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This guidance provides an overview of the impact pathway methodology, which is the central methodology for valuing changes in air quality.
It values the air quality impacts of proposed decisions by estimating how changes in the ambient concentrations of air pollutants affect a range of health and environmental outcomes.
This guidance sets out best practice appraisal methodologies for all projects, programmes and policies. The impact pathway approach has been developed by Defra with support from the Interdepartmental Group on Costs and Benefits (IGCB), a Defra-led group of government analysts that provides advice relating to the quantification and valuation of local environmental impacts.
September 2015: Since this guidance was published the impact pathway approach has been updated to reflect the health impact of emissions of NOx and concentrations of NO₂. These changes are explained in the interim guidance