A false report of "grossly" high lead contamination at a historic downtown Halifax military building forced hundreds reservists and teenage cadets into temporary facilities.

Military officials declared much of the Halifax Armoury off limits in early November after an October report found lead dust concentrations were 20,000 times greater than health guidelines.

But the internal report, obtained by CBC News, contained a serious mathematical error.

CBC News notified military public affairs officials of the mistake. Instead of 20,000-times greater than health guidelines, the actual testing data found lead levels were likely 20-times greater, said Carol Lee Giffin, the Department of National Defence's safety and environment officer for the Maritime region.

"It is significant," Giffin said. "In the conversion between foot-squared and centimetre-squared, it caused them to be off by a factor of 1,000."

Soldiers left the Halifax Armoury for more than a month. (Brett Ruskin/CBC)

In 2012, analysts measured levels approximately 20-times above acceptable guidelines, she said.

"They, at that time, had deemed the building safe for occupancy," Giffin said.

The 116-year-old Halifax Armoury is owned by the Department of National Defence and has had a history of hazardous material contamination that has led to quarantines.

In 2010, the Armoury's basement was put under quarantine to clean up high levels of lead and asbestos left over from a closed indoor firing range.

Paint is peeling off the Halifax Armoury's interior walls. (Brett Ruskin/CBC)

The current issue stems from rain leaking through the roof, trickling lead from painted walls onto the floor. Lead paint from the drill floor also has rubbed off and turned to dust, officials said.

When the October report first came out, army officials told hundreds of reservists and cadets who used the space to immediately keep off the dusty floor.

The parade floor is used for meetings, exercises, drills and even floor hockey. Reservists and cadets also sometimes sleep overnight on cots on the floor, the October report said.

Alternate sites were arranged for their meetings, drills and exercises, and in mid-November cadets and reservists moved to temporary facilities elsewhere.

About two dozen office workers remained in the building and were told not to sweep dusty floors for fear of making the lead airborne.

Staff at the Halifax Armoury were told not to sweep the floors for fear of contaminates becoming airborne. (Supplied to CBC )

The military hired remediation crews to scrub and use vacuums with high-quality air filters to clean the expansive cement drill floor.

Displaced groups are now allowed back into the Armoury, CFB Halifax's base commander said.

"I'm confident, based on the information that we have, to reopen access to the building," said Capt. Chris Sutherland, who is in the navy.

Overall, most of the building meets all health standards, Sutherland said. Several basement areas are not safe to do prolonged work in, but can be used to access other parts of the building, he said.

Black mould is growing on the walls in parts of the Halifax Armoury. (Supplied to CBC)

There have been concerns in the past about mould and asbestos, Giffin said. But when asked if any spaces in the building are currently quarantined, she said, "Nope."

Photos taken before and after the interview with military officials show otherwise.

"Do not enter," a sign posted to a basement door said. "This room is quarantined for mould. Do not enter."

These were taken at two separate times during which DND said there were no quarantine areas. The top photo was taken during the first week of December and the bottom on the third week. (Supplied to CBC)

Cadet groups and reservists are making arrangements to return to the Armoury, Sutherland said.

The base commander has offered to meet with the leaders of each group to discuss the confusion over the lead levels.

Paint is rubbing off ceiling tiles, which are made from asbestos. (Supplied to CBC)

An 11,000-square-metre building to replace the Armoury is slated to be built at Windsor Park. There is no word yet on what will happen with the old Halifax Armoury space.

The Halifax Armoury may receive $15 million this coming June for cosmetic and structural repairs to its the west-facing wall, according to an advance procurement notice issued in October.