Environmental groups and one of France's largest labour unions called Monday for a containment shield and other safety measures to ensure decontamination work at Notre-Dame Cathedral does not expose workers and residents to unsafe levels of lead.

The Paris regional administration suspended the job of removing hazardous substances from the fire-ravaged cathedral last month under pressure from labour inspectors concerned about health risks for workers.

The administration had said that when the lead-removal work resumed, stricter safety procedures, new equipment and allowing much fewer workers inside at a time would "prevent any release of polluting elements to the outside."

But representatives from environmental groups and the CGT union said at a news conference Monday they don't think the government safeguards go far enough.

They asked for a regularly updated chart showing the level of lead in the air. Labour and environmental groups are also pushing for the creation of a medical centre to monitor firefighters, workers and residents.

Paris Deputy Mayor Anne Souyris said updated measurements of lead levels are set for release Tuesday.

Watch: Aerial view of Notre-Dame fire damage

Investigators still don't really know the extent of the damage from the Notre-Dame fire. There are concerns that the walls may no longer be strong enough to support the rest of the frame, which raises questions over what a rebuild of the famed cathedral may look like. 2:18

The decontamination work is scheduled to resume Wednesday, starting with the square in front of Notre-Dame and adjacent streets, Souyris said.

Hundreds of tonnes of lead that was in Notre-Dame's spire and roof melted during the April 15 fire, which came close to destroying the cathedral.

Lead levels remain elevated at some spots inside and in the soil of the adjacent park and forecourt, according to the Paris regional health agency. Those areas have been closed to the public since the fire.

The environmental activists and union officials said they want a containment shield built over Notre-Dame to keep more lead from being released into the atmosphere.

Watch: What it will take to rebuild Notre-Dame Cathedral

"For the efficiency of the decontamination measures within the area, it is absolutely necessary that the site is confined," said Annie Thebaud-Mony, co-founder of health and environment group Henri Pezerat.

Notre-Dame rector Patrick Chauvet acknowledged that lead can escape into the environment from a big hole in the cathedral's roof but ruled out building a containment shield before the decontamination work resumes.

Paris authorities ordered new checks of schools and daycare centres in the Notre-Dame neighbourhood and recommended blood tests for children under age seven and pregnant women who live nearby.

Children are especially vulnerable to health problems from lead poisoning and exposure.