With residents set to start returning to Fort McMurray tomorrow, the premier has announced that hundreds of undamaged homes in neighbourhoods hardest hit by the wildfire are not safe to live in.

Tests done near those homes show ash and soil in the area contain substances like arsenic and other heavy metals.

"Unfortunately, I have to report today that the outcome of these tests indicate that undamaged homes in certain neighbourhoods are not immediately safe for reoccupation," Premier Rachel Notley said Monday afternoon.

"I realize this will be difficult news for people to hear who were expecting to return to their homes later this week," Notley said. "But as always, safety and health remain our top priority."

'It's the craziest thing'

Jessica Rejman's home in the Waterways neighbourhood is one of those declared unsafe.

"If you look at the whole block, our homes look OK," she said. "There doesn't look like there is anything wrong with them."

However the homes across the street from hers were destroyed, she said.

"There are only two on that block that made it. It basically burned a semi-circle around (our homes), and I don't how it did that. It's the craziest thing.

Jessica Rejman's home as the wildfire advanced on May 3. (Supplied) Notley's announcement did not come as a complete surprise, she said.

When Rejman bought her home she knew she was buying one in a historical district. "I already knew it would be pretty hazardous for that reason alone."

Nonetheless she said she was shocked by the hazards of the ash.

"I knew it was going to be bad, but maybe not that bad."

Rejman said she didn't plan to stay in Fort McMurray in the near future because she has asthma, but she hopes to visit her home next month.

Her home is one of about 560 not directly damaged by fire, but declared unsafe for habitation.

The homes are spread between Abasand (350 houses and 10 apartment complexes), Beacon Hill (183 houses and one apartment complex) and Waterways (27 homes and one complex).

Residents of those homes will be escorted in and allowed to inspect their homes and gather personal belongings, but will not be able to stay.

The premier estimated the decision will affect close to 2,000 people.

Residents urged to take precautions

"The ash has a very high pH which makes it caustic and may cause both skin and respiratory irritation and burns," said Dr. Karen Grimsrud, Alberta's chief medical officer of health.

"There's also heavy metals like arsenic in these samples. As well, polyaromatic hydrocarbons, dioxins and furans have been detected at concentrations above what has been recommended for public health."

(Government of Alberta)

Grimsrud is advising all returning residents to take precautions when around ash or debris such as wearing long-sleeved shirts, pants and gloves.

"Parents should not allow children to play in ash and should clean all toys before use."

Destroyed homes under protective shell

The homes destroyed by fire have been sprayed with a composite material that hardens into a protective shell, acting as a barrier keeping contaminated ash and other debris from spreading through the air.

The material acts like a safety blanket, said Scott Long, executive director of operations with the Alberta Emergency Management Agency.

The blanket prevents cross-contamination by wind that could pick up the toxic ash and spread it into uncontaminated areas.

Australia has used the material for similar reasons, while California uses it to prevent soil erosion. Landfills around the world also use the material, including the one in Wood Buffalo.

Heavily damaged areas of Fort McMurray have also been closed off with more 30 kilometres of security fencing, the premier said.

The staged re-entry plans for the rest of the 90,000 evacuees is on schedule to begin Wednesday.

The fire is still burning, covers just under 580,000 hectares, although it is not expected to grow significantly in

coming days due to cooler and wetter conditions.

Once 281 South African firefighters join the fray, they will bring the total number to 2,000.