently, there has been evidence of a decline in vehicle emissions, following increasingly strict European Community directives. Improved engine design, with fuel injection and engine management systems, the introduction of catalytic converters, and the adoption of emission standards in the annual MOT test will reduce levels further, although this favourable trend will be opposed by any increases in traffic.

Table 2 Estimated United Kingdom emissions of carbon monoxide by emission source, 1990-92, thousand tonnes per year*

Source	1990	1991	1992	Percentage of total in 1992**
Road transport: Petrol Diesel	6,152 157	6,140 164	5,865 164	87 2
Power stations	50	47	45	1
Domestic	286	293	258	4
Commercial/public service	8	8	7	-
Refineries	1	1	2	-
Iron and steel	28	26	25	-
Other industrial combustion	41	42	42	1
Extraction and distillation of fossil fuels	33	35	36	1
Waste treatment and disposal	220	220	220	3
Railways	12	12	12	-
Aircraft	11	11	11	-
Shipping	20	20	19	-
Agriculture	1	1	1	-
Total	7,020	7,021	6,708	100

*     Figures rounded tothe nearest 1,000tonnes.

**    Figures rounded to the nearest 1%.