How 'Le Pong' engulfed Kent and Sussex
"Nobody smoke" people joked on Facebook when they first started to whiff what is being called "Le Pong".
Some have described it as smelling like rotten eggs while have others have detected notes of stale cabbage.
But for most people, the odour is reminiscent of diesel fumes.
It all stems from a gas leak at a chemical plant in Rouen, which is more than 60 miles north west of Paris.
The substance responsible is mercaptan - a harmless additive to natural gas.
But while it might not be toxic - it doesn't half smell.
Tunbridge Wells in Kent is 120 miles north of Rouen but on Tuesday morning, Le Pong was certainly in evidence.
It seemed to come in waves, wafted across the Channel on the chill January air.
A traffic warden working in the town centre said it was a motorist who first alerted her to the smell.
She said: "They'd parked up and I asked them to move on but he said: 'Can you smell oil, we might have an oil leak?'
"They said it was like a diesel smell, but I said I can't smell anything more than usual and it's probably the pollution from London Road.
"But then I do smell diesel all the time, so it didn't seem like anything out of the ordinary to me."
However, if your job involves the more delicate scents of lavender, ylang ylang and neroli, then the stench of mercaptan is more jarring.
Keri Bond, manager of Champneys spa in Tunbridge Wells High Street, said: "I could definitely smell burning. We thought something must be on fire.
"Then we got messages about this gas cloud from France.
"We were going into every room and smelling it to see if there was a fire. It smelled like burning or as if the air conditioning system had broken down."
On Facebook, Nigel Sharp wrote: "My employer has told me I'm not allowed to smoke anywhere in Sussex or Kent until the gas cloud from Northern France clears.... Aargh!"
The National Gas Emergency Number received so many calls, as the gas cloud enveloped the region that people were warned they could have difficulty getting through.
An update from the National Grid on Facebook said the service would normally expect 8,000 calls from around the country but had received 60,000 by 10:00 GMT.
Police in Sussex, Kent and Surrey also received anxious calls from the public - but they issued a statement calming fears about any potential dangers.
Sussex Police said: "The smell is from an additive to the gas which has an unpleasant aroma but is not toxic and there is no danger to the public."