Aeroplane-style emergency masks have been installed by environmentalists at a Leeds bus stop to highlight the dangers of roadside pollution.
They said politicians should "act now on climate change" and they were "deeply worried" about the health risk.
The drop-down masks, which do not pump oxygen, were attached to a bus stop on Infirmary Street by members of campaign group AirTeam.
Leeds City Council has been asked to comment.
According to Friends of the Earth (FoE), figures show the annual average nitrogen dioxide (NO2) level in and around Infirmary Street exceeds the legal limit of 40ug/m3.
In February, FoE said Neville Street, a short distance away from Infirmary Street, was the most polluted street outside London.
Caroline Thomas, one of those involved in the bus stop protest, said: "Increasingly over the last few years it's very noticeable in the city.
"As an asthma sufferer I notice the impact - the feeling in my lungs straight away when I come into the city centre.
"I am deeply concerned about the health effects of air pollution not only on myself, but for my 11-month-old daughter."
The campaigners dressed up in cabin crew outfits and handed out leaflets highlighting the "toxic air" on the streets.
Ellen Clarke said: "I feel so worried about what Leeds' air pollution is doing to our health.
"I have young children and we know that little lungs are really vulnerable to inhaled toxins."
Earlier this year, the council said owners of polluting lorries, coaches, taxis and private hire vehicles would be charged for entering parts of city from January 2020.