This time the workers were wearing disposable underwear and other protective gear after a delay prompted by fears of lead contamination.

Activity resumed under strict lead-protection measures for the stonemasons, clean-up workers and scientists working on the site, according to the Culture Ministry.

They include throwaway full-body clothing, obligatory showers and a new decontamination zone to ensure that they do not track pollution outside the site.

The workers are clearing out hazardous debris and studying and consolidating the medieval monument - a crucial first step to prepare the fragile cathedral for a long, multimillion-euro reconstruction effort.

But even this first step is taking longer than expected because of lead worries.

Hundreds of tonnes of lead melted in the April 15 fire that decimated Notre Dame's roof and toppled its spire, spewing toxic dust into the air.

Some environmental activists and residents say French authorities underplayed the lead poisoning risks in the aftermath of the blaze.

Under pressure from labour inspectors, the Paris regional administration ordered the consolidation work to be halted in July pending new worker-protection measures.

Now multiple lead-prevention operations are under way in the area around Notre Dame. Experts are carrying out a deep clean of neighbourhood schools and are spraying chemical agents and vacuuming surrounding streets to remove any residual lead.

The regional health authority said last month that the main lead risk was inside the cathedral itself and its forecourt, and that no dangerous lead levels had been registered since the fire in the surrounding streets, where tourists and residents circulate.

Didier Durand, whose stone-working company Pierrenoel has eight employees working on the cathedral, said he is eager to get back to work.

"Notre Dame hasn't been saved yet," he said. "We haven't stabilised the buttress structure yet... and we're losing a lot of time."

The cathedral, its adjacent park and its forecourt have been closed to the public since April 15 and are likely to stay that way for years.

French President Emmanuel Macron has said he wanted Notre Dame rebuilt within five years but reconstruction experts doubt that is possible.

Australian Associated Press