Bed bugs themselves are not deadly to humans. Their bites are usually only mildly irritating, while their presence is psychologically the stuff of nightmares. For travelers to cities with known infestations, the health concern shouldn’t be whether or not the bed bugs will bite, but whether the hotel room has been doused in deadly pesticides.

Five foreign tourists and a Thai guide died this year in Chiang Mai in northern Thailand, while several others fell ill. Authorities conclude that most of the deaths were related since they occurred in proximity to each other, but the exact cause is still unknown.

The Thai Department of Disease Control recently released a report stating that it believes the tourists died of chemical exposure, but they don’t know exactly which chemical was the culprit. But other experts point the finger at bed bug pesticides.

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The department says more than 350 drug, chemical, and organism tests were performed on the victims, who came from New Zealand, Canada, the United Kingdom, and the United States. In the case of New Zealander Sarah Carter, 23, who died in February, 1,250 tests were performed.

“The three New Zealand women had severe metabolic acidosis (abnormal acid level in circulation) and two of them suffered myocardial injury (injury to the heart muscle),” the report said.

Though the Thai government has been unable to pin down the exact chemical responsible, a New Zealand television station found traces of the insecticide chlorpyrifos in the room where the New Zealand tourists had stayed before passing away, reported the BBC. reported that it was the television show 60 Minutes that found the traces of pesticide.

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The report states that chlorpyrifos was not the likely cause since the clinical tests did not show evidence of “organophosphate, organochlorine and carbamate group pesticides.” Chlorpyrifos is an organophosphate.

A United Nations chemicals expert, Ron McDowall, told Checkpoint on Radio New Zealand that he believed it may have been a bed bug pesticide.

“I think they’ve got it right – it’s a chemical, its likely to be pesticide, whether it’s an organo-phosphate or an organo-chlorine and at the end of the day Sarah Carter’s been killed by an over-enthusiastic spraying company,” he said.

The victims included:

An American woman, aged 33, who died on January 11

A Canadian, 29, who became ill

A Frenchwoman, 25, who died on January 19. Her death has since been ruled as unrelated since she had a fever and other symptoms before arriving in Chiang Mai.

A Thai woman, 47, who died on February 3

Sarah Carter, who died, and two friends, both 23, who fell ill on February 6

A British couple, aged 78 and 74, who died on February 19