Investigations are being carried out at three areas of Gloucestershire identified as pollution "hotspots".
The study involves analysis of traffic fumes across the county
Tests at Barton Street and Priory Street in Gloucester and at junction 10 of the M5 showed that nitrogen dioxide levels failed to meet national targets.
The air quality study involved test-tube analysis of traffic fumes over a six-month period.
Other sites at Tewkesbury High Street, Birdlip and London Road, Cirencester were only just within agreed limits.
Julie Wight, environmental health manager at Gloucester City Council, said: "Generally, pollution in Gloucester is reducing, but we do have slightly elevated levels in a couple of areas in the city.
"We will now be producing an action plan to tackle it."
Last week, increasing traffic volumes were highlighted at the launch of the "Great Gloucestershire Debate".