Mysterious Ailment Takes Heavy Toll Among Thais : Southeast Asia: The Sudden Unexplained Nocturnal Death Syndrome has killed hundreds of young men. Doctors remain baffled<i> .</i>

BANGKOK, Thailand â€”  A decade-old medical mystery that has confounded investigators in the United States and Southeast Asia by killing young, apparently healthy Asian men in their sleep has taken a new turn with the deaths in a single day of two Thai construction workers in the island republic of Singapore.

The two men, Wichit Khamwaen, 45, and Sitthi Sataisong, 27, apparently died in their sleep before dawn Monday.

Sitthi’s roommate told authorities that Sitthi had trouble breathing during the night, but he could not revive him.

A third Thai died Wednesday, bringing to 13 the number of Thai workers who have died this year in Singapore under similar mysterious circumstances--heart failure in the middle of the night, with no apparent symptoms or causes.

The Singapore Ministry of Health said in a statement Sunday that about 200 young Thais have died of the mysterious illness since 1983; about 20 young and seemingly healthy Thais died last year.

Officials of the U.S. Centers for Disease Control in Atlanta have told the Singaporeans of similar deaths in the United States involving Southeast Asian immigrants. The U.S. deaths from the so-called Sudden Unexplained Nocturnal Death Syndrome average nearly four a month.

In addition, the Thai Ministry of Labor reported Wednesday that of the 1,000 Thais who have died in Saudi Arabia in the past two decades, about 60% died of unexplained heart attacks.

The unexplained deaths have stunned the estimated 200,000 Thais in Singapore, where they provide mainly low-cost labor as domestic servants and construction workers. Concern is so great that the Thai government dispatched a team of medical investigators from the Public Health Ministry to Singapore last week, but they returned home Tuesday without having solved the mystery.

“We didn’t come up with any conclusion,� said Ong-art Klamphaiboon, the public health official who led the delegation.

Ong-art said the circumstances of the deaths in the Singapore cases appeared similar to those reported among Southeast Asians in the United States, a large percentage of whom live in Southern California.

One clue that was examined and then rejected was a method used by Thai construction workers to cook sticky rice, a staple of their diet. The workers used sections of plastic pipe as a steamer for the rice. When burned, the plastic material, also known as PVC, can give off poisonous gases.

But Handrick Ng of the Singapore Health Ministry said the Thai workers were not raising the pipes to sufficiently high temperature to produce those gases.

“Besides, an autopsy would show toxicity in the tissues but we found no such toxicity,� Ng said.

Another possible link being investigated is that all of the men who have died in Singapore are apparently from the same northeastern region of Thailand.

Ng said researchers are considering the possibility that the workers succumbed because of some reaction to stressful work in the different climate. Northeast Thailand is primarily highland terrain, while Singapore is flat and tropical.

Thai authorities, meanwhile, have arranged for the installation of more telephones at work camps in Singapore to help communicate with workers in case of emergency. They are also urging Thai workers there to learn cardiopulmonary resuscitation in the event further cases occur.

“It’s like playing Russian roulette, not knowing when a bullet will hit me,� a young Thai worker told the Singapore Straits Times.