Glasgow City Council has released new figures showing a rise in the number of drivers repeatedly breaching Low Emission Zone (LEZ) rules.
Figures for September show 89 vehicles received the maximum fine of £960, imposed when the same vehicle is penalised for the fifth time.
Under LEZ rules, drivers whose cars do not meet clean-air standards are fined £60 but this doubles each time the vehicle enters the restricted city centre zone.
In July, traffic cameras detected just one driver on their fourth offence, costing £480 - this soared to 83 in September.
Since the LEZ's introduction on 1 June, more than 20,000 fines have been issued.
If a charge is paid within 14 days of it being issued, the cost reduces by 50%.
The fines double every time the same car or van enters the zone.
Although cars and light goods vehicles can only be fined a maximum of £480, buses and HGVs can be fined £960.
Despite the increase in repeat offences, overall fines in September were down almost 800 on the July figure - from 6,139 to 5,341.
Glasgow City Council is yet to release its full revenue from the zone.
What area does the LEZ cover?
The LEZ covers an area from the M8 motorway to the north and west of Glasgow, the River Clyde to the south, and the Saltmarket/High Street to the east.
Edinburgh, Aberdeen and Dundee are to introduce similar schemes to Glasgow's LEZ in Summer 2024.
Like Glasgow, the restrictions will affect the owners of diesel vehicles over eight years old and petrol vehicles dating from before 2006.
The £60 base rate is set nationally by the Scottish government and other LEZs will follow the same pay structure as Glasgow.
How do fines work?
The owner of any non-compliant vehicle detected within the low emission zone will initially face a fine of £60.
The penalty charge will then double with each subsequent breach, rising to a maximum of £480 for cars and light goods vehicles and £960 for buses and HGVs.
Once that figure is reached, the maximum fine will be imposed for each day that the vehicle enters the zone.
The rate is reset to £60 if there are no breaches for 90 days.
The council says the size of the fine will only start to increase after the vehicle's registered keeper can be expected to have received the first penalty charge notice.
'Harmful air pollution'
A council spokeswoman said: "Glasgow’s plan to phase in a Low Emission Zone was announced in 2018 to tackle the harmful air pollution that has blighted the city centre for decades."
She said air pollution was "creating and exacerbating people's health conditions and the city's health inequalities, disproportionately affecting the most vulnerable".
"We would remind drivers that Glasgow’s LEZ is now fully in force and to familiarise themselves with its emissions requirements," the spokeswoman said.