After the Conservative Party won the Uxbridge and South Ruislip by-election last night - Boris Johnson's old seat - the Tories pointed towards Labour Mayor Sadiq Khan's plans to expand London's ULEZ as the reason behind their victory.
Outside of London, a number of cities have introduced clean air zones in the past few years, including Birmingham, Bradford and Portsmouth.
All of these zones, which put financial penalties on less environmentally friendly vehicles, have been implemented in a bid to improve air quality in these areas.
But how can air pollution damage our health - and how do clean air zones help alleviate this?
Health problems caused by air pollution
Breathing in dirty air, unsurprisingly, has a negative impact on our respiratory system.
According to a study into clean air zones by Imperial College London, published by Rosemary Chamberlain MPH earlier this month, found that air pollution can contribute to "cardiovascular and respiratory diseases, birth outcomes, dementia, lung cancer [and] diabetes."
Tim Dexter, clean air lead at Asthma + Lung UK, said: 'The devastating impact that air pollution can have on our lives should not be underestimated. Toxic air affects everyone's lungs, it can stunt children's lung growth, worsen long term lung conditions, and can cause asthma attacks.
"Recent research provides further evidence that air pollution also has a role in causing lung cancer in non-smokers. The latest data shows that toxic air contributes to up to 43,000 premature deaths in the UK a year.
"This is why schemes like the ULEZ, which meaningfully reduce air pollution, are so needed. The ULEZ scheme across inner London to date shows that where action is taken, things can get better, and millions of people can breathe cleaner air. It is crucial the government and local political leaders now take urgent action to reduce the excessive levels of air pollution across the entire country, and that the government provide adequate financial support to help people transition to cleaner modes of travel."
How do clean air zones help?
Implementing a clean air zone makes drivers of older and more polluting vehicles think twice about entering the area, due to the associated fee. While some may of course pay this, others will simply avoid the area, thus meaning fewer pollutants in the vicinity.
In Leeds, the clean air zone was deemed to be "no longer required" as early as 2020, due in part to businesses switching to cleaner vehicles faster than expected.
In her study, Ms Chamberlain concluded: "Available evidence suggests observable health benefits from schemes restricting private vehicles in cities. Evidence for low emission zones is most consistent for cardiovascular disease, whereas evidence for congestion charging zones is restricted to road traffic injuries in London. Further research could usefully investigate how to optimise the design of such schemes to improve health."