Old buses emitting 10 times more pollution than newer vehicles are still on the streets of Norwich, despite the council pledging to end their use more than three years ago.
In September 2015, Norwich City Council vowed to work with bus companies to get all vehicles up to the Euro 5 emission standard within three years - and get as many as possible to the newest and cleanest standard called Euro 6.
The target was later changed to March 2019, but that has been missed too with dozens of city buses still at the old Euro 3 standard.
It comes after we revealed older buses from other cities were being moved to Norwich when those cities got cleaner vehicles.
Under the Euro 3 standard, vehicles can emit 10 times as much particulate matter, or soot, than the newer Euro 6 vehicles
The older emission standard also allows six times as much Nitrogen Oxide emissions which contributes to asthma.
At First, the city's biggest bus operator, almost 30pc, of buses are on the Euro 3 standard, while the rest are Euro 5 and 6.
Steve Wickers, managing director, said: "Air quality and our carbon footprint are subjects we take very seriously and we constantly work towards improving both in a sustainable, manageable and cost-effective way.
"Our vehicles are much cleaner than most other modes of transport on our roads."
He added that the firm works closely with the city and county councils which are hoping for government funding to fit more buses with cleaner exhausts.
Other cities have already taken stricter steps to improve air quality. In Glasgow all vehicles in the city centre will have to be at Euro 6 or better by 2022.
And in Leicester, the council will charge bus companies to run any vehicles which are not Euro 6 standard from next year.
Jeremy Cooper, from bus operator Konect, said they would support a similar project in Norfolk, if funding was provided to upgrade buses.
A spokesperson for Transport for Norwich, a partnership between the city and county councils, said it would be applying to the Department of Transport for money.
"We're exploring all options so we don't know at this stage whether it will include measures such as banning or charging for diesel engines," a spokesman said.
-'Massive health crisis'
It is "outrageous" Norwich is having to make do with ageing buses cast-off to the city when other areas replace their fleets, former Green Party leader Caroline Lucas has said.
During a visit to Norwich, the Green Party MP said: "It is massively important that bus companies take their commitments seriously and it is outrageous that Norwich is being used as a kind of dumping ground for other people's old buses that, crucially, don't have the most up to date technology when it comes to air pollution.
"Air pollution is a massive health crisis."
First said its bus fleet was much cleaner than most other modes of transport, particularly after it upgraded some of its older vehicles to reduce particle emissions.
Its vehicles also have an 'engine cut out' mode meaning if a bus is stationary for more than two minutes the engine will stop.
Konect said 24 of its 37 buses in Norwich were at the Euro 5 standard and three Euro 6 buses would arrive in October.
-Air pollution in Norwich
Last year Norwich was named as one of 32 cities in the UK exceeding air pollution levels.
Data from the World Health Organisation (WHO) showed fine particle air pollution was just above target levels.
The WHO report said millions of people globally were dying every year from air pollution.
Last week, meanwhile, leading doctors told The Times that government ministers were ignoring medical advice and doing the "minimum possible" to tackle air pollution in the UK.
The latest air quality annual report for Norwich, shows levels of pollutant nitrogen dioxide have fallen since 2013, but another pollution, called particulate matter has stayed at similar levels.
Green Party councillor Denise Carlo, said: "I would like to see Norfolk County Council use some stick against the bus operators in getting them to clean up the toxic emissions from their fleets."
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