RESUMEN
AIMS: A reduction in the carbon monoxide content of domestic gas and car exhaust gas has been associated with a decrease in gassing suicides in many western countries. In Japan, a reduction in the carbon monoxide content of domestic gas supply began in the early 1970s, and carbon monoxide emissions standards of new passenger cars were significantly strengthened in 1978. However, little is known about the impact of detoxification of these gases on gassing-related suicides in Japan. Therefore, we examined the changing patterns of suicide due to domestic gas or car exhaust gas inhalation by gender and age in Japan between 1968 and 1994. METHODS: Suicide mortality data were obtained from the Vital Statistics of Japan. In this study, age was divided into four groups: 15-24, 25-44, 45-64 and 65+ years. Method of suicide was divided into three groups: domestic gas, car exhaust gas and non-gases. We calculated method-specific age-standardised suicide rates by gender within each of the four age groups. We applied joinpoint regression to the data and quantified the observed changes. RESULTS: Suicide rates by domestic gas, regardless of gender and age, increased from 1968 to the mid-1970s and then decreased sharply. The proportion of all suicides accounted for by domestic gas was comparatively high in the mid-1970s among females aged 15-24 and 25-44 years, while for other gender-age-groups the proportion of domestic gas suicides remained small, even at the peak. For females aged 15-44 years, the decrease in domestic gas suicides appeared to cause a substantial decrease in overall suicides in this gender/age group. Car exhaust gas was a more common method for males, particularly those aged 25-64 years. Car exhaust gas suicide rates for males aged 25-64 years peaked in the mid-1980s, followed by a sharp decrease. CONCLUSIONS: A reduction in the carbon monoxide content of the domestic gas, which began in the early 1970s in Japan, was associated with a decrease in domestic gas suicides for both genders of all ages. Concerning females aged 15-44 years, a decrease in domestic gas suicides caused a substantial decrease in overall suicides in this gender/age group since the proportion of domestic gas suicides among all suicides combined was comparatively large. However, it remains uncertain whether the introduction of catalytic converters in the 1970s in Japan resulted in a reduction of suicides from car exhaust gas inhalation.