ARTICLE AD BOX
By Jonah Fisher & Katy Austin
BBC Environment Correspondent and BBC Transport Correspondent
The expansion of London's Ultra Low Emission Zone was a big issue in the Uxbridge and South Ruislip by-election.
What is it, and why is it controversial?
What is Ulez?
A £12.50 daily charge applies for driving in the Ultra Low Emission Zone, commonly referred to as Ulez, if the vehicle doesn't meet certain emission standards.
Boris Johnson decided to introduce it when he was the Mayor of London. It then came into effect in April 2019. By this time, Labour's Sadiq Khan had become Mayor.
Initially it covered the same central area as the Congestion Zone, before widening out to the North and South Circular roads in 2021.
In November 2022, a further expansion to cover all London boroughs was confirmed. This is due to start on 29 August 2023.
The mayor's office said it was needed to tackle air pollution, congestion and the climate emergency.
Some other UK cities have their own, different clean air zones.
Why are such zones introduced?
Low emission zones have been introduced to clean up the air, with the aim of making people healthier and reducing the burden on the NHS.
Petrol and diesel vehicles emit the CO2 that warms our planet, but this scheme is principally aimed not at fighting climate change but reducing levels of two key air pollutants - nitrogen dioxide (NOx) and fine particulate matter (PM2.5).
Those pollutants have been blamed for causing thousands of premature deaths and stunting the growth of children's lungs.
What scrappage schemes are available?
For the Ulez expansion, £110m has been set aside for a scrappage scheme, for people on low incomes or disability benefits, some very small businesses and London-based charities.
Eligible drivers can get up to £2,000 for scrapping a car. The amount of money on offer differs for vans and motorbikes.
Changes to the scheme's criteria were made in June.
There is also a grace period for small businesses and charities who have already ordered a new compliant van or minibus, or booked retro-fitting.
Why is Ulez controversial?
The latest expansion of Ulez has proved divisive. Clean air campaigners support it.
However, some residents, businesses and politicians have expressed serious concerns. They include the financial impact during a cost of living squeeze, and the nine month-long notice period.
Some people affected say the scrappage scheme won't help them. The cost of second-hand Ulez-compliant cars has also risen.
Five Conservative-run councils launched legal action over the decision to expand Ulez. There have also been public protests.
The mayor has defended the plan, arguing it is necessary to help prevent health conditions related to air pollution and even excess deaths. Preparations such as installing cameras have continued.
Has the existing London Ulez made any difference?
The number of older polluting vehicles that have entered the Ulez has fallen dramatically with 97% now meeting the cleaner standards.
Within that area it is estimated that NOx emissions have fallen by 26% since 2019 with PM2.5 emissions falling by 19% over the same period.
Air quality has improved as a result but all Londoners still live in areas exceeding World Health Organization guidelines for both pollutants.