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Background air quality is the level of contaminant across the airshed (a geographic area for air quality management) from all sources. This includes contributions from natural sources (for example volcanoes, forest fires and wind-blown dust) and from human-generated sources (such as industry, domestic heating and ‘remote’ roads).

Background air quality is used to assess the cumulative impact of a discharge to air. In the case of a roading project, the air pollution coming from the road being improved (the relative impact) needs to be added to the background concentration of air pollution (that is, background air quality excluding nearby roads) to allow the cumulative (or absolute) impact of the project to be calculated.

We have developed default background air quality values by census area unit. This information can be used as an input into an air pollution dispersion model, such as our Air Quality Screening Model to estimate the cumulative impact.

Air Quality Screening Model 

These default values are provided in the spreadsheets below:

Supporting information on the background methodology is available upon request from

In order to reduce uncertainty and comprehensively assess the air quality effects of a roading project, the default background air quality values may need to be supplemented with more site-specific information. Suitable data may be available from regional councils, Waka Kotahi or research organisations. However, projects with a high air quality risk, typically large projects in major urban centres, may require a dedicated monitoring campaign to determine background air quality.