University of Bristol researchers said that contributory factors to the drop in suicide probably includes less unemployment and laws that have reduced the risks from car exhaust fumes.

Using data on suicide mortality, population statistics and surveys, the study found that for 15 to 24 year-old men, the overall suicide rate dropped from 16.6 per 100,000 people in 1990 to 8.5 per 100,000 in 2005. Among men ages 25-34, overall suicide rates declined from 22.2 per 100,000 in 1990 to 15.7 per 100,000 in 2005.

Car exhaust emission legislation in 1993 has contributed to falling suicide rates, because it has led to a marked reduction in car exhaust gas poisoning due to catalytic converters, the authors said in a statement.

"Favorable 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," study co-author David Gunnell said in a statement.