The suicide rate among young men in England and Wales is at the lowest level for 30 years, say researchers.
The suicide rate was 8.5 per 100,000 in 2005
One key factor has been a cut in toxins in vehicle exhaust fumes because of catalytic converters - making it harder for people to kill themselves.
For women, suicide rates are at their lowest level since 1968, but more are killing themselves by hanging, the Bristol University team reported.
Charities said more still needed to be done to prevent suicides.
Suicide rates among young men more than doubled between the early 1970s and the 1990s.
But since then, there has been a steady decline, the research published in the British Medical Journal found.
Using data on suicides from 1968 to 2005, the researchers found that for males aged 15 to 24, the overall suicide rate dropped from 16.6 per 100,000 people in 1990 to 8.5 per 100,000 in 2005.
Amongst men aged 25 to 34, overall suicide rates declined from 22.2 to 15.7 per 100,000 over the same period.
The study said car exhaust emission legislation introduced in 1993 contributed to falling suicide rates from carbon monoxide poisoning, because of an increased number of cars with catalytic converters, which make fumes less toxic.
Suicide rates were also found to mirror unemployment rates.
For women, suicide rates have been fairly steady. However, the proportion of women aged 15 to 34 committing suicide by hanging has increased "massively" - from 5.7% of all suicides in 1968 to 47.3% by 2005.
More research is needed to find out why this is, the study says.
Since the 1990s, antidepressant prescribing in the general population has increased, which some believe may have also contributed to falling suicide rates.
Lead researcher Professor David Gunnell said the reasons behind falling rates of suicide were complex.
"Favourable changes in several different factors - levels of employment, substance misuse and antidepressant prescribing, as well as policy focus on suicide and vehicle exhaust gas legislation - may have contributed to the recent reductions," he said.
The Department of Health, which has introduced a strategy and targets to reduce suicide, said it would continue to work to find ways of supporting people in distress.
Marjorie Wallace, chief executive of the mental health charity Sane, said the figures were encouraging.
"However, simply removing one means of suicide may reduce the numbers but as the evidence indicates people can turn to other - possibly more disturbing - ways, such as hanging.
"One unnecessary loss of life is one too many, and we must fight to raise awareness of the complex triggers to suicide, and make available treatments such as talking therapies immediately the warning signs are identified."
The Samaritans said: "We encourage people to seek help as early as possible and we are developing new services such as our SMS service, particularly aimed at younger age groups."
Alison Cobb, policy officer at Mind, said there was still much work to be done to reduce the stigma associated with mental health problems.
"One in three men with stress, depression or anxiety say they feel embarrassed about seeking help, choosing instead to bottle up their emotions. We need to break down this barrier."
A separate study by the same team, also published in the BMJ, found that a recent substantial drop in the use of antidepressants in children and adolescents had not led to an increase in self-harm or suicide, as some had feared.
Prescriptions of a type of antidepressants called selective serotonin reuptake inhibitors (SSRIs) in the under-18s have fallen by almost half in the past few years, after drug regulators issued stronger warnings on their use.
Professor Gunnell said concerns about the dangers of reducing antidepressant use in children, who might therefore be at an increased risk of committing suicide, had been raised by US research.
"There's been a greater fall in antidepressant prescribing in the UK but we have seen none of the potentially alarming upturn in suicides."