Norwich is being used as a 'dumping ground' for old buses which are no longer needed in other parts of the country, sparking fears about air pollution.
The owners of the city's two biggest bus operators, First and Konect, both send old buses to the city from other parts of the country when those areas get new vehicles.
It is common for large bus companies to move fleets around, but the extent that it is happening in Norwich means some recently arrived buses in the city are already 14 years old.
Older buses, from before 2005, emit up to ten times more pollution than newer vehicles, unless they go through a revamp known as "retrofitting".
Buses are one of the major contributors to air pollution in the city, according to a council report from last year.
But bus companies have hit back, saying it is cheaper to make older vehicles cleaner than buy new buses.
Last year the Go-Ahead Group, which owns Konect Bus, bought new, low emission buses for its fleets serving the south coast and London.
It then sent 23 old vehicles it no longer needed to Norfolk, some of which had been on the road for 14 years.
The last time it bought a new bus for Norwich was 2015.
Managing director of Go-East Anglia Jeremy Cooper said the buses had to be brought in from other areas to replace older vehicles in Norfolk.
"Using buses from elsewhere in the Go-Ahead Group helps to keep fares down for customers and keeps these routes commercially viable," Mr Cooper said.
He added the London vehicles were retrofitted - meaning they emitted fewer pollutants than other vehicles their age.
He also said Konect's fleet was on average just under nine years old.
"We will invest in more new buses with the latest technology if our contract to run these services is extended as originally planned," Mr Cooper added.
First Group also sends old buses to Norwich when other cities where the company runs services get new vehicles.
One regular passenger said he had seen the word "Bradford" still on the side of an old First Bus driving along Dereham Road while waiting for his bus earlier this year.
"Why is Norwich the dumping ground for old buses and why do we rarely see brand new ones here?" he asked.
First said it was upgrading its older vehicles to make sure they emitted fewer pollutants.
But it still has more Norwich buses on the old emission standards, known as Euro Three, than the new emissions standards, called Euro Six.
It also has more buses from 2002 than any other year operating from its Norwich depot.
Its oldest buses are 18 years old, but 15 of its older buses have been retrofitted, thanks to a grant from the Department for Transport (DfT).
First said that meant its older buses were not necessarily dirtier.
The retrofitted buses are the level below Euro Six.
Steve Wickers, managing director at First Eastern Counties, said: "People talk about dirty buses, but even though you are still using diesel it's as clean, or cleaner, than any modern car filters.
"We are socially responsible, and we have invested quite a lot in our vehicles.
"Some of that solution will be retrofitting, as we have to be realistic."
He said a new double decker bus cost between £220,000 and £240,000, while retrofitting a bus cost around one-10th of that.
First bought new buses in 2015 and 2016 and said it was putting together a business case to buy more new vehicles.
In 2012 an air quality action plan was launched in Norwich to reduce pollution and a key part of that was encouraging bus firms to update their fleets.
A spokesman from Transport for Norwich, a partnership between Norfolk County Council and Norwich City Council, said buying new vehicles was a commercial decision for the companies, but added it would apply to the DfT for funds.
A council report from last year into Norwich air pollution said: "It is the keen intention of Norfolk County Council to continue to encourage bus operators to replace older, polluting vehicles for buses using cleaner technology and especially for buses using the Castle Meadow Low Emission Zone."
But it warned: "The upgrading of bus fleets is a slow and costly process."
-'Give us electric buses'
The former Green Party leader at the city council said "dumping" old buses on Norwich was "absolutely outrageous".
Andrew Boswell, an environmental consultant, said the city's buses needed to be replaced with electric vehicles, rather than bringing in old diesel ones.
"Dumping smoky and polluting old buses in Norwich harms our children," he said.
"If they are getting buses form elsewhere they should only take electric buses."
He called on the bus companies, city and county councils to work in closer partnership to find the funds for electric buses.
Mr Boswell said public money should be diverted from building the Western Link Road to new buses.
A fleet of carbon neutral buses, which ran on biogas, where brought to Norfolk and Suffolk in 2012 by the Go-Ahead Group.
But four years later they moved them to Plymouth saying they were "unsuitable" for rural areas.
-Bus pollution
The oldest engines in buses on Norwich's streets are emitting up to ten times more pollution than the newest engines.
The cleanest emission standards, called Euro Six, allow 0.0005 grams of particulate matter, or soot, per kilometre.
But the oldest bus engines are at a much lower standard called Euro Three which allows ten times more soot.
The older emission standard also allows six times as much Nitrogen Oxide emissions (NOx).
They contribute to acid rain and can also trigger asthma and inflammation in lungs.
The annual health cost of the impacts of particulate matter in the UK is estimated to be around £16 billion.
A city council report into air pollution last year said seven sites in the city centre recorded air pollution above target levels.
All drivers can now be fined for leaving engines running.
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