Where do London's air pollution alerts come from?

Londoners received their fourth high pollution alert this year, after the city's mayor triggered a warning on Tuesday.

To try to help the situation, people were asked to walk, cycle and use public transport more often, instead of driving.

Direct alerts were also sent to schools, councils and care homes from 5 to 8 November.

Despite the alert, actual levels of pollution were lower than expected, but dense fog, fireworks and particulates from shipping were unable to escape the cold air and pushed the concentration up, experts at Imperial College London (ICL) say.

The capital's air quality alerts rely on monitors that can detect particles invisible to the human eye.

In a wooden cabin on Marylebone Road, devices take readings on air pollutants that are analysed by teams at ICL.

One is a particulate monitor that gauges the concentration of particulates in the air.

"From a health view point, that is the most important piece of equipment," says Prof Frank Kelly, who leads the team at Imperial.

"It measures tiny particles, less than 2.5 microns in diameter. A human hair is 60 microns - so you have no chance of seeing them."

Over the weekend, monitors alerted the team that the particle concentrations were "very high".

"At the weekend, we had that really cold weather, the inversion with all the fog, the pollution couldn't escape," he explains.

On the top of the cabin, chimneys capture air particles and feed them to the monitors below.

The monitors showed pollutants were not a result of the traffic, Kelly says, with some also coming from the Continent, shipping in the English Channel and "obviously from bonfires and fireworks".

The alert, which ended on Tuesday evening, also asked Londoners to avoid driving or using wood burners where possible.

Jemima Hartshorn, founder of air pollution campaign group Mums for Lungs, says the alerts are a wake-up call to the reality of London's low air quality.

"We could all smell it, we could feel it, we could taste it - London is very, very polluted," she says.

"We need to be really frank and think about and talk about the government's responsibility here.

"We need to see much bigger change than each of us as an individual can do to truly clean up our air."

Although London's air pollution levels remain high, BBC analysis shows the capital's air quality has improved substantially over the past decade.

Average nitrogen dioxide (NO₂) levels have fallen by almost 38% since 2014 and every neighbourhood now meets the UK's legal limit, an annual average of 40 micrograms per cubic metre (μ g/m³).

But fewer than 1% of neighbourhoods meet the World Health Organization's recommended guideline. At 10 μ g/m³, it is a quarter of the UK's legal limit.

Nevertheless, Kelly is positive about how air quality is changing in the city.

"London is really leading the world in basically getting its air quality under control and that's mostly because of what's happened since 2019 when the ultra low emission zone was introduced in the city," he says.

"Initially it was just in the central area but now it goes right out to the M25, so the biggest controlled emission zone in any city in the world."

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