The Welsh Government says the speed limit - which covers the A483 between Junctions 5 and 6 at Wrexham - has helped reduce pollution but it now wants to make sure more people stick to the rules.
"We need to act now to make Wales a safe place to live with clean air for everyone," said Lee Waters, the deputy minister for climate change who also has responsibility for transport.
While he wants speeds to reduce he says he wants to go "further and faster" despite the unpopularity of enforcement.
"We know that slower speed limits are not a popular choice," he said. "But we need to do things differently and be bold if we are to stand a chance of tackling climate change.
"It’s clear that the speed restrictions we’ve introduced on our most polluted roads are working - the results speak for themselves – but compliance with these limits is essential if we are to achieve the reductions we need to make in the shortest possible time."
The other parts of the road network in Wales to be covered by the 50mph limits are the A494 between the Wales/England border and St David’s Interchange Deeside; the M4 between junctions 41 and 42 Port Talbot; the M4 between junctions 25 and 26 Newport; and the A470 between Upper Boat and Pontypridd.
The decision to introduce environmental speed limits to improve air quality levels in five locations across Wales in 2018 was the first of its kind in the UK.
A reduction of up to 47 per cent in levels of nitrogen dioxide has been attributed to the limits.
The Welsh Government says air pollution is considered to be one of the largest environmental health risks of our generation, according to the World Health Organisation, increasing risks of heart and lung disease and contributing towards the worsening of pre-existing health conditions such as asthma.
Assistant Chief Constable, Mark Travis, the roads policing lead for Wales at South Wales Police said the move is an "important issue to tackle pollution and to give Wales and its communities a cleaner and healthier future.
"All four Welsh Forces are supporting its implementation, with the enforcement of these speed limits approached in a consistent, proportionate and transparent manner.”
Safety campaigners also say it should make the roads safer.