Unusually low winds caused the build-up of fumes in the south-east of England before being blown north to mix with Edinburgh’s emissions.
Environmental group Friends of the Earth Scotland have warned that people with lung and heart complaints are particularly vulnerable.
“The air pollution episode is creating dangerous conditions, especially for vulnerable people with lung or heart problems,” said Emilia Hanna of Friends of the Earth Scotland.
Levels of particulate matter – tiny hazardous particles in the air from traffic – exceed both World Health Organisation and Scottish regulatory safety standards.
Such excessive amounts of toxic pollution only occur a few times a year in Edinburgh – while Ozone gas can cause problems in summer smog.
Forecasts show the pollution to be heading north over the weekend before dispersing early next week.
Traffic-derived air pollution, mainly composed of fine particles and toxic gases, has been linked with a number of serious health issues, including cancer, heart problems and dementia.
“Air pollution is an important risk factor for asthma attacks, which can be severe,” said Edinburgh University asthma expert Prof Aziz Sheikh.
“It is important that people with asthma are regularly using their preventer inhalers to minimise this risk and that they have their reliever inhalers readily available for use if they experience symptoms.”
Friends of the Earth estimates pollution causes 2500 early deaths in Scotland each year.
“We have a right to breathe clean air, and the toxic levels of pollution are seriously damaging our health, despite a legal obligation on the Scottish Government to tackle this problem,” said Ms Hanna.
“Toxic air pollution is a public health crisis, and motorised transport is the key culprit.”
She called on the government to encourage more environmentally friendly modes of transport and to fund its promise of creating Low Emission Zones by next year. A Scottish Government spokesman said: “We are determined to improve air quality and are working to ensure Scotland’s first low emission zone is in place next year.
“The government is liaising closely with local authorities and other partners to meet this timetable.”
The government has invested in greener transport initiatives, including electric cars, and has led the world in adopting air quality laws, added the spokesman.
Green Party councillor Chas Booth, meanwhile, asked the city council to commit to LEZs this week, after Glasgow did the same.
“We are in discussions with the Scottish Government on this very issue. We are committed to it,” replied council leader Andrew Burns.