Air pollution in Swaffham has breached national standards for a second year running, but proposed new air quality policies could be too late to affect plans for hundreds of new homes and a superstore.

The government will force Breckland Council to declare an air quality management area (AQMA) – the first in the district – after nitrogen dioxide levels in the town centre exceeded its objective in 2010 and 2011.

Sensors recorded an average of 40.4mg per cubic metre last year, compared to a national target of 40mg, with the problem at its worst where cars queue on the A1065 from traffic lights near the Market Place towards Sporle Road.

Breckland councillors at yesterday's Overview and Scrutiny Commission recommended the council adopt new air quality guidance drawn up by the Norfolk Environmental Pollution Group, which also being considered by other councils in the county.

However, it might not come into effect before a decision is made on Abel Homes' plans for 250 new homes in the south of the town, submitted last week, and proposals for a Tesco store in the north. An application by developers Ben Bailey to build 335 homes beside the Abel site was approved in March.

Swaffham councillor Frank Sharpe said: 'We are going to potentially have 600 new houses just down the road and with the increase in traffic, if we have a problem at the moment that we appear to be unable to reduce, how on earth are we going to cope with the increase in traffic?'

Phil Cowen warned the pollution problem could spread to other market towns with new housing developments in the pipeline, with an application for 375 homes in Attleborough due to be debated on Monday.

Zandra Waterford, scientific officer for environmental protection, said the council would have to ask developers to make financial contributions to help investigate air quality, and encourage them to take air quality guidance into account early when drawing up their plans.

The council has already appointed consultants to propose the borders of the AQMA in Swaffham, and is considering options such as personal travel plans, rephasing the traffic lights and a park and ride scheme to reduce pollution.