The crowd chanted, “Lubrizol, guilty; the state, complicit,” cars honked in solidarity and the citizens marched through the deserted streets, past the looming Gothic cathedral famously painted by Monet at different hours of the day.

“Now we have learned that we live on top of a volcano,” Dr. Fleury said.

“We could die, and we could die in an industrial accident,” she added.

And yet nobody has died, and many of the initial symptoms appear to have worn off. Few at the rally complained of having felt anything at all, or of feeling anything now. Some in Rouen are puzzled at the depth of the reaction.

“This is a subject that mobilizes a kind of tension,” said Guy Pessiot, a local historian who once ran Rouen’s tourism office. “There’s a kind of permanent anxiety. It’s the number one subject of conversation.”

“And yes,” he said, “there’s a kind of hyper-reaction: ‘We’re all going to die 15 minutes ago.’ ”

He worried that the image of “rotten Rouen,” industrialized and smoke-filled — Gustave Flaubert, the town’s most famous native son, described “smokestacks of factories pushing out immense brown plumes” in “Madame Bovary” — would return.

Bottled water still packs local supermarkets because residents are fearful of drinking what comes from the tap, and slight coughs set off alarms long after ailments linked to the blaze are not present.

The local prefect, the government’s top representative, said soon after the fire that there was nothing to worry about.