The air quality in Bath City Centre has improved since a clean air zone was brought in, according to new data.
A report submitted to the council shows that nitrogen dioxide levels have fallen by more than 12% compared to the same period in 2019.
It also shows the number of non-compliant vehicles entering the zone has reduced.
But the report identified four areas in the zone where levels of the gas may still exceed government limits.
The report is the first in a series which will regularly monitor the performance of the CAZ.
All but three of the 220 buses that operate on scheduled routes in the zone and more than 90% of HGVs and 90% of taxis travelling into the zone are now compliant with emission standards.
Councillor Sarah Warren, Deputy Leader and cabinet member for Climate and Sustainable Travel, described the findings as "very encouraging news".
"The aim of the Clean Air Zone is to improve air quality and reduce pollution which can have a devastating impact on people's health, triggering asthma attacks and making heart and lung conditions worse. So, I'm pleased that the early indications are that the Clean Air Zone is working.
"It is, however, early days and more time is needed. Normal traffic volumes in the city have been affected by Covid and the closure of Cleveland Bridge has caused some temporary changes to traffic patterns.
"I am also concerned that despite significant progress in reducing NO2 levels, there are still four locations in the city - Cleveland Place East junction, Dorchester Street, Victoria Buildings and Wells Road near the Churchill Bridge gyratory - where NO2 levels have the potential to exceed the government target we are aiming for.
"Our aim is to meet the government target in all locations, whilst minimising the social, economic and distributional impact of the zone on our residents and businesses. To achieve this, our current focus is on upgrading a relatively small cohort of commercial vehicles and, in particular, older highly polluting vans.
The Clean Air Zone was launched in March to tackle harmful levels of air pollution caused by the most polluting taxis, vans, buses and larger commercial vehicles regularly driving in Bath.
It was the first charging CAZ to be launched outside of London and levies a charge on anyone driving certain higher emission vehicle in the zone. This excludes private cars and motorcycles which are not charged.
Grants and interest-free finance are available to encourage owners of non-compliant polluting vehicles to replace them with cleaner ones.