Aluminum phosphide is a stable substance but when exposed to moisture, it breaks down and releases a toxic gas called phosphine, said Jim Keher, a toxicologist and dean of pharmacy at the University of Alberta.
"It's pretty bad stuff," he said.
"Clearly it's going to be effective in terms of the insects but the danger to humans and animals, pets and so forth, cannot be overstated."
RCMP and fire officials have said a woman took her five children to the Fort McMurray hospital Sunday after they started vomiting. An eight-month-old girl died and the other children were in critical condition.
Deputy fire chief Brad Grainger said the mother had with her a bag of green pellets, slightly smaller in size than quarters, and suspected they were the cause of the trouble.
The family had recently brought a type of aluminum phosphide back from a trip to Pakistan, he said. The tablets were placed around their apartment, particularly in one bedroom, to try to kill bed bugs.
At one point, Grainger said, the mother vacuumed the home and disturbed the pellets.
"They broke down and got into the air, which then caused the problem."
Keher explained that the tablets would have been dangerous as soon as they were exposed, since moisture in the air would have started breaking them down, but vacuuming would have accelerated the process.
Phosphine is a heavy gas and would have affected children playing on the floor more than the adults in the apartment, said Grainger
There is no antidote for the poison, Keher added, and it can cause long-term damage to a body's liver, heart and kidneys.
The Canada Border Services Agency said it is working with the RCMP on the case, but neither agency would comment on whether charges could be laid.
According to Health Canada's website, imported pesticides must be regulated under the Pest Control Products Act and bear a Canadian label.
An agency spokesman said in an email that within Canada, such pesticides are restricted, meaning they can only be sold to individuals holding an appropriate certificate or licence.
The spokesman said Health Canada would not comment on the Fort McMurray case, calling it an "ongoing investigation."
Charity Wallace, spokeswoman with Alberta Environment, said aluminum phosphide is listed as a Schedule 1 substance in the province, meaning its availability and use is restricted to commercial applicators and trained farmers with licences. Each province has its own classification system.
Nicholas Holland, owner of Peregrine General Pest Control in Calgary, said no one should be using aluminum phosphide to kill bed bugs. He has only used it before to wipe out insects in grain elevators.
Unfortunately, he said, people sometimes resort to home remedies.
Blaine Timlick with the Canadian Grain Commission said the insecticide is primarily used to combat infestation in grain, although he's also heard of farmers dropping pellets down gopher holes.
"It's really unfortunate that whoever it was that was giving these people this product didn't explain to them that it should be only used in a certain situation and it shouldn't be used where people are living, at all."
— By Chris Purdy in Edmonton.