A top government official reassured the public on Friday that the black smoke unleashed by a fire at a chemicals factory in the northern city of Rouen, "did not reveal any acute toxicity in the main molecules".

Speaking on a public radio station, Prefect Pierre-Andre Durand said he understood the anxiety around it because the fire, which broke out in the early hours of Thursday, "was quite impressive".

The fire was completely put out by Friday morning, however, firefighters stayed on-site to monitor any hot spots left behind, which "could take several days" according to Durand.

Local authorities said there no casualties on-site.

But despite the prefect's reassurances, an acrid smell lingered on Friday morning and several people complained of throat irritation.

General practitioner Stéphane Pertuet, who was also interviewed by the radio station, said that "beyond the eye irritation or respiratory mucous", linked to "hydrocarbon odours", "there is no acute toxicity. We are not confronted with a chlorine cloud."

Besides Rouen, residents of 12 surrounding municipalities were asked to stay home, while schools in the area will remain closed until Monday. People living in a 500 perimeter around the factory were able to go back home on Thursday evening, said the prefecture.

After the deployment of dams, pollution of the Seine was avoided, according to the authorities.

This is not the first time that the factory, which makes oil additives and is classified as a SEVESO site, i.e. industrial establishments in the European Union where dangerous substances are used or stored in large quantities, makes headlines.

In 2013 a gas leak spread a rotten-egg smell all the way to Paris and the southern parts of England.

Unions are calling for the Lubrizol group to tighten restrictions in their factory to avoid any future catastrophes. But the prefect said that it was already following compliance.

The prosecutor's office has announced the opening of an investigation for "unintentional destruction" for Thursday's fire.

The environment protection association Robin des bois called for "factories like Lubrizol to be more closely monitored" and for the prefect to launch an environmental investigation.

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