The effects of Canterbury’s “toxic air” are laid bare to see in blackened filters removed from a cyclist’s face mask.

Martin Baker wore the protective gear for two weeks on his daily commute from Sturry to the University of Creative Arts, where he works as a teaching assistant.

The 29-year-old, of Mill Road, says he was left stunned by the level of discolouration on the filters.

“I got a mask because I heard statistics about Canterbury’s air pollution being way above national guidelines, but I never expected it to be this bad,” he said.

“What scares me is that these filters only catch the larger particles, but apparently it’s the smaller particles that do the most damage.”

Photos of the filters were used on 250 leaflets handed out in the city by Canterbury Green Party members.

They targeted motorists in St Dunstan’s – a “dirty air blackspot” – urging them to cut off their engines while waiting at the busy level crossing.

Exhaust fumes billowing from vehicles stopped at the gates contribute to some of the highest levels of pollution recorded in the city.

Green party campaigns manager Nathan Tough said: “Most of the drivers we spoke to were very aware and sympathetic to what we were trying to achieve and switched their engines off.

“In fact, there was a tangible difference in the air quality, which became less pungent as the day went on.”

Professor Stephen Peckham, director of health policy at the University of Kent, estimates that at least 100 deaths a year in Canterbury are caused by air pollution.

Of 27 sites monitoring nitrogen dioxide levels in the city, he said nine showed readings above the limit set by the EU, with St Dunstan’s, Broad Street and Wincheap the most toxic areas.

Councillors across all parties supported the idea of a car-free day in the city in an effort to encourage people to use greener forms of transport.

But Canterbury City Council says its counterparts at Kent County Council will take the lead if any such day occurs.

Green party parliamentary candidate Henry Stanton said: “In light of the city council running scared of a car-free day, and with no plan in place for fixing Canterbury’s illegal air pollution, we are doing what we can to encourage drivers to make a difference to levels of pollution, which is damaging health and claiming lives.

“Tests on children in polluted areas of London show that they have 8% less lung function than children raised in an unpolluted environment and parts of Canterbury are approaching similar levels of pollution.

“It is unacceptable that our council continues to be so complacent over this invisible killer.”