Nine-out-of-ten roads in Northampton scored illegally high levels of a toxic air pollutant in November last year.
A year-long survey by the borough council has shown how levels of poisonous nitrogen dioxide (NO2) have stayed "stubbornly high" throughout the town in 2017.
But the council failed to produce any results for the Drapery, a known pollution hotspot where regularly queue and where the Chronicle & Echo found NO2 levels were reaching nearly double the legal limit in a test in November.
It comes ahead of a borough council clean-air strategy set to take effect in 2018, which includes phasing out diesel and petrol taxis by 2025 and supporting electric cars.
Leader of the Labour group Danielle Stone said: "I'm shocked. I didn't think it would be this bad.
"I'm very critical of the borough council's upcoming clean air strategy. It's not immediate enough and has no action against idling traffic, taxis and buses, which is one of the biggest causes of pollution."
A borough council report in 2017 linked one-in-20 deaths in Northampton to air pollution.
Using specialist diffusion tubes, the local authority has recorded the average levels for NO2 (micrograms per metre cubed, or ug/m3) across the town. The legal limit set by WHO for an area is 40ug/m3.
Now, figures for 2017 shows how high-pollution areas like St James Road, Campbell Square and Harborough Road are producing up to 70ug/m3 of NO2.
In November 2017, only nine of the 77 diffusion tubes across the town recorded a legal level.
And a Chronicle investigation in November recorded levels close to double the legal limit in the Drapery.
A borough council spokesman said: "We do have some data for the diffusion tubes in Bradshaw Street and Sheep Street which covers around seven months of the year. This will be published in due course, once we have been able to produce estimates for the missing months via technical guidance provided by DEFRA.
"The raw air quality data provided on our website will likely be adjusted using DEFRA’s formula in the coming months and of course at that point, any areas of concern across the town will be part of the focus of our low emissions strategy."