The wildfires that tore into Fort McMurray have left a toxic legacy, with mounds of ash across the city containing harmful levels of contaminants, according to tests conducted over the past month.

Data on soil and ash obtained by The Globe and Mail show that the level of benzene and arsenic found in the three Fort McMurray neighbourhoods hit hardest by the wildfires were 20 times higher than the limit deemed safe by the Alberta government. In all, 19 different metals and compounds were found above recommended limits. The last test data were collected six days before residents were allowed to return to the city.

These findings are why Alberta's Chief Medical Officer directed the province to stop 2,000 residents from returning to housing still standing in the three worst-hit neighbourhoods. Researchers contacted by The Globe say that while the contaminant levels are high, residents can avoid much of the risk by not touching or inhaling the ash.

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Destroyed areas have been fenced off and emergency crews have sprayed a coat of hard resin to stop ash from becoming airborne.

"In the heavily damaged neighbourhoods of Waterways, Abasand and Beacon Hill, testing has confirmed contaminants in the ash are at levels that make these neighbourhoods unsafe for occupation until debris can be cleared," Karen Grimsrud, Alberta's Chief Medical Officer, said in a statement.

"Ongoing monitoring of particulates in the air has found no evidence of movement of ash."

The Alberta government and the local municipality collected 85 ash samples over five days in May. Most of the samples came from residential areas. The findings show that the toxic material found in the ash piles will pose a challenge for officials planning the removal of tonnes of contaminated debris from the city.

Many of the toxins revealed by these tests reflect modern construction materials – the complex polymers and processed materials used to build homes create harmful compounds when exposed to fire.

The tests were carried out in once-bustling neighbourhoods, alongside burnt cars and shattered homes. One test in the Abasand area, carried out on May 12, was made alongside a flowerbed and gravel walkway. It found levels of benzene, a compound linked to cancer, nearly 40 times higher than the recommended limit.

While residents of the three neighbourhoods singled out by Dr. Grimsrud have been kept out of their housing indefinitely, more than 40,000 people have moved back to Fort McMurray, many in areas where nearby homes were lost to fire.

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More than 250 households adjacent to destroyed properties have been warned to have their homes tested before moving back permanently. Residents returned to find pink notices posted to their front doors warning them to contact their insurance companies for the testing.

While Melanie Galea's neighbour in downtown Fort McMurray had a pink notice on his door, her home, only a few metres further down the street, did not. It's an unsettling situation for the realtor, who remains worried about what contaminants might be in the piles of ash only a stone's throw from her front door.

"I'm worried about the health risk; that's my top worry. If we had children, I don't know if I'd bring them back here," she said.

Maen Husein, a professor at the University of Calgary, looked at the data at the request of The Globe. He cautioned that contaminated soil lies under the toxic ash, and will require removal, too.

"That debris needs to be removed by a very professional company and the employees need to be very well protected. They need high-tech breathing masks; those typical masks you see won't work," he said.

Similar cleanups have been undertaken in California and Slave Lake, Alta., following wildfires that destroyed residential areas. Officials in Fort McMurray say they don't have a timeline for the cleanup, but the ash will be sent to the city's landfill.

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Tom Moore has lived through the situation Fort McMurray is about to face. He's the manager of Slave Lake's landfill. More than 400 buildings were destroyed in that city during wildfires in 2011 – about 2,400 have been lost in Fort McMurray.

The landfill in Slave Lake received about 14 tonnes of debris from each burnt structure. That contaminated debris and soil is now entombed in clay. Five years after the fire, the town has only recently started detecting heavy metals reaching the bottom of the landfill.

"This is a challenge that'll last years," Mr. Moore said.