Paris health official Arnaud Gauthier said that the cleaning had been ordered “to reassure us that the risk is minimal,” adding that the levels of lead pollution caused by the April fire at Notre-Dame were not a cause for alarm.

There have been several scares over lead pollution since the blaze, with many locals fearing that the melting lead from the roof had ended up in the air they were breathing.

READ ALSO IN PICTURES: See the latest images from inside fire-ravaged Notre Dame

The rescue operation inside the cathedral. Photo: AFP

Environmental groups warned soon after the fire that 300 tonnes of lead in the roof of the Paris landmark had gone up in flames, posing a danger to residents in the area, particularly to children.

A report from the Mediapart investigative website on Thursday reported that dangerous levels of lead – as much as 10 times higher than the safe limit – had been detected in schools and creches surrounding the cathedral.

It added that Paris authorities had waited until May before conducting tests in the 10 creches and schools that are within 500 metres (yards) of the monument on the Ile de la Cite island in central Paris.

One result — in the private Sainte-Catherine elementary school — showed lead at a level of 698 micrograms per m2, 10 times higher than the limit of 70 microgrammes which is considered dangerous.

During a tour of the scorched edifice on Wednesday, reporters were told by Culture Minister Franck Riester that workers inside had to wear special masks because of the presence of lead which had seeped into some of the stone during the fire.

In June the French health authorities advised parents living in central Paris with young children and pregnant women to get their lead levels tested after an abnormally high level was detected in a child in the area.