Carbon monoxide (CO) is a colourless, odourless and tasteless gas with relatively poor solubility in water. Anthropogenic emissions of CO originate primarily from incomplete combustion of carbonaceous materials. The largest carbon monoxide emissions are produced as exhaust of internal combustion engines, especially of vehicles with petrol engines.

The lungs are the only significant routes for CO uptake from the environment. Carbon monoxide diffuses rapidly across alveolar, capillary and placental membranes. Inhaled CO has no direct toxic affects on lungs but rather appears to exert its effects by interfering with oxygen transport through the formation of carboxy-haemoglobin (COHb). Exposure to CO is often evaluated in terms of COHb levels in blood measured as percentage of total Hb bound CO. Approximately 80% to 90% of absorbed CO binds with haemoglobin to form carboxy-haemoglobin, which reduce the oxygen carrying capacity of the blood and impair the release of oxygen from haemoglobin (Table 7). COHb levels in non-smokers range between 0.3% to 0.7% and 5% to 7% in smokers. COHb levels in excess of 15% in a significant proportion of urban non-smoking populations can be considered as evidence of widespread exposure to environmental carbon monoxide.

In a study conducted under controlled laboratory conditions, healthy subjects exposed to CO, sufficient to result in 5% COHb levels exhibited reduced duration of exercise performance and consumption of oxygen. Studies involving subjects with deficient blood supply to the heart (Ischemic Heart Disease) who were engaged in exercise during exposures have shown that COHb levels as low as 2.2% can lead to:

1. earlier onset of electrocardiograph change indicative of increased deficiency of oxygen supply to the heart;

2. earlier onset of chest pain;

3. increase in the duration of chest pain; and

4. decrease in oxygen consumption.

Table 6 Symptoms Based on Blood Carboxy-haemoglobin Levels

Percent Hb	Medical Symptoms and Consequences
10%	No symptoms. Heavy smokers can have as much as 9% COHb
15%	Mild headache
25%	Nausea and serious headache. Fairly quick recovery after treatment with oxygen and/or fresh air
30%	Symptoms intensify. Potential for long term effects especially in the case of infants, children, the elderly, victims of heart disease and pregnant women.
45%	Unconsciouness
50%+	Symptoms Leading to Death

Table 7 Symptoms Based on Carbon monoxide in Ambient Air

CO (PPM)	Exposure	Symptoms
35	8 hours	Maximum exposure limit allowed by OSHA in the workplace over an eight hour period.
200	2-3 hours	Mild headache, fatigue, nausea and dizziness
400	1-2 hours	Serious headache and other symptoms intensify. Life threatening after 3 hours
800	45 minutes	Dizziness, nausea and convulsions. Unconscious within 2 hours followed by death within 2-3 hours

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