More than 120 people fled their homes yesterday when a 14-year-old Japanese schoolgirl took her own life by mixing a deadly cocktail of household products and creating clouds of highly toxic gas.

Her death, just a few days into the new school year, was the latest tragedy in what some in Japan see as an epidemic of copycat suicides among the young and internet-obsessed.

The girl’s death brought to 70 the number of young people in Japan who have brewed the fatal concoction and killed themselves with hydrogen sulphide gas this year.

Police fear that worse is to come: one of the products used to generate the gas has sold out in many stores in the past few weeks.

Despite Japan’s extremely high suicide rate, the worst among developed countries, the use of hydrogen sulphide as a means of suicide has been virtually unknown. Its sudden popularity has baffled experts, although many believe that it represents a significant attitude change because of the danger the method poses to innocent people.

The poisonous gas, which lingers in the room where it is produced, remains potent long after the suicide. In some cases the person finding the body has been taken seriously ill; a mother who discovered her son breathing the gas also died from the fumes.

Ryoji Matoba, a specialist in forensic medicine and suicides, said: “I suppose each generation has its popular method of suicide. I fear very much that people will copy what they have seen on the news. The people who do this believe, wrongly, that they will die still looking all right, but this gas turns the body brown and green.”

Despite efforts by the authorities to crack down hard on Japan’s many suicide websites, the victims were all led through their final moments of life with the aid of step-by-step guides readily available online in Japanese. The sites explain which cleaning products and bath additives to buy, how to mix them, and how quickly the resulting toxic gas will take effect.

Yesterday, after police evacuated every resident from the five-storey apartment block in the southern town of Konan where the girl was found, they discovered the macabre calling card that attested to the role of the websites.

Pinned outside the bathroom where the girl died was a poster that has begun to appear with grim regularity across the country: “Do not open! Poison gas being generated on the other side of this door!” The poster, always in the same vocabulary and typeface, is available for download on the most visited suicide sites – an apparent effort by the website creators to make the process easier.

Japan has been an unhappy leader of so-called internet suicides. Four years ago a spate of suicide pacts began that had been forged online between strangers who would arrange to meet and end their lives together.

Plague of death

— World Health Organisation figures show that Japan has the second-highest suicide rate in the Group of Eight nations, beaten only by Russia

— At least 30,000 people have killed themselves in Japan every year since 1998

— The 1998 peak in suicides coincided with a nationwide economic slump

— Last year the Japanese Cabinet approved measures to cut suicides by tackling unemployment, promoting counselling and filtering suicide websites

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