First West Yorkshire, part of FirstGroup, has retrofitted exhaust aftertreatment systems to 21 buses operating on three routes in Calderdale.
The operator is fitting the SCRT system, from Eminox, which combines a continuously regenerating trap (CRT) and selective catalytic reduction (SCR).
The operator’s investment, which was matched by government funding, sees it upgrading around a fifth of its fleet to Euro VI emission standards, cutting emissions of NOx and particulate matter by around 95%.
Martin Hirst, commercial director at First West Yorkshire, said: “We are completely committed to reducing bus emissions across West Yorkshire and supporting the local campaigns for cleaner air. With the latest green exhaust system technology, we are reducing emissions significantly. Greener buses mean cleaner town and city centres and a healthier environment for everyone.”