Motorists from Ross and Herefdordshire travelling on the M4 between junctions 28 and 24 will have to drive at 50mph or face being fined.
The new enforcement speed was made law on Thursday November 17.
The move is the final stage of a phased approach that the Welsh Government has taken to improve air quality on some of the most polluted roads across Wales, as well as reduce congestion and improve safety along this section of the M4.
Since the Welsh government took the decision to introduce environmental speed limits to improve air quality, levels at five different locations across Wales - the first of its kind in the UK – there has been a successful lowering of nitrogen dioxide levels in these areas.
Enforcement in four of the five areas began in October last year and Thursday marked the completion of this rollout with M4 junction 24-28 going live.
Air pollution is considered to be one of the largest environmental health risks of our generation, according to the World Health Organisation.
Driving at 50 mph will not only help us all protect our families from serious illnesses like heart disease, lung cancer and asthma, but it also helps manage congestion, improve journey times and reduce accidents.
Deputy minister for climate change, Lee Waters, who has responsibility for transport said: “We’ve made significant progress in bringing emissions levels down in recent years, but we now have to go further and faster.
“We know that slower speed limits are not a popular choice, 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.
“We need to act now to make Wales a safe place to live with clean air for everyone.”
Superintendent, Michael Richards of Gwent Police said: “The environmental 50mph speed limits are playing an important part in tackling pollution on some of Wales’ most polluted roads, helping Wales and its communities build a cleaner, safer and healthier future.
“All four Welsh Forces are supporting the enforcement of these speed limits.”
Teresa Ciano, partnership manager at Go Safe added,: “‘GoSafe fully supports the environmental 50mph speed limits. In addition to reducing NO2 levels and building cleaner environments for our communities.’’