Brad Grainger, deputy chief of operations for the Fort McMurray fire department, said the family brought the insecticide, which is similar to aluminum phosphide pellets used in North America, home from Pakistan, where they were on vacation about 10 days ago.
The pellets, when put into fumigator, emit a phosphine gas, which can be toxic.
"We're not sure how long the family was exposed," Grainger said. "The younger that you are, the more susceptible you can be."
One of the children, an eight-month-old baby, died in hospital and four others were in critical condition Monday. Ages and names were not released.
Grainger didn't say what type of insect the family was trying to kill, but the Edmonton Journal quoted a relative who said it was bed bugs.
Grainger said that type of insecticide is a controlled substance in Canada.
He said an acceptable level of exposure is one part per million over 15 minutes or 0.3 parts per million over eight hours. One room in the apartment had concentrated levels of the insecticide at four parts per million.
"(That's) about four times the average short duration exposure (that's safe)," he said.
The property manager of the apartment block said the children's mother grew concerned when they started vomiting.
Sandy Mijajlovic, who runs the four-storey building, said she spoke to the mother Monday morning.
"She doesn't know yet why it's happened, what's happened. It's a difficult situation," said Mijajlovic. "I know the kids ... I see them. It's not easy to see someone pass away. It's such a small baby."
RCMP said the baby was taken for treatment Sunday afternoon after the entire first floor of the building was evacuated following word of a substance spill.
Cpl. George Cameron said a two- and a six-year-old were in Stollery Children's Hospital in Edmonton, while a four- and a seven-year-old were in hospital in Fort McMurray.
The children's mother was also in hospital under observation, he said. The father was with the two children in Edmonton.
Tenants who were forced from the building on Sunday returned home later that evening after air-quality tests indicated there was no danger.
"The building is safe," Mijajlovic said.
Mijajlovic, who also lives in the building, said other tenants haven't raised health or safety concerns with her.
"It's just sad," Mijajlovic said. "I want police to check out what is going on."
Grainger said the insecticide was limited to the family's apartment and authorities were working to clean the area.
— By Clare Clancy in Regina
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