UK cities are among the worst in Europe for nitrogen dioxide pollution, which is linked to breathing difficulties such as asthma.
Six British towns and cities are in the top 20 in Europe for NO2 pollution, a study by monitoring service Airly found.
These were London, Edinburgh, Dundee, Leeds, Newcastle-upon-Tyne and Slough.
The findings come five months before Britain will attempt to rally the world into ambitious action on climate change at the Cop26 conference in Glasgow.
Nitrogen dioxide is produced mainly by the burning of fossil fuels, which the UK wants other countries to curb as world leaders strive for net zero emissions by 2050.
Breathing air with high levels of NO2 can aggravate respiratory problems such as asthma.
It can lead to coughing, wheezing or difficulty breathing and prolonged exposure can make people more vulnerable to respiratory infections.
"The data isn't great for large parts of Europe but there is certainly an opportunity to tackle the problem," Airly chief executive Wiktor Warchalowski said.
"People need to change their habits, especially with the use of cars, and local authorities need to start by monitoring the problem then putting in place appropriate policies to manage problem hotspots."
Cluj-Napoca in Romania was the most polluted city in the NO2 rankings, with Newcastle second.
Romania's Timisoara, Ploitesti and Bucharest were also in the top 10, along with Naples, Athens, Marseille and Granada.
The figures were based on air quality sensors set up in towns and cities to monitor pollution.
As part of global efforts to curb emissions, G7 leaders agreed at their summit last week to end government funding for fossil fuel energy as soon as possible.
This is intended to help meet the Paris Agreement target of limiting global warming to well below 2°C and ideally 1.5°C.
Britain wants to reduce emissions by 68 per cent by 2030 compared to 1990 levels.
Cop26 is a key political milestone for the UK as it tries to project global leadership after Brexit.
The UK's Cop26 president Alok Sharma this week described the conference as "our last hope" of meeting the 1.5°C target.
"I have faith that world leaders will rise to the occasion and not be found wanting in their tryst with destiny," he said.
But the pollution findings come after a climate panel found fault with the UK for failing to keep pace with global warming.
The UK’s Climate Change Committee said the government failed to heed previous warnings on the subject.
Episodes of extreme heat were becoming more frequent, it said in its latest report.
Meanwhile, Scotland's government was criticised for failing to meet its own climate targets in the run-up to its hosting of Cop26.
Emissions fell by 51.5 per cent up to 2019, below a target of 55 per cent.
Jamie Livingstone, the head of Oxfam Scotland, said ministers could make “no more excuses”.
“On the eve of the most significant global climate talks for years being hosted in Glasgow, the Scottish government has scored a hat-trick of own goals by missing its annual emissions targets three years in a row,” he said.