It wafted over northern France late in the night and reached southern England by morning on Tuesday, a noisome cloud that roused inhabitants from their sleep with its nauseating stench. There were thousands of frantic calls to emergency services, from Normandy to Paris, with residents describing the smell of household gas or rotten eggs, but the authorities moved quickly to calm fears.

The cloud, officials said, was one of a group of substances called mercaptans, foul-smelling but largelyharmless chemicals — at low doses, at any rate. It had escaped from a chemical plant near the northern city of Rouen.

“Given the absence of danger,” the Interior Ministry said in a statement, “the inhabitants of the affected regions are invited to not call emergency services.” In a separate joint statement, the ministers of the interior and ecology noted that the chemical is dangerous only at concentrations 20,000 times that at which the nose detects it.

Often described as smelling of rotten cabbage, mercaptans are used as a marker for household gas, which is odorless, so that leaks do not go undetected. They are present in feces and some cheeses, and can cause headaches and nausea, according to the Centers for Disease Control and Prevention. Those symptoms were described by a number of people in France on Tuesday.

The fumes also crossed the English Channel and settled Tuesday over the English coastal town of Hastings, due north of Rouen. The police there suggested on Twitter that residents use “Vicks on a tissue” or carry a “scented pomander” to combat the smell. In France, the authorities proposed few such handy solutions, though the regional prefecture did put together a crisis team. The ecology minister, Delphine Batho, announced that she would be heading to the chemical plant to be sure the leak was plugged.

Lubrizol, the additives company that operates the plant, offered little explanation for the problems, which began when the production of another chemical went awry, said Nathalie Bakaev, a company spokeswoman. The leak, which still going on Tuesday afternoon, began Monday at 8 a.m., Ms. Bakaev said, and in the beginning, the smell was largely concentrated around Rouen. But the winds picked up in the evening, and the stench drifted into Paris by 2 or 3 a.m. Tuesday.

The government deflected health concerns raised by journalists and citizens. At the observed levels, mercaptans present “no threat to the populace,” Pierre-Henry Brandet, the Interior Ministry spokesman, told France Info radio. About 50 similar mercaptan leaks have occurred in France in the past 25 years, the newspaper Le Parisien reported.

The French are not necessarily inclined to trust their public officials on such matters, though, and it would be wrong to say that their doubts are completely unfounded. In 1986, the authorities implied that a radioactive cloud drifting west from the stricken nuclear plant at Chernobyl had halted, as if by some miracle of geography and science, just before France’s eastern border.

The mercaptan leak is “not at all the same thing,” Mr. Brandet said Tuesday, and official analyses of mercaptan levels in the air will be made public, he said.

“You can’t start from a principle, and from an assumption, that the authorities are lying to you,” Mr. Brandet said, sounding a bit exasperated. “There’s nothing to hide.”