Green campaigners have accused the council of dragging its heels over a study into how to slash pollution in the heart of Oxford.

It comes as thousands more cars clog Oxford's streets as the 'school run' returns in earnest.

And figures show that it is not only congestion which rises, compared to the quieter summer months. So do levels of poisonous gases - which exceed levels set by the European Union.

Tony Payne, Oxford City Council's environmental protection team manager, said: "You can calculate emissions from vehicle flows. On a day with more vehicles, clearly there will be more emissions. Traffic congestion exacerbates that."

The stark contrast has renewed calls for a ban on ageing, pollutant and fume-belching vehicles in the city centre.

It has thrown calls for a Low Emission Zone (LEZ) back into the spotlight, but the council has been accused of brushing the idea under the carpet.

Statistics show city traffic levels increase by up to 17 per cent per cent in September, compared to August.

Meanwhile, levels of nitrogen dioxide - which can spark respiratory problems - can rise by anything between 10 to 30 per cent. This is against a backdrop of a city that is still failing to meet important European air quality standards.

The last recorded yearly figure showed 40 of the 60 nitrogen dioxide testing sites in central Oxford failed to meet targets.

The top option for cleaning the city's air is a LEZ - which could see all but the greenest vehicles banned from the heart of the city.

Local Green party leader Craig Simmons said: "We have been calling for the city centre to become a Low Emission Zone - where you ban all vehicles that don't meet certain standard air quality targets - like old buses that push out fumes."

But Mr Simmons said action on an LEZ was taking too long.

He said: "It's being brushed under the carpet, but improving air quality is important. There is a legal requirement to improve air quality and it's not being taken seriously."

He believes the county council needs to invest more in its safe routes to school plans.

Mr Payne pointed out the impact on air quality in the city was also dependent on weather and physical location.

He said: "If you have a period of still, stable weather conditions, there's not much wind about to disperse the pollutants. When it's windy the pollutants are dispersed. The more open the location, the easier the dispersion. In St Aldates, with tall buildings, the pollutants are more likely to be trapped."

Last year there was a 23 per cent increase at one station in the city from 52 microgrammes per cubic metre of air in August up to 64 microgrammes in September. At some other stations it was up to 30 per cent and the council expects the figures to be similar this year.

Mr Payne said the city was struggling to meet air quality standards, as set by the European Union.

A feasibility study into a clean hub, commissioned by the city and county councils in April last year, is still being carried out - results are due later this year.

Mr Payne said: "We need to improve air quality and that is based on emissions. So the way to improve air quality is to cut emissions."

As part of LEZ plans for London, from 2008 diesel-engined lorries, coaches and buses would have to meet a minimum standard, with a range of penalties for those who ignore the rule.

A LEZ would be welcomed by campaign group Cyclox, which is worried about the health of people cycling around Oxford.

But member David Jacques also believes the council should suggest alternatives to clogging the roads with cars on the school run. He said ideas such as walking school-organised buses should be encouraged.

TOP POLLUTANT

NITROGEN dioxide is one of the most prominent air pollutants and can cause respiratory problems and irritation to the throat, nose and eyes.

People with asthma are particularly badly affected.

The main source of the gas is vehicle emissions - estimated to account for 80 per cent of measured levels.

Nitrogen dioxide levels, in Oxford, are monitored continuously at three city centre locations and at a further network of 60 central sites, including kerbside, intermediate and background locations.

Tests at those 60 locations have shown that Oxford has failed consistently to meet Air Quality Standards at about two thirds of the sites.

At several sites the annual average Nitrogen Dioxide levels have been more than twice the air quality standard of 40 microgrammes per cubic metre of air.

FUME SITES

NITROGEN dioxide levels in Oxford (2006 annual mean in microgrammes per cubic metre.)

Top Five Sites

Queen Street - 101;

High Street -100;

St Clements Street - 87;

Frideswide Square (Bus Stop) -87;

High Street (Covered Market) -83.

Other Parts of the City:

Abingdon Road (Weirs Lane) -52;

Green Road Roundabout: - 65;

Iffley Road (Boundary Brook Road) - 48;

Parks Road (Science Library) - 46;

Botley Road - 42.

Air Quality target set by the European Union: 40

Source: Oxford City Council; Airwatch Annual Report 2006