Research by the University of Birmingham's Medical School shows that successful suicides through CO poisoning have been falling since 1992, when catalytic converters became compulsory on new cars sold in the UK.

The researchers say that, if their data on the West Midlands area were applied to England and Wales, 950 people would not have died between 1991 and 1994.

Led by Dr Richard Wilson, they say people who choose CO poisining - mostly from car exhaust fumes - as a suicide method are more than twice as likely to be successful as those who choose other methods.

They call it a "depressingly successful way of taking one's life".

They believe people will start to resort to other methods once they realise how catalytic converters reduce CO levels.

They want more research into this area and more monitoring of poisoning cases.

Accidental poisoning

Suicides involving carbon monoxide have tripled since 1974 and reached a peak in 1991.

The seven-year study, published in Occupational and Environmental Health, also found that the number of people being poisoned by their gas heaters and boilers is rising in the West Midlands, while it is falling elsewhere.

The researchers say this may be because coroners in the region are more likely to record accidental death verdicts because of the stigma attached to suicides.

They say that, because of high-profile advertising campaigns, many people may think students in bedsits are at highest risk of accidental poisoning.

However, their research shows the elderly and very young are most at risk, particularly in winter months when gas fires are most in use.

Chest pains

CO reduces the amount of oxygen in the blood. The severity of poisoning depends on a variety of factors, including lung capacity and age.

The elderly and very young are most at risk from faulty gas appliances

Common symptoms of low-level poisoning range from headaches, nausea and chest pain to diarrhoea.

The researchers found 43% of CO poisoning cases treated in West Midlands' hospitals were accidental.

The number of admissions has been rising in the 1990s, although the fatality level has remained stable.

They said over half of all cases are due to gas pipe leaks, mobile gas fires or coke, coal or paraffin heaters.

The researchers say most cases are preventable and are calling on health and safety and gas fitters' organisations to target the whole population.

They also say officials should consider whether to promote the use of CO alarms to the public.

Winter campaign

The Health and Safety Executive (HSE) has just begun a winter campaign aimed at the general public.

As well as continuing to target students through adverts in student publications, it is also putting warnings in all big regional papers and advice on gas bills.

Leaflets will also be sent out with bills, warning people to get their gas appliances checked.

A spokesman said it could not force people who own their own houses to check their gas appliances - as it can with landlords - but could warn of the dangers.

It says people should not think that CO alarms can replace the need for regular checks of appliances, particularly old ones.

The HSE runs a Gas Safety Advice Line from 8am to 9pm from Monday to Friday. This gives advice on poisoning symptoms, how to maintain your gas system and how to report negligence by gas fitters.

The freephone number is 0800 300363.