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Air Emissions Risk Assessment for Environmental Permits

Good air quality is essential for a good quality of life, helping to maintain human health and wellbeing, the climate, habitats and achieve sustainable economic development. This page/document provides guidance on the air emissions risk assessment required for a Pollution Prevention Control (PPC) Part A or Part B permit application or to vary an existing permit if air emissions are affected by the change.

Permit requirements for emissions to air

In your permit application/variation for a site in Scotland you need to include the following technical information:

1. A description of the activities or proposed changes to your existing activities.
2. Identify releases to air, including the nature, quantity, source and location of pollutants*.
3. Identify all sensitive receptors (people and protected sites) within the relevant screening distance from your site. Check the Screening Distance Table. For combustion activities, use the Proximity Screening tool**.
4. Assess whether there is a likely significant effect or whether it can be screened out as insignificant (following the steps and using the screening tools detailed below).
5. Submit your air emissions risk assessment as part of your permit application/variation.

* Irrespective of the activity there is a minimum stack height requirement which must be adhered to: no discharge stack should be less than 3 metres above either the roof ridge height of the building on which it is located, or the ground if it is separate from any building.

**Use the Proximity Screening Tool (hosted on the APIS website) to screen for relevant ecological receptors, based on fuel type, thermal capacity and distance between receptor and the emission source; if no relevant ecological receptors are found by this tool, then the habitats impact assessment is concluded and no further assessment of impact to ecological receptors is required.

How the risk assessment works

You need to compare the impact of your emissions to air to the following environmental benchmarks, and where relevant odour thresholds:

  • Air Quality Standards (Scotland) Regulations (which give effect to EU Limit/Target Values).
  • UK Air Quality Strategy Objectives for Scotland.
  • Critical loads and critical levels for ecological receptors.
  • Environmental Assessment Levels (EALs).

See environmental benchmarks by substance for more information. The information on this table is subject to amendments as new benchmarks are identified or additional benchmarks are added. Further information on critical loads and critical levels is available: Critical Loads and Critical Levels - a guide to the data provided in APIS | Air Pollution Information System.

Steps to carry out a risk assessment

To complete an air emissions risk assessment, the following steps need to be followed:

1. Use screening tools such as H1 software tool or SCAIL to calculate the concentration of each substance released into air from the proposed activity – known as the process contribution (PC).
2. The tools will identify PCs with insignificant environmental impact so that they can be ‘screened out’ – this means that no further assessment is needed.
3. For substances not screened out, calculate the predicted environmental concentration (PEC) for each substance released to air – the PEC is the PC plus the concentration of the substance already present in the environment at the receptor (background concentration*).
4. Identify substances that have insignificant environmental impact – these can be screened out.
5. Carry out detailed dispersion modelling for the substances that cannot be screened out.
6. For each substance released to air, compare the PC and PEC with the relevant environmental standard and summarise the results.
7. Mitigation must be considered if there is an unacceptable impact, and you should take advice from SEPA.
Steps 1-4 are not required for energy from waste applications as detailed dispersion modelling (Step 5-7) is required for all parameters. More information on energy from waste is available.

*You can find out about background concentrations from:

This information will usually be shown as a long term (annual) average concentration.

Background concentrations may already include PCs from your site. To avoid your PCs being double-counted, use a background concentration from a source that is not affected by the direction that the wind predominantly blows from (that is the prevailing wind direction). For example, if the prevailing wind comes from the west, do not use a background concentration from a source to your east.

When you calculate background concentration, you can assume that the short term background concentration of a substance is twice its long term concentration.

H1 software tool

You should use SEPA’s H1 risk assessment tool to complete steps 1 –4 of the risk assessment, except:

  • Permits that only regulate Medium Combustion Plant (MCP) between 1-20 MW or permits that regulate Intensive Agriculture. They require the use of the SCAIL tool: and
  • most PPC part B applications depending on their nature and location. This must be confirmed by discussing with SEPA prior to making an application.

A copy of the H1 software tool and user guide.

The tool:

  • Carries out the calculations required for the risk assessment of emissions to air and screens out insignificant emissions.
  • Presents evidence that can be easily reviewed and assessed to help determine your application.
  • Quantifies impacts of emissions deposited from air to land.

You do not need to take further action for any air emissions screened out as insignificant in relation to assessing impact on human health.

Screening criteria:

  • For the tool to screen out a process contribution (PC) for any substance, the PC must meet both of the following criteria:
    • the short-term PC is less than 10% of the short-term environmental standard for the substance,
    • the long-term PC is less than 1% of the long-term environmental standard for the substance.
  • If either of the above criteria are not met, then the tool carries out a second stage of screening to determine the impact of the predicted environmental concentration (PEC). For the tool to screen out a PEC for any substance, the PEC must meet both of the following criteria:
    • the short-term PC is less than 20% of the short-term environmental standard minus twice the long-term background concentration for the substance,
    • the long-term PEC is less than 70% of the long-term environmental standard for the substance.

