London (CNN) London is to get a much-needed breath of fresh air on its most extensive car-free day to date, with 12.4 miles (20km) of road to be closed in the centre of the British capital.

Mayor Sadiq Khan unveiled the plans for September 22 on Thursday -- Clean Air Day -- in a bid to help tackle London's air pollution problem. The car-free day would also help residents "reimagine" their city, he said.

More than two million Londoners live in areas that exceed legal limits for nitrogen dioxide -- NO2 -- including more than 400,000 children under the age of 18, according to the mayor's office.

According to the 2019 State of Global Air (SOGA) report , air pollution is the fifth leading factor in mortality across the world, responsible for more deaths than alcohol, malnutrition and drugs.

Vehicles are responsible for around half of harmful nitrogen oxide air emissions in the British capital, the mayor's office said in April. They contribute to a toxic air health crisis that increases the risk of asthma, cancer and dementia as well as causing thousands of premature deaths every year , it added.

On September 22 -- a Sunday -- hundreds of activities will take place on the car-free streets, while 18 of London's 32 boroughs have also confirmed events, including the creation of "Play Streets" where children and communities can gather safely.

"This will be a great opportunity for us all to leave our cars behind and explore our streets by foot, or by bike," said Khan in a statement Thursday.

"I encourage as many Londoners as possible to join in the fun and see the city from a different perspective. I will continue to work with those boroughs who are forward-looking and want to think differently about how to use road space. Hopefully this will shame into life those boroughs with outdated views who are dragging their feet," he said.

A recent Transport for London survey found that almost one in two Londoners did not realise vehicles were the main cause of the city's air pollution.

"Days like this are a great way to open people's minds to all the possibilities our urban spaces have when we put people before cars. If you see car-free cities as a sacrifice, go and catch a glimpse of our city as it could be if it as designed for and around people...Reducing toxic air pollution and carbon emissions are vital, but they are far from the only benefit of reclaiming our streets," said Areeba Hamid, clean air campaigner at Greenpeace UK.

In April, London became the first city in the world to implement a 24-hour, seven day a week Ultra Low Emission Zone , inside which vehicles will have to meet tough emissions standards or face a charge.

CNN's Jack Guy and Ben Westcott contributed to this report.