Science: Molecule of the Month: Silent killer in the air we breathe: John Emsley explains the perils of carbon monoxide

ON THE first Saturday this spring, Michael and Deborah Mason and their two sons, Christopher, four, and Jeremy, two, left Fulham to spend a weekend at their cottage in Muddiford, north Devon. Sadly they were not aware that a bird had been busy building its nest in the flue of the cottage's gas heater.

That weekend they all died of carbon monoxide poisoning. The haemoglobin in their red blood cells had been rendered useless by breathing this gas, and so was incapable of carrying out its essential function of transporting oxygen around the body.

Carbon monoxide (CO) is a colourless, odourless, highly toxic gas. We are all exposed to it because of trace amounts in the atmosphere, with the highest levels in cities, mainly from car exhausts. The CO in the air can tie up 5 per cent of the haemoglobin in our red blood cells, and if we smoke, this figure can be 10 per cent. If it reaches 30 per cent, we experience the symptoms of carbon monoxide poisoning: drowsiness, headache, dizziness and chest pains. When it gets to 50 per cent, we quickly die. CO is so avid for haemoglobin that we can reach this level if there is as little as 0.1 per cent CO in the air.

Victims of CO poisoning turn bright pink from the carboxy-haemoglobin in their blood. If they are caught in time, they can be saved. Treatment is simple: fresh air, or better still, oxygen.

According to John Timbrell, a toxicologist at the London School of Pharmacy, 'It takes about four hours to reduce the level of CO in the blood by half, but only one hour if you breathe pure oxygen. But even if a person survives CO poisoning they may well suffer permanent damage to their heart and brain.'

The worst mass poisoning by CO occurred on 2 March 1944 at Balvano, Italy, when a packed train stalled in the Armi Tunnel and 521 people died. The build-up of CO can be rapid if coal, oil or gas is burnt in an inadequate supply of air. These fuels use up oxygen as they combust and give off mainly carbon dioxide (CO2 ), plus a little CO. In a restricted supply of air they produce more CO.

David Jenkins, product safety adviser of the Royal Society for the Prevention of Accidents, says CO causes two hundred accidental deaths in the UK each year: 'People are most at risk when they move into new accommodation or holiday apartments - they may not realise that previous occupants have sealed ventilators or that a chimney has become blocked.'

In Britain, gas appliances must be fitted and serviced only by firms approved by the Council for Registered Gas Installers (Corgi), and regular maintenance is important. However, the Corgi code cannot protect holidaymakers abroad and several people have died of CO poisoning from gas water heaters in unventilated apartments.

Car exhausts can produce CO levels as high as 50ppm (parts per million) in heavy traffic. Winding up your windows does not protect you from the CO emitted by the car in front - the level of CO inside a car can be more than double that on the outside.

Gases from a car's exhaust contain 4 per cent CO when it is being driven, but this rises to 8 per cent when the car is in neutral. Suicides who attach a hose to the exhaust and pipe the fumes into the car die very quickly.

Carbon monoxide makes up between 0.05 and 0.2ppm of the atmosphere and there are about 500 million tonnes of the gas circling the globe. Each year we produce 450 million tonnes, and it stays in the atmosphere for about two months. About half the CO produced by humans comes from burning fossil fuels, and half from burning wood and straw. A lot of CO is generated naturally, from the oxidation of organic molecules such as methane in the atmosphere. Where all the CO ends up is still not resolved, but soil is a natural 'sink' and soil micro-organisms are thought to absorb the largest amounts.

The International Geosphere Biosphere Programme is studying all naturally occurring gases in the atmosphere. Carbon monoxide is being measured at sites around the world, but at concentrations of less than 0.1ppm it is not easy to monitor. To find better ways of analysing CO at these low levels, Norwegian scientists are looking at methods based on tuneable lasers, while at Europa Scientific of Crewe, Cheshire, they are investigating photoacoustic spectroscopy. Gary Atkins, a development scientist at Europa, says: 'We need a portable meter to measure CO rapidly. It can be done with infra-red light and this will be at the heart of our new detector.'

Carbon monoxide is manufactured on a large scale by the chemicals industry, and in Britain most of it is made by reacting methane gas with steam. The mixture of hydrogen gas and CO thus produced is used to make methanol, a liquid with many applications. In California it is blended with petrol to give a cleaner- burning motor fuel. In the UK, much of the methanol produced is reacted with yet more CO to form acetic acid and acetic anhydride, chemicals used to make plastics, paints and painkillers, and in pickling onions.

John Emsley is science writer in residence at the Department of Chemistry, Imperial College, London.