Area buses going through Glasgow city centre must now meet European emissions standards.
A target of 20 per cent of the vehicles have to achieve the goals as the first low-emission zone (LEZ) in Scotland.
All vehicles entering the city centre must meet the standards by 2022 but campaigners argue it will not achieve the change to air quality needed.
Campaigner Gavin Thomson told the BBC said: “The toxic, illegal levels of air pollution causes hundreds of premature deaths and life-limiting conditions.
“Unfortunately, the weak terms of this low-emission zone - only applying to one in five buses - won’t achieve any significant change for air quality.
“This ‘no ambition zone’ means fumes from transport in the centre of Glasgow will carry on poisoning people’s lungs for many more years.
“Around 18 per cent of buses in Glasgow already met the emission standards before the zone was decided, so there will be very little change in the fleet.”
Glasgow City Council said the plan was “manageable, yet robust”.
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