The second inquest for Ella Kissi-Debrah is thought to be the first time that air pollution has been recorded as a medical cause of death, but the damage done to people’s health has long been known.

Pollutants, including nitrogen dioxide and small particles known as particulate matter, or PMs, play a role in the equivalent of 40,000 early deaths a year in the UK, experts have said.

Air pollution can create a catalogue of health problems: it triggers strokes, heart and asthma attacks, increasing the risk of hospitalisation or death, causes cancer and can stunt lung growth in children.

It has been linked to premature births, damage to children’s learning and even dementia.

Older people, the young and those with chronic illnesses are more vulnerable to air pollution and those on low incomes and from ethnic minorities are disproportionately affected by it.

Legal limits for pollution to protect people’s health should have been met by 2010.

But across the UK levels of nitrogen dioxide still breach the rules today, and while particulate matter pollution is within legal limits, it is still above World Health Organisation guidelines.

The Government has lost three court actions in the last decade brought by environmental law charity ClientEarth, over its failure to tackle the problem of illegally dirty air.

The court cases have prompted requirements for action by local, regional and devolved governments – and a new clean air strategy last year to tackle the problem.

In 2019, 33 of the UK’s 43 air quality zones were still above the legal limit for nitrogen dioxide, analysis of government figures by ClientEarth has revealed, including London, Manchester, Sheffield, Glasgow, Belfast and Bristol.

Nitrogen dioxide and PMs come from sources such as road traffic and domestic heating systems including boilers and wood burners, with vehicles – particularly diesel vehicles – a key part of the problem.

In the long-term, the shift to electric vehicles and heating systems will help, though particle pollution is still caused by tyres and brakes, but those improvements are still many years off.

Traffic’s role in nitrogen dioxide emissions was highlighted earlier this year when levels of the pollutant fell in many cities and towns as people stayed home and roads emptied of vehicles in lockdown.

But the benefits were short-lived, as pollution has already returned to pre-pandemic levels or even higher in the majority of towns and cities.

And evidence suggests that air pollution could play a role in at least some of the deaths from Covid-19, compounding its other health impacts.

As people continue to stay away from public transport, efforts to encourage active travel such as walking and cycling in towns and cities – which would also benefit health and reduce the pollution that drives climate change – have been mixed.

Low traffic neighbourhoods and cycle lanes have been installed, but a backlash from some motorists has seen them removed in places such as Kensington High Street.

Campaigners want to see measures to speed up the use of electric vehicles, including e-bikes, and the introduction of clean air zones, which charge drivers of the most polluting vehicles in certain areas.

But efforts to bring in clean air zones have been delayed by the pandemic in some areas.

Part of the lack of urgency on the issue may be because, unlike the old pea-souper smog events that prompted the UK’s first Clean Air Act to clean up pollution in the 1950s, today’s pollutants are an invisible killer.

But now the fatal impact of pollution has a human face – an active nine-year-old girl who died because of the toxic air she was surrounded by.