Pollution levels for nitrogen oxide in Scottish cities are running at more than double the Government's recommended guidelines in some city-centre streets.

The UK's National Air Quality Strategy, designed to protect human health, has set targets for cleaner air by the year 2005. However, the findings of a study show levels of nitrogen dioxide are currently too high and the cities will not meet the new standards.

Environmental group Friends of the Earth has called for immediate action.

A safe level for the harmful gas is 21 parts per billion (ppb), according to the Government, but research shows one site in Glasgow had 48ppb and one in Edinburgh 46ppb. The location with the lowest recorded amount of pollution, 23ppb, was in Aberdeen.

Most action is needed in Glasgow. Of a dozen sites monitored in the city, six are likely to break the nitrogen dioxide limit in 2005.

Glasgow city centre also had the highest concentration of the gas which is responsible for many respiratory problems, including asthma and hay fever, and has been blamed for increasing the risk of bronchitis and pneumonia.

It is predicted 18 out of 38 sites in Edinburgh will fail to meet the required standard when it comes into practice in six years time but levels are lower than in Glasgow.

Air quality at these sites has improved since 1996. One of the city-centre hotspots, Princes Street, attracts hundreds of thousands of tourists every year.

High levels of traffic-related pollution were found in Dundee city centre, despite large areas being pedestrianised. Aberdeen recorded the best results and the biggest improvement since the last survey, in 1996.

Government figures show 90% of nitrogen dioxide, which is also responsible for the formation of ozone, smog, and acid rain, comes from vehicle exhaust emissions.

Dr Richard Dixon, head of research at Friends of the Earth, said: ''These reports reveal the true challenge facing Scotland's urban areas. Even changes in technology will not be enough - the simple truth is we have to stop and then reverse the predicted growth in traffic.

''I think many councils have not yet realised how important the Air Quality Strategy is going to be. They may argue about pedestrianising this street or that but soon they will have to reduce the traffic over a broad area to meet these legal targets on air pollution.

''Of course a level of nitrogen dioxide which is bad for you in 2005 must also be bad for you today. It is clear many people are living with unacceptable levels of pollution and action should be taken today.''

The Government has predicted traffic will grow by 50% over the next 30 years.