Two environment experts have resigned in protest from the Scottish government's clean air strategy team.
Prof James Curran and Emilia Hanna said they were frustrated by slow progress and lack of "ambition" in the planned Low Emission Zone (LEZ) for Glasgow.
They were advisers from Scottish Environment Link which represents 35 environmental groups.
The Scottish government said it remained committed to protecting the public from harmful air pollution.
Ministers have promised to deliver low emission zones in four Scottish cities by 2020, but detailed plans for the first one in Glasgow have been met with disappointment from many environmentalists.
The LEZ will only target the most polluting type of buses, and will not result in any vehicle being banned from the city centre until the end of 2022.
Link has now withdrawn from the Cleaner Air for Scotland Governance Group which oversees the government's clean air strategy.
In its resignation letter, Link said: "For two years the Link representatives have made every constructive effort to inject ambition and urgency into the creation of Low Emission Zones in Scotland.
"At nearly every single stage they have felt frustrated by lack of progress."
Prof Curran, a former chief executive of the Scottish Environment Protection Agency, contrasted the approach taken to air pollution with the resources invested in flood prevention.
He said: "Recently the Scottish government created an ambitious, strategic and tightly managed national system to tackle flood risk - and now funds it with over £40m per year.
"Sadly there is no similar determination to tackle air pollution which is directly causing thousands of premature deaths."
Emilia Hanna, an air pollution campaigner for Friends of the Earth Scotland, claimed toxic air contributed to 2,500 deaths in Scotland a year but there was little sense of urgency from the clean air group.
She said: "There has been a culture of denial on the group, with members keen to ensure we spoke only about 'poor air quality' than 'air pollution'.
"Denialism, apathy and foot dragging on the group has meant missed opportunities and unnecessary loss of life."
The Scottish government said it was "fully committed" to protecting the public from the harmful effects of air pollution and would "continue to take firm action to combat the illnesses that air pollution can contribute to".
A spokesperson said the government was disappointed by the resignations.
She added: "Success requires collaboration across a number of different interests if we are to produce the best outcomes for Scotland and the group has provided valuable input into the development of low emission zones and other parts of the strategy's delivery."