Fewer people have killed themselves using car fumes since rules on cleaner car emissions were introduced, US researchers have found.

Researchers from the Centre for Disease Control and Prevention, Atlanta looked at data from before and after the introduction of the 1970 Clean Air Act which set low carbon monoxide (CO) emission rates.

They also looked at the effect of the introduction of catalytic converters, which improve how efficiently engines work, in 1975.

In the UK, catalytic converters have had to be installed on all new cars since 1992.

Anything that goes towards reduces people's ability to take their own life is to be welcomed

Spokeswoman for The Samaritans

The researchers looked at 116,703 deaths where CO poisoning was listed as a contributing factor from the late 60s to the late 90s. Cars were mentioned in 70% of cases.

Almost 74,000 were suicides; the rest unintentional deaths.

Researchers found car-related carbon monoxide suicides fell by 43% between 1968 and 1998.

The overall CO-related death rates fell by an estimated 57.8%.

The introduction of the catalytic converter in 1975 saw unintentional car-related CO deaths fall by around 81%.

Health benefit

CO is formed during the incomplete combustion of fuel, including gasoline used in cars.

In addition to CO poisoning from cars running in enclosed or semi-enclosed spaces, people can also be at risk from heating systems and improperly vented combustion appliances.

The researchers, led by Dr Joshua Mott, said: "An estimated 11,700 unintentional motor vehicle-related poisoning deaths may have been averted since the year of introduction of the catalytic converter, reflecting what appears to be a public health benefit associated with enforcement of the 1970 Clean Air Act."

NHS figures for England and Wales looking at the period 1960 to 1998 indicate a fall in young male suicides, thought to be because of the introduction of catalytic converters.

A spokeswoman for The Samaritans told BBC News Online: "One of the things we know about suicide is that access to lethal means of killing yourself increases the propensity to commit suicide.

"Anything that goes towards reduces people's ability to take their own life is to be welcomed.

"We find that time and space to work out a way of moving forward can make the difference between people taking their own life, and not doing."

The research is published in the Journal of the American Medical Association.