Accompanied by reports of some sort of fog, the stench – or at least awareness of it – seemed to spread east and west along the river, inviting comparisons with “chlorine meets TCP/Dettol meets battery acid” near Waterloo Bridge, and prompting Billy Kingsmill in East Ham to demand: “What the eff is it?”

As reports of the “burning/chemical/metallic” smell crept past the Houses of Parliament towards Victoria and Paddington stations, comparisons were made with the Great Stink of London of 1858, which forced even MPs to do something about the dreadful state of sanitation in the capital.

Wednesday, then, was the wrong day for Kew Gardens to start boasting about its corpse flower with “one of the foulest odours in the plant kingdom”.

By Thursday morning the smell had gone and the air quality had returned to good, but the questions remained, along, in some cases, with concerns about sore throats, sore lungs and headaches.

What exactly had caused the great(ish) stink of 2018? How worried should the citizens of London be? And could the Metropolitan Police have shown a little more interest when one concerned Twitter user drew their attention to the foul smell enveloping the capital?

For answers, The Independent turned to Dr Thomas Smith, assistant professor of environmental geography at the London School of Economics (LSE).

He, like many others, had noticed the weird smell.

Unlike many others, however, Dr Smith had a “laser egg”.

This small device can measure air quality by counting particles in the air that are two-and-a-half microns or less in width and therefore small enough to be breathed into the lungs.

The laser egg was stationed on the roof of the LSE’s St Clement’s building, near Lincoln’s Inn Fields, and was readable via an app on Dr Smith’s mobile phone.

When Dr Smith noticed the stink while working in his fourth floor office, he checked the reading on his phone. Then he went onto the roof to check that no students were skewing the results by smoking near the laser egg.

No one was having a crafty fag near the laser egg. The result was real.

As Dr Smith duly tweeted at 9.48pm, his machine had given an Air Quality Index (AQI) reading of 102, in the “unhealthy” range for people with pre-existing lung conditions and the highest pollution level that particular device had ever recorded.

Alas, however, Dr Smith’s relatively inexpensive laser egg could only count the offending particles, not say what they were.

He could, though, offer some speculation about where they came from.

London, it seemed, had detected what previous generations called, and what some Brexiters may still refer to as ‘the smell of the continent’.

Although Dr Smith didn’t put it like that.

“I suspect,” he said, “That since the wind was coming from the south and south east, it was probably passing over some rather industrialised areas of [mainland] Europe and there were probably some chemicals in the air.”

“Whatever it was,” he added, “It probably came from places beyond the remit of the UK Environment Agency.”

When we consulted the experts at the London Air Quality Network at King’s College, London, they confirmed that Dr Smith’s hunch was broadly correct.

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Timothy Baker, the principal air quality analyst, said London had received a dose of Austrian and German air, although home-grown emissions, especially from traffic during the evening rush hour, had also been partly to blame for Wednesday’s poor air quality.

Mr Baker said: “What our monitoring equipment picked up on was a marked increase in the types of particles that are formed as a result of distant air pollution reacting in the atmosphere as it comes towards us.

“The air [that came to London] spent two days – Monday and Tuesday – travelling slowly over Austria and southern Germany, picking up quite a chunk of whatever urban areas there were pumping out.”

Mr Baker said that on Wednesday a high nitrate count – produced from particles originating in Austrian and German cars and domestic heating – was largely to blame for producing poor air quality in terms of potential to irritate lungs.

But in terms of what so irritated the noses of Londoners, Mr Baker was unable to identify the chemical culprit for that industrial-seeming stench.

What causes unhealthy air quality and what causes a nasty smell can sometimes be two completely separate things, Mr Baker explained.

The irony with air pollution was, he said: “The side of it that is causing a lot of damage to people’s health is often undetectable.

“The issue tends to attract attention when there is something people can see or smell, but there can be worse particle episodes that most people don’t notice.”

Mr Baker was willing to speculate that the Austrian and German urban areas that produced pollution from vehicles and domestic heating may also have contained industrial units pumping out things that ended up stinking like “chlorine meets TCP”.

But he admitted: “We don’t know what caused the precise nature of the smell.”

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Back in the LSE with the laser egg, however, Dr Smith did offer some reassuring context – about both the smell and the unhealthy particles.

“I wouldn’t want to be too alarmist,” he said.

Although that 102 measurement was his laser egg’s highest recording, he has only had it since May.

Apart from Wednesday, his laser egg had been recording good air quality of between 10 and 30 on the index all summer, and on Thursday morning it was back down to 25.

And, Dr Smith said, the AQI reading of 102 had only just made it into the unhealthy range. This categorisation, denoting air quality that may adversely affect people with asthma or other lung conditions, starts at 101.

An index reading that is unhealthy for the whole population starts at 151, very unhealthy at 201 and “hazardous” is classed as anything from 300 upwards.

Dr Smith said: “While researching wildfires in Southeast Asia. I’ve had readings of 2,000 in the smoke.

“Cities like Beijing go over 100 frequently. London has had far worse than this over the years, and whatever it was, it’s over now.”

“People who live in more industrial parts of the UK, he added, “Are probably accustomed to these kinds of smells all the time, but people in central London are not used to it, because it is a very de-industrialised city.