Most people know a thing or two about toxic gas. It's usually green, or possibly yellow, like the poison gas you hear of in First World War accounts, or see the Joker using in Batman movies. It will make you cough and choke, but thankfully the effects can be avoided by the simple expedient of holding a handkerchief over your mouth.
Unfortunately, none of the above applies to one of the most poisonous gases on the planet. Like many really nasty toxins, this one is a gift from Mother Nature. Carbon monoxide is so simple that it can be made by anyone with access to a car whose catalytic converter is not yet warmed up. Or, even easier, just find a gas fire and block its flue.
Until the mid-Sixties, thousands of people every year killed themselves with carbon monoxide (CO), usually - like the poet Sylvia Plath - by sticking their heads in unlit gas ovens. Murderers, like the wartime killer John Christie of Rillington Place, had a handy tool literally on tap. Then, most domestic gas was "coal gas", made by partly burning coal in huge retorts and piping the resulting fumes - which held high concentrations of CO - into people's homes.
Then the UK changed over to the new, clean, "natural", North Sea gas. It burnt with such a bright, clear flame - rather than the yellow flicker of coal gas - that users were warned of the difference, lest they mistakenly thought that their appliance had gone out. Great stress was placed on preventing leaks (in case of explosion, not poisoning), and nasty smells were added to the pure gas so that any leaks could be detected. Much was made of the purity of the new gas and its lack of toxicity. Natural gas is hardly poisonous at all. Until, that is, you start to burn it.
Just like the partly burnt coal whose gases were used to heat the nation's ovens, partly burnt natural gas will also contain carbon monoxide (when burnt completely, like all hydrocarbon fuels, it degrades to water, and to CO's famous cousin, the greenhouse-gas carbon dioxide, CO2). The easiest way to partly burn natural gas is to light a gas fire that is connected to a blocked chimney. Scenarios like this kill about 50 people in the UK every year and poison hundreds more, at times causing permanent disability.
Cases of carbon monoxide poisoning peak in cold weather when heating is turned up high. Most dangerous is an unexpected cold snap, when heaters, boilers and fires that have not been used for months are sparked up for the first time. Things are even more dangerous when the air is still outside and there's no breeze to assist decrepit chimneys. And, of course, every winter there are TV campaigns that encourage us to seal our doors and windows against draughts. Very good at keeping out the cold; very good at keeping noxious fumes in.
So how was a gas traditionally associated with cold winter nights able so ruthlessly to devastate a family on holiday in sunny Corfu? It seems the culprit in this case was a faulty water-heater, powered by gas and venting its fumes into their hotel room. The case has resonances with that of the Eighties tennis star Vitas Gerulaitis, who was killed by a gas-powered heater used to warm the water in the swimming pool of the Long Island mansion where he was staying.
All you need to risk exposure to CO is a closed environment, and something - anything - that burns or even just smoulders. In February last year, a 34-year-old man, Lewis Dunton, died in Morocco after taking embers from a camp fire into his tent to keep warm.
Even if your home is totally free of any potential CO source, be it gas, paraffin, coal or wood, you can be in danger from your neighbours if you have an adjoining wall. When the Cheetham family, including two boys aged eight and 10, were killed by a faulty gas fire in Chesterfield in 1999, the same fumes also killed their 79-year-old neighbour. Their terraced houses shared a chimney.
The picture is made more frightening by the attitude of some physicians. Many doctors, confronted with more than one member of a household exhibiting similar symptoms, will put those ailments down to some mysterious "virus".
Just for a change, "food poisoning" or "poisonous mushrooms" were initially blamed for the Corfu tragedy. Unfortunately, the idea of a toxic gas in the house is regarded by some doctors as rather eccentric, even verging on the paranoid.
The false diagnosis of "virus" is particularly dangerous in cases of CO poisoning (which can cause the classic "flu-like" symptoms of headache, lethargy and muscle pains, and also gastrointestinal upset, especially in children), because the advice is often to go home and keep warm - a recipe for disaster if the source of warmth is also the source of your illness.
Thankfully, there is something all of us can do to avoid the horrible effects of this ubiquitous gas. As well as making sure that gas appliances are checked and chimneys swept, invest in a carbon monoxide alarm. A decent CO alarm, one with a real-time digital readout, will cost about £40. They can be battery-powered and portable, so easy to take on holiday with you. What better way to show someone you care at Christmas than to buy them a present that could save their life?
Actually, given that between now and the end of the year another 10 people or so are going to die from carbon monoxide in the UK, it might be better to splash out on one today.
CO Awareness Week starts on 4 December (www.co-awareness.co.uk)