Where do London’s air pollution alerts come from?

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.”