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“She died from breathing failures, circulatory collapse due to brain edema [brain swelling].… No toxic traces have been found in her blood and gastric fluids,” he said, adding police would not be pursuing criminal charges.

Chlorpyrifos is notoriously hard to detect, owing to its short half-life. After only one day, the chemical is effectively eliminated from the body.

The Vietnamese results do not “rule out” a poisoning, says Dr. Ross. “It also depends on the sophistication of the analysis.… I doubt that they would have the same resources that you would find in a Western toxicology lab.”

Born in Hamilton to Vietnamese parents, Ms. Huynh had been teaching English in South Korea when she and Ms. Bowerman, her co-worker, decided to fly to northern Vietnam, where they checked into the Son & Daughter Guesthouse.

After Ms. Bowerman’s death, Ms. Huynh was discharged from hospital and returned to the guesthouse, where she told the owner she was “still OK,” according to theTuoi Tre News. A day later, she returned to the hospital by taxi and died early on Aug. 2.

Perhaps Cathy Huynh would have lived if she had gone to another hotel

“Perhaps Cathy Huynh would have lived if she had gone to another hotel,” wrote Dr. Ross in an August blog post.

Chlorpyrifos is banned for household use in Canada and the U.S., yet it is still used in parts of Asia as an indoor pesticide.

“It was probably used to kill bedbugs in the rooms that the tourists slept in, and the rooms were probably not well-ventilated and/or the windows were closed during the night,” Dr. Ross wrote in the email.