Toxic fumes from vehicle exhausts are breaching legal safety limits on 23 busy streets and endangering the health of people across Scotland, according to the latest Government monitoring.
Hazardous concentrations of nitrogen dioxide and tiny particles belched out by cars, lorries and buses have been found in Glasgow, Edinburgh, Dundee, Aberdeen, Greenock, Rutherglen, Kilmarnock, East Kilbride, Falkirk, Perth, Crieff and elsewhere in 2014.
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On 11 streets the annual average levels of pollutants have increased between 2013 and 2014. The worst pollution was detected on Hope Street in Glasgow, where nitrogen dioxide levels were 62% above the European legal limit - which was meant to have been met five years ago.
Health experts have said the levels are high enough to cause breathing problems, heart attacks and cancer. They attribute more than 2000 deaths a year in Scotland to the pollution - far more than are road accidents, drugs or alcohol.
Air pollution, they say, has now become a major public health crisis, which is estimated to cost the National Health Service in Scotland up to £2 billion every year.
Simon Gillespie, chief executive of charity the British Heart Foundation, said: "Exposure to certain air pollutants can make existing heart conditions worse and can cause heart attacks among children and the elderly.
"The current, illegal, amounts of certain pollutants are unacceptable. The Government must act to clean up dirty air and protect our hearts."
Professor David Newby, chairman of cardiology at the University of Edinburgh, agreed. Particulate pollution was linked to heart failure, he said. "We have demonstrated that this is because it causes abnormalities in blood vessels and starves the heart of blood."
Fintan Hurley, the scientific director of the Institute of Occupational Medicine in Edinburgh and a Government adviser on air pollution, warned that the breaches of safety limits were just "the tip of the iceberg". Health was also being put at risk by pollution that was below the limits, he said.
"Air pollution is a big public health problem because everybody is exposed and there is no completely safe level," he said. "The regulatory limits for fine particles don't give good protection - even if these were met, there would still be a big problem.
"We need to get a whole social movement behind cleaner air, as part of the wider movement for a better and fairer Scotland."
Friends of the Earth Scotland, which is leading the campaign against air pollution, accused the Scottish Government of being "painfully slow" to act. A national low-emissions strategy promised before the end of 2014 has still to appear.
Emilia Hanna, the environmental group's air pollution campaigner, said: "Scotland's streets have dangerous levels of toxic pollution which are breaking legal limits that were due to be met in 2010.
"The time has come for our polluted air to be treated as the public health crisis it really is."
She pointed out that children were particularly vulnerable, with pollution known to restrict lung development and cause long-term health problems.
Local authorities also needed to be given more resources to tackle problems in their areas, Hanna argued. She said: "It is very worrying that the draft budget shows no increase in funds for action on air pollution and suggests the Scottish Government plans on spending 200 times as much money next year on building new roads as on tackling deadly air pollution."
However, the Scottish Government insisted it was making "excellent progress" in reducing air pollution in partnership with councils. It said new data showed that nitrogen dioxide levels had dropped by 65%, and particulates by 58%, between 1990 and 2011, and further decreases were expected.
But a spokesman said:"We recognise there is more to be done to deliver further benefits for human and environmental health where areas of poorer air quality remain."
A consultation of the low-emission strategy was to be launched "later this month", the spokesman added. "This will set out the contribution that reduced air pollution can make to delivering sustainable economic growth and enhancing the quality of life for communities across Scotland."
Local authorities pointed to all the measures they were taking to try to improve air quality. Glasgow City Council had required bus operators to invest in cleaner fleets, joined a car-rental club, launched a successful bike-hire scheme and encouraged electric vehicles.
City of Edinburgh Council's transport and environment convener, Lesley Hinds, said the city had seen a 10% fall in nitrogen dioxide levels across much of the city.
"There are still a small number of areas that need particular attention, and we are making efforts to address them," she said. "Through implementation of the local transport strategy and meetings of the city's air quality working group we regularly review air quality management areas to assess pollution, in order to create a cleaner, greener city for everyone."
Aberdeen City Council's transport spokesman, Ross Grant, highlighted a car-sharing club, a fleet of "pollution-free hydrogen buses" and the new western peripheral route as among the council's improvements.
But he also commented: "While we understand that clean air is essential for health and helps to make the city a pleasant place to live, it is not just the responsibility of the city council. Everyone can play their part by considering their travel habits and making small, meaningful changes."
A spokesman for Dundee City Council said: "The measures that the council have brought in over the past few years are starting to bear fruit with certain of the pollutants that are tested for."
Dr Sean Semple, an air pollution expert and senior lecturer at the University of Aberdeen, agreed that improvements had been made across the country, but said the latest figures showed that concentrations of pollutants in many towns and cities still exceeded regulatory standards designed to protect public health.
He said: "To improve air quality further we will need to look carefully at how we organise transport, how we power the vehicles we use and how we can encourage more journeys by walking and cycling.
"Given that many of the roads listed have exceeded European standards for several years there is a need for stronger action to ensure we comply.
"Every year of delay means more people suffering unnecessary asthma exacerbations or heart attacks."