Autumn will soon be in full swing, and it won't be long before the central heating is cranked up in many British homes. But as well as warming people up, the big switch-on could also bring with it a silent killer.

Faulty appliances that use fossil fuel (coal, gas, oil or wood) can produce carbon monoxide, a difficult to detect colourless, odourless and tasteless gas - which can also cause sudden collapse, loss of consciousness and even death.

It's estimated that more than 4,000 people in the UK attend A&E departments each year because of carbon monoxide poisoning, with at least 40 dying from it.

The Department of Health estimates the true number of people exposed to sub-lethal amounts of carbon monoxide is even greater, however. Older people, children, pregnant women and their unborn children, and those with breathing problems or cardiovascular disease, are at increased risk of its effects.

Yet, research suggests that 43% of Britons don't have their gas appliances checked annually, as is advised, and 10% have never had them checked at all.

Gas Safety Week (September 15-21) aims to draw attention to this potentially fatal safety check omission, in a bid to cut poisoning cases.

When inhaled, carbon monoxide reduces the oxygen-carrying capacity of blood, and so starves vital organs of oxygen. As more carbon monoxide is breathed in, less oxygen can be carried in the blood and symptoms, which can include headaches, drowsiness, dizziness, chest pains, nausea and vomiting, worsen. High levels can potentially lead to organ failure and can kill, sometimes quite rapidly.

Long-term exposure can also be associated with lasting neurological problems, like difficulty concentrating.

Symptoms can often mimic flu or food poisoning, and Scott Darroch, spokesperson for the Gas Safe Register, the register of qualified, legal gas engineers which runs Gas Safety Week, warns: "People with carbon monoxide poisoning might not necessarily put two and two together. You're feeling dizzy and a bit ill - is it flu? Carbon monoxide poisoning can be very hard to detect."

Darroch explains that Gas Safety Week aims to highlight the fact that carbon monoxide poisoning, as well as fires or explosions, can occur when a gas appliance is unsafe, usually because it hasn't been serviced regularly or fitted and maintained properly.

The Gas Safe Register estimates there are around 7,500 illegal gas fitters operating across the UK, and up to 250,000 illegal gas jobs may be carried out each year.

"Carbon monoxide is a risk people should really be aware of," warns Darroch. "It's pretty easy to spot a fire, but you might not be aware that you've been exposed to carbon monoxide."

Dr Simon Bouffler of Public Health England's Centre for Radiation, Chemical and Environmental Hazards (CRCE), says: "Many carbon monoxide poisoning deaths are preventable.

"To lower the risk, people should ensure their fossil fuel and wood burning appliances are regularly checked by an appropriately registered engineer. "

He recommends that appliances and flues are checked before the start of winter, and that rooms in which appliances are used are adequately ventilated.

He also advises people with potential carbon monoxide poisoning symptoms to visit their GP, and tell the doctor that they may have been exposed to the gas.

Carbon monoxide alarms, which can be bought from most DIY stores, are increasingly fitted in British homes. However, the Gas Safe Register stresses that the most important precaution, with regards to avoiding leakage of the lethal gas, is to make sure appliances are well maintained.

"A carbon monoxide alarm is a good second line of defence, but getting an appliance installed, serviced and maintained by a qualified person on an annual basis is the best thing you can do," stresses Darroch.

Carbon monoxide can also be produced when fuels such as oil, charcoal, coal and wood don't burn completely. It can also build up when flues or vents are blocked, for example, where a bird builds a nest over a flue.

KNOW THE RISKS

The dangers associated with carbon monoxide poisoning should never be underestimated, as Stacey Rodgers knows from personal experience, as it caused the death of her 10-year-old son, Dominic, in 2004. Extra shockingly, the leak came from a faulty flue and boiler in the house next door to theirs. The devastated mum, from Huddersfield, set up the Dominic Rodgers Trust in memory of her son, in a bid to raise awareness of the dangers of carbon monoxide poisoning.

"The carbon monoxide seeped through brickwork underneath Dominic's bedroom, and killed him while he was sleeping," she explains. "Be aware people, make sure you get all your fuel burning appliances checked by a Gas Safe registered engineer, and in the second line of defence, fit a carbon monoxide alarm. The alarms have proven to save lives; this is the only way you will know carbon monoxide is present."

It's also important to know the symptoms of carbon monoxide poisoning, she stresses. "Recognising the early symptoms will save your life. Carbon monoxide is called the silent killer because it has no smell and no taste - you can't see it, so you don't know it's there. Some people could be getting poisoned now, so we need to get the message out and stop innocent victims from being killed."

CARBON MONOXIDE CLUES

Watch out for these potential warning signs in your home:

:: Black, sooty staining on or around an appliance.

:: A yellow gas flame from gas appliances, rather than a blue flame - although this doesn't apply to fuel-effect, living-flame or decorative-flame gas fires.

:: A lot of condensation inside.

:: Smoke accumulating in rooms due to faulty flues.

:: People, and even pets, living in the same house are displaying symptoms which could indicate poisoning.

:: You're experiencing symptoms which improve when you're outside.

:: For more information, visit www.gassaferegister.co.uk