After a trying month for tens of thousands of people evacuated from wildfire-ravaged Fort McMurray, Alta., there’s mixed news this week. For some, it's time to begin the journey home. For others, the waiting continues.

More than 500 undamaged homes in Fort McMurray have been deemed unsafe after traces of toxic substances, including arsenic, were found in samples of ash. Officials say it could take until September for the harmful ash to be cleared.

“Specifically, the ash has a very high pH (level) which makes it caustic and may cause both skin and respiratory irritation and burns,” said Alberta’s chief medical officer Dr. Karen Grimsrud. “There’s also heavy metals like arsenic in these samples, as well as polyaromatic hydrocarbons, dioxins (and) furans have been detected at concentrations that are above what is recommended for public health.”

The news further delays re-entry plans for at least 2,000 residents, Alberta Premier Rachel Notley said at a press conference on Monday.

“It means people who live in those neighbourhoods should not plan to return permanently on June 4 as originally planned,” Notley said.

The good news, Notley said, is that officials are working to find a way to allow those residents to briefly return home to “inspect their residences and retrieve belongings.”

“We are actively working on a plan to allow this to happen and will release details as soon as they are available,” she said.

The affected neighbourhoods include:

357 homes and 10 apartment complexes in Abasand

27 homes and one apartment complex in Waterways

183 homes and one apartment complex in Beacon Hill

The wildfire continues to burn eastward and is estimated to cover about 5,800 square kilometres, Notley said. That’s about nine times the size of Toronto.

Arrangements are being made to provide debit cards loaded with relief funds for evacuees who left Alberta after the blaze broke out. The cards will be made available between June 1 and June 15, Notley said.

The Fort McMurray hospital has been cleaned and will offer “basic services” starting June 1, Notley said. It is expected to be fully operational by June 21.

Starting Wednesday, displaced residents will be allowed to return to the city for the first time since the evacuations at the beginning of the month.

The phased re-entry plan is:

Zone 1: Lower Townsite, Anzac, Fort McMurray 468 First Nation, Gregoire Lake Estates (June 1)

Zone 2: Parsons Creek, Stone Creek, Timberlea, Eagle Ridge, Dickinsfield (June 2)

Zone 3: Thickwood, Wood Buffalo (June 3)

Zone 4(a): Gregoire, Prairie Creek, Saprae Creek Estates (June 3)

Zone 4(b): Waterways, Abasand, Beacon Hill, Grayling Terrace, Draper (June 4)

Stopping the spread of toxic ash

In efforts to prevent toxic particles from spreading, a substance called Posi-Shell is being sprayed on charred remains to keep ash from catching in the wind.

The non-toxic substance will blanket the wreckage of burned-out houses in efforts to contain the ash. Scott Long with Alberta Emergency Management Agency described the substance as “very light” and said it has been used in landfills and in wildfire response efforts in Australia.

Despite the efforts, traces of ash could remain in Fort McMurray until the end of summer, officials say.

Challenges not over

While the re-entry is being hailed as a major step forward in the recovery from the wildfires that devastated the region, experts caution there could be some bumps ahead.

"It takes time after a big event like this to get everything set up and running 100 per cent," Kim MacLean, Director of Disaster Management for the Canadian Red Cross, tells Canada AM from Fort McMurray.

"There is a team here in Fort McMurray that's really working to get people ready and get the community ready. [They] are doing everything possible that they can in creating a safe place."

Co-ordinating support systems to meet the needs of returning residents has required cooperation on all fronts.

Notley laid out several criteria for re-entry including: the fires must no longer be an "imminent threat" to the community, local infrastructure and essential services must be prepared to provide "basic service" and local government must be re-established.

Local businesses in Fort McMurray are working to make sure they have enough food and water and the Red Cross is encouraging those returning residents who can, to stock up with enough supplies to last two weeks.

Hospitals and doctors' offices may take a few days to become fully operational as well – something residents with precarious health should keep in mind, says MacLean.

"There could be still some smoke. If you have respiratory problems you should potentially consider staying out of the community."

But there is also the issue of the intangible needs, MacLean says the Red Cross is ready on that front too.

"When many of these people left, their memories of exiting the community are certainly not going to be good memories. When they return they are going to be thinking about it and there's the potential for anxiety," says MacLean.

Information centres will be set up and staffed with support workers to help those who may find themselves getting overwhelmed.

With files from Emma Jarratt