The number of young male workers with no history of health problems who died suddenly in their sleep has risen sharply over the past decade in the manufacturing hub Dongguan, research shows.
The city's police recorded 893 cases of sudden unexplained nocturnal death syndrome from January 2001 to October last year.
It is a more than triple the 231 cases recorded from January 1990 to December 1999.
The sharp increase came to light after researchers at Zhongshan School of Medicine based in Guangzhou released their analysis of police records of deaths in Dongguan over the past two decades, the official Guangzhou Daily reported.
The syndrome covers the deaths in their sleep of otherwise healthy adults with autopsies revealing no potentially fatal disease or injury. The men who died usually experienced an abrupt difficulty in breathing before death, the school's studies into the syndrome said, but the cause remained unclear.
The syndrome is mostly noted in the Southeast Asian countries including Thailand, Vietnam and the Philippines.
The newspaper reported that it first drew the attention of mainland medical scholars when an increasing number of migrant workers in Dongguan were reported to have died suddenly in their sleep in the 1990s.
The recent analysis of cases in Dongguan revealed a similar pattern to international studies, suggesting that young male manual labourers were at greatest risk.
More than 90 per cent of victims in Dongguan were manual labourers, the newspaper quoted the analysis as saying.
It said that long working hours as well as poor sanitation and ventilation in their living and working spaces might have put the workers at higher risk. Workers' rights groups in Guangdong have long been concerned about the sudden deaths of migrant workers, which they believed were caused by overwork, according to Zeng Feiyang, the director of the Guangdong Panyu Migrant Worker Centre in Guangzhou.
"It is especially hard to help the victims get compensation since the concept of overwork has not been recognised by Chinese law," said Zeng.
"Factories owners could easily argue against the accusation, saying the victim's colleagues were working for the same number of hours a day healthily."
The analysis found people aged 20 to 40, usually the breadwinners under heavy pressure, were at highest risk.
More than 80 per cent of the 328 people who died during 2001 and 2006, for instance, were aged between 21 and 40. More than nine out of every 10 victims were male, according to the analysis.