A temporary 50mph speed limit will come into force on the M4 at Newport from Monday 18 June.
The new limit is one of five being introduced across Wales in an effort to tackle pollution, It will apply between junction 25 for Caerleon and junction 26 at Malpas, including through the Brynglas Tunnels.
The Welsh Government’s environment minister Hannah Blythyn said: “I am committed to taking action to reduce air pollution in Wales to support a healthier future for our communities and protect our natural environment.
“Current levels of nitrogen dioxide exceed the legal limit at the five road locations, so we need to act as soon as possible.
“We have been carrying out investigations into all potential measures for reducing nitrogen dioxide levels.
“Our initial studies tell us that 50mph speed limits should have the greatest potential to improve air quality at each location.
“The actions we take to improve air quality in Wales have a key role in supporting the right conditions for better health and well-being.”
And economy and transport secretary Ken Skates said: “I want to see concrete measures put in place by the UK Government which focus on removing the source of the problem.
“They have prevented us from taking our preferred course of action – it is now time for them to step up and use the powers which they have so closely guarded for the benefit of the people of Wales.
“The root cause is the emissions from vehicles, but legislative competence for vehicle standards and fiscal measures, such as the Vehicle Excise Duty and Company Car Tax, remain with the UK Government. Despite repeated calls, the UK Government has consistently refused to devolve powers which allow us to improve standards and incentivise improved emissions through the tax regime.”
New limits will also be in place between junctions 41 and 42 of the M4 at Port Talbot, on the A494 at Deeside, the A483 at Wrexham and on the A470 between Upper Boat and Pontypridd.
Evidence has shown nitrogen dioxide emissions are at their lowest when light vehicles, the main contributors to emissions, travel at speeds between 40 and 50mph.