PARIS — Months after fire engulfed Notre-Dame and the 460 tons of lead on its roof and spire, alarming levels of lead are still being found after decontamination efforts, including at the Paris Police Headquarters — raising new concerns that the authorities have not fully tackled the problem.

The environmental group Robin Hood said on Friday that lead levels up to 20 times above the safety guidelines had been found on the balcony of an apartment in the Saint-Germain-des-Prés area, on the left bank of the Seine.

And at Police Headquarters on the Île de la Cité, law enforcement measurements in early September showed lead levels reaching 40,000 micrograms per square meter in the courtyard, or over 3,700 micrograms per square foot, the group said.

Such levels are eight times above the safety guidelines issued by regional health authorities.

“The levels we obtained are extremely worrying, and we can be sure that more apartments are in the same situation,” Jacky Bonnemains, the head of Robin Hood, said of the apartment.