1 Carbon monoxide (CO)

• A colourless, odourless gas (so people cannot detect it — "silent killer"), commonly produced by the incomplete combustion of fossil fuels (petrol, coal, natural gas) when there is not enough oxygen: 2C + O₂ → 2CO, 2CH₄ + 3O₂ → 2CO + 4H₂O (compared with complete combustion CH₄ + 2O₂ → CO₂ + 2H₂O). Sources: car exhaust, faulty gas heaters and water heaters (closed bathrooms), generators, burning of waste, cigarettes.
• Its small molecule penetrates rapidly from the lungs to the blood, where it competes with oxygen (O₂) to bind with the haemoglobin in the red blood cells. CO binds to haemoglobin about 200 to 250 times more strongly than oxygen, forming carboxyhaemoglobin (HbCO), a fairly stable compound that prevents the oxygen from binding to haemoglobin.
• The reduced oxygen supply to tissues causes hypoxia (cells do not get enough oxygen for cellular respiration). The cells shift to anaerobic respiration, producing and accumulating lactic acid, which causes fatigue and headache.
• With higher HbCO concentration in the blood the symptoms become serious: dizziness, loss of consciousness, brain damage and death.

First aid: move the person into fresh air immediately, call for emergency medical care, give oxygen.

Problem. The affinity of haemoglobin for CO is 220 times that for O₂. Assume that the ratio HbCO/HbO₂ at equilibrium = 220 × (partial pressure of CO / partial pressure of O₂). Estimate the percentage of haemoglobin bound to CO when the air contains 100 ppm CO (0.01 %) and 21 % O₂ (use fractions in place of partial pressures).

Solution. Ratio = 220 × (0.0001/0.21) = 220 × 4.76 × 10⁻⁴ = 0.105. Fraction HbCO = 0.105/(1 + 0.105) = 9.5 %. So even 100 ppm of CO (only 0.01 % of the air) can disable about a tenth of the blood's oxygen-carrying capacity; at 1000 ppm the ratio becomes 1.05 and HbCO is about half, which causes unconsciousness. (This is a simplified model — it ignores the gradual approach to equilibrium — but it explains the extreme danger of CO.)