Welcome to the Decision-Making Threshold for Air Pollution homepage. As progress is made, resources resulting from the project will be available from this page and provided on the Resource Hub.
Project aims
Decision-Making Thresholds (DMT) inform the assessment of air quality impacts on designated conservation sites to 2030.They are an evidence-based approach for determining when a proposal and the emissions it gives rise to are considered “nugatory” in the context of ecological risk assessment.
DMTs have a range of uses, from decision-making about new or existing proposals to broader policy development. They aim to focus effort to address air pollution effects on ecosystems where it will have most impact by filtering out lower risk projects.
Why is this important?
Current screening thresholds are typically based on professional judgment supported by the best available evidence.
Case law has outlined that whilst necessary, professional judgment alone is insufficient to provide certainty that the proposals being screened out should not require further assessment. This leaves decisions open to challenge, generates extra cost for case specific analyses and can cause significant delays.
A strong evidence base will supplement this professional judgment and address some of the challenge to current approaches. Better evidence also helps practitioners strike a balance between operational applicability and case law compliance.
Decision-Making Thresholds 2030
DMTs 2030 were established in 2021 following completion of the DMT project. This project was led by DTA Ecology in a consortium with Air Quality Consultants (AQC), Ecological Planning & Research Ltd (EPR) and UK Centre for Ecology and Hydrology (UKCEH). This report produced a Main Report and Technical Report, containing criteria underlying the DMT’s evidence basis and guidance on how to use the DMTs. These are available on the Resource Hub.
Use of the DMTs has been mixed amongst the UK Country Nature Conservation Bodies (CNCBs) and regulators, with uptake in Northern Ireland, and within Natural England’s standing advice (from October 2025). Other agencies are considering their position or have decided DMTs are not going to form part of their processes.
Decision-Making Thresholds 2050
JNCC is leading a programme to extend the DMTs to 2050. This reflects on changes to policies, data and tools that have become available since the establishment of the DMTs 2030. This programme also explored the varied uptake across the UK CNCBs and gathering user feedback.
The scoping phase of the DMT 2050 programme (2025 to 2026) was comprised of three projects delivered by Ricardo, UKCEH and AQC, which ran until March 2026.
1: Assessing Barriers and Exploring Baseline for the Decision-Making Thresholds 2050 (Ricardo-AEA Ltd) will:
- Raise awareness and aim to increase adoption and acceptance of the DMT 2030 by the four UK CNCBs, regulators and planning authorities through stakeholder workshops, webinars and development of guidance materials.
- Establish the 2050 baseline modelling scenarios and development pressure categories to derive a 2050 DMT and Site Relevant Thresholds (SRT).
2: Review of available data and gaps, including modelling, on nitrogen pollution in the four UK countries, to support the development of the Decision-Making Thresholds (DMT) to 2050 (UKCEH) will:
- Assess the currently available policies affecting UK emissions and baseline modelling that have been undertaken to 2050 for nitrogen deposition, ammonia, nitrogen oxides, sulphur dioxide and other pollutants relevant to effects on ecosystems.
- Review available data and gaps, with recommendations on what baseline modelling should be completed for the DMT 2050 and indicate limitations or strengths/weaknesses and risks for policy and modelling usage.
3. Development of the Decision-Making Thresholds (DMT) 2050 Methodology (AQC) will:
- Review and refine the methodology underpinning the DMT, Objective Compliant Change (OCC), Site Relevant Threshold (SRT) and road Relevant Threshold (RRT) to 2050.
- Highlight opportunities with new data or modelling developments, gaps or risks to inform the updated DMT 2050 methodology.
- Provide advice on risks and opportunities with the proposed baseline that DMT 2050 is likely to be based on.
We are continuing to work with the Inter-Agency Air Pollution Group to support the implementation of the DMT 2030 across the UK, and to develop the evidence underpinning the DMT 2050.
We are now delivering phase 2 of extending the DMT beyond 2030, which incorporates the findings and recommendations developed during phase 1.
How can I get involved?
The DMT 2050 programme will include stakeholder engagement and expert consultation.
If you would also like to join our stakeholder list and hear about working groups and pollution related events, please contact us.
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