1.1 Background

The UK Government and devolved administrations are taking active measures to improve air quality through the Air Quality Strategy for England, Scotland, Wales and Northern Ireland (AQS) (DETR et al 1999a). This Strategy defines Air Quality Standards and Objectives for eight pollutants and identifies their major sources. The AQS gives the following objectives for nitrogen dioxide (NO₂) to be achieved by the end of 2005:

Annual mean: The annual mean must not exceed 40 m gm^-3.

Hourly mean: The hourly mean must not exceed 200 m gm^-3, not to be exceeded
more than 18 times a year.

In addition the European Union 'Daughter Directive' gives the following limit values for NO₂ to be achieved by 1 January 2010:

Annual mean limit value of 40 m gm^-3.

1-hour limit value of 200 m gm^-3, not to be exceeded more than 18 times a year.

The annual mean objective and limit value are likely to be the most stringent of the targets, particularly at the roadside.

The Government has recently published a consultation document on the Review of the Air Quality Strategy (DETR et al 1999b). This report sets out proposals for amending the Air Quality Strategy following a review carried out during 1998.

The national modelling of roadside NO₂ concentrations (Stedman et al, 1998), carried out as part of this review, indicated that policies currently in place or to take effect before 2005 will lead to the annual average objective being achieved at all background locations, except inner London, and at most roadside locations by 2005. However, the national modelling identified a number of major urban road links where concentrations at the roadside may exceed the objective.

One of the key uncertainties within the national modelling of NO₂ is the question of public exposure. The approach adopted in the Strategy is to apply the objectives where members of the public are likely to be exposed over the averaging time of the objective. In the case of annual mean, this refers to roadside locations where residential buildings schools or hospitals etc are close to the kerb. The national models used for the review of the Air Quality Strategy simply predicted NO₂ at the roadside and did not consider the location of housing. NO₂ concentrations also decrease significantly with distance from the roadside. If housing is located more than about 10 m from the edge of the road, then NO₂ levels there are likely to be lower than the roadside levels predicted by the model.

A pilot study to assess the number of households adjacent to road links with the highest predicted roadside NO₂ has been undertaken (King et al 1999). This considered 102 road links, including a case study of the North Circular Road in London, and involved visits to each road and the use of a postcode to determine numbers of residential addresses. The site visits found that 23 road links have houses within 10m of the kerb, with a total estimate of more than 1,200 houses. Using the postcode data, a total of over 2,600 houses were found to exist along these road links, but the data sets were not detailed enough to determine how many of these are within 10 m of the kerb. The current study aims to consider the location of the houses in more detail through the use of more detailed digital map data.

The objectives of this study were:

to develop an accurate Geographical Information System (GIS) based method for estimating numbers of households exposed to exceedances of the annual mean air quality objective for NO₂ at the roadside;


to quantify the number of households within 10m of kerb on these links;


to enable similar assessment of exposure to other pollutants using this method.

For this study, only those road links surveyed in the pilot study (King et al 1999) were considered.

Executive Summary         Chapter 2