In the basement were three heaters to keep the swimming pool water warm

In the basement were three heaters to keep the swimming pool water warm. The third was not properly installed, and was venting fumes directly into the basement, rather than outside. The following morning, the tennis player was dead, with a post mortem carbon monoxide level in his bloodstream of 75 per cent. The central nervous system appears to be the system most sensitive to the effects of the gas.Another misconception is that only the poor or those in bedsits owned by dodgy landlords are at risk. But it poisons the body in many other ways, that are only now being recognised. It interferes with the ability of the body's cells to use oxygen.

It makes some parts of the blood more sticky, causing it to sludge up in tiny capillaries, and it damages the lining of blood vessels, giving rise to sometimes irreversible damage in delicate structures around them. For years, surviving victims of exposure who claimed to have symptoms were dismissed as neurotic because no carbon monoxide could be measured in their blood. Now it is clear that they have real, permanent damage that can be seen on brain scans like Denise's, with changes strikingly similar to those found in multiple sclerosis. Carbon monoxide is well known for binding strongly to haemoglobin, the oxygen-carrying protein in red blood cells. It binds so strongly that it will displace oxygen, causing death by chemical asphyxia And that's as far as most books go.

One day, a gas boiler may be working perfectly well - the next, a combination of factors such as debris falling into the chimney and the wind blowing in the wrong direction, may be enough to spill the gas into your house.The books are wrong about other things too. If you burn gas, coal, oil, paraffin or wood without sufficient fresh air around, carbon monoxide (rather than the dioxide, the greenhouse gas) will be produced. You can't smell it, see it or taste it (at least that's what the medical textbooks say; some victims will tell you it has the faint scent of almonds.) It's certainly deadly, and also capricious. She is still plagued by poor memory, and exhibits other strange phenomena like a body temperature always two degrees below normal, and a pulse rate of 48 beats per minute, which is normal for a trained athlete, but not for a "forty-year-old smoker."Carbon monoxide is produced from the incomplete combustion of any hydrocarbon fuel.

The idea of poisoning by an invisible gas seems a little preposterous, and victims who suggest the idea themselves are sometimes labelled paranoid. While Guy's hospital admits that "the carbon monoxide poisoning was not identified as the cause of her symptoms until her faulty gas fire had been detected," it also says Denise "had a range of very unusual symptoms which were not typical of carbon monoxide poisoning".According to Debbie Davis, who runs Carbon Monoxide Support, a Leeds- based charity, this is far from true Denise exhibited classic symptoms of CO poisoning. Although chronic exposure can mimic a lot of other ailments, it is relatively easy to test for. The limiting factor, and the reason people are given the wrong diagnosis again and again, is that no-one thinks of the possibility.

When she spoke to a junior doctor at the hospital, the penny dropped. It was carbon monoxide poisoning all along.Denise's story is far from unique. People with chronic carbon monoxide (CO) poisoning are misdiagnosed all the time. It was only then that Denise, a fully-trained general nurse who now works in palliative care, realised the true cause of her symptoms.