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Air quality assessment kit for small and large schemes released by Defra

18 March 2020
Figures from transport emissions
Figures from transport emissions


Revised guidance from Government details the process for assessing the air quality impact of a policy or project. The mortality burden of the air pollution mixture (based on both particulate matter (PM2.5) and nitrogen dioxide (NO2) in the UK, is equivalent to 28,000 to 36,000 deaths at typical ages. 'You must factor the effects on air quality into policy making, where relevant,' says the advice.

This guide sets out the various methodologies for appraising air quality.

The impact pathways approach (IPA) is the best practice approach to valuing changes in air quality. This approach uses atmospheric modelling to estimate the impact of changes in the ambient concentrations of air pollutants for a range of outcomes.

Full IPA can be resource and time intensive. It needs you to estimate emissions, dispersion, population exposure and outcomes.

The Department for Environment, Food and Rural Affairs (Defra) have developed ‘damage costs’ to enable proportionate analysis when assessing relatively small impacts on air quality.

Damage costs are a set of impact values, measured per tonne of emission by pollutant, which are derived using the more detailed IPA. These values estimate the societal costs associated with small changes in pollutant emissions. Combine them with emission change forecasts to provide an approximate valuation of the aggregate impacts of a policy.

This guidance outlines how you can work out the effects of air pollution within economic appraisals. The quality of the air can have an impact on:

  • human health

  • productivity

  • wellbeing

  • the environment

Use the toolkit to calculate the net present value of the change in air pollution for your scheme. If the impacts are less than £50 million in total, then use the damage costs approach to:

  • value the changes in air pollution in a ‘light touch’ way

  • understand the methodology of the toolkit

  • ‘Damage costs’ are the health impact on society of a change in emissions of different pollutants.

Read the damage costs approach before including air pollution analysis within your appraisal.

If the impact is more than £50 million, or the main objective of the policy or project is changes in air quality, use the impact pathways approach. The impact pathways will value the impacts of air pollution where they are ‘localised’ - that is focused in one specific area, for example, the impact of emissions around a specific power station.

There are various methodologies for air quality appraisal. As well as the impact pathways approach (IPA), changes in air quality may be appraised using:

  • damage costs (a set of monetary impact values per tonne of emission)

  • activity costs (monetary value per KWh energy used)

  • abatement costs (the cost of any emission mitigation measures), if a policy is expected to push emission concentrations above legal limits

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