The role of the screening tools is to filter out the low-risk pollutants. Therefore, screening tools are set up to be precautionary i.e. their results are conservative estimates and the figures obtained may be higher than the PCs or PECs calculated using ADMS or AERMOD software.

The risk assessment report needs to include the following information:

  • The height and effective height of the release (chimney, vent height).
    • The effective height of release could be considered as zero where the point of discharge:
      • is less than 3m above the ground or building on which it is located, or;
      • is greater than 3m above the ground or building on which it is located but less than the height of any building within a distance 5L from the point of discharge (where L is the lesser of the building height and the maximum projected width between two points the same height in the building).
    • When the effective height of release is more than 3 metres above the ground or building on which it is located, but less than 2.5 times the height of the tallest adjacent building, estimate it by following these steps:
      • Take the actual height of release.
      • Subtract the height of the tallest building within a distance of 5L (this can be the building where the emissions are coming from, if it’s the tallest).
      • Multiply the figure that’s left by 1.66.
  • When both scenarios above are not relevant, the effective height of release should be the same as the height of release.
  • The efflux velocity and total flow of the release.
  • A list of screened and screened out substances.
  • Each assessed substance’s long and short-term PC and Environmental Quotient (EQ). When calculating release rates, the concentration of the Emission Limit Value (ELV) should be combined with the worst-case (maximum) volumetric release rate as this will provide the maximum potential emission that can be emitted under the permit.
  • The percentage of each operating mode for the year.

SCAIL screening tools (Combustion and Agriculture)

The Simple Calculation of Atmospheric Impact Limits (SCAIL) screening tools are designed to assess the impact of emissions on sensitive habitats within a site with a statutory nature conservation designation (e.g. Sites of Special Scientific Interest (SSSIs)) and human health receptors.

A SCAIL screening tool should be used to inform whether or not to move to step 5 (‘ detailed dispersion modelling’) of the risk assessment process. It applies to all PPC part A and B applications/substantial variations, including Medium Combustion Plants (MCP).

If a likely significant effect to a site with a conservation designation (made or proposed) under the Conservation Regulations 1994 – known as European sites (e.g. Special Area of Conservation – SAC) – or a national SSSI cannot be ruled out, then this potential impact needs detailed assessment.

There are 2 versions of the SCAIL screening tool:

  • SCAIL-Agriculture: This version focuses on emissions from agricultural sources (e.g. poultry sheds; pig housing).
  • SCAIL-Combustion: This version estimates the impacts from small and medium combustion sources on sensitive habitats sites.

More information on SCAIL can be found at: SCAIL - Simple Calculation of Atmospheric Impact Limits ( Or email

The SCAIL screening tools link to the Air Pollution Information System (APIS) database in order to access background air pollution data and site relevant critical loads and levels. In addition, the APIS website provides further information on pollutants and their impacts on habitats and species: Air Pollution Information System.

Detailed Dispersion Modelling

In most cases, you will be required to provide the results of detailed dispersion modelling for any releases to air at receptors not screened out as insignificant. To do detailed modelling, you will need to use an air quality modelling package that models the transport of a substance as it travels through the atmosphere and predicts the process contribution at relevant receptors.

Detailed modelling requires specialist knowledge and if this is something you cannot do in-house you can find a consultant to do it for you (however, they will charge for the work). We strongly recommend any methodologies for carrying out detailed modelling are agreed with SEPA prior to any work being carried out.

Special Treatments

Note: SEPA doesn't accept the use of the plume depletion modules within the model alone; any assessment considering deposition impacts should present a scenario which doesn't use the plume depletion modules as a minimum). The deposition must be calculated from the process contribution concentration at the receptor, using the conversions in "Technical guidance on detailed modelling approach for an appropriate assessment for emissions to air" AQTAG06, 2014.

Method Statement

SEPA strongly recommends that an air quality modelling method statement is submitted to us in advance of any detailed modelling work being carried out. This has the advantage of allowing agreement of the methods and input parameters in advance and reduces the likelihood of further work, saving time and resources.

SEPA does not prescribe any particular air quality model, but it should at least be:

  • Appropriate for the intended purpose;
  • Based on established scientific principles;
  • Be fully validated and independently reviewed;
  • Have a full technical specification with associated validation and review documents made available.

There are currently two leading models used for regulatory purposes in the UK – the Air Dispersion Modelling System (ADMS) , developed by Cambridge Environmental Research Consultants (CERC), and AERMOD, developed by the United States Environmental Protection Agency (USEPA).

A method statement should include:

  • Choice of model to be used.
  • Pollutants of interest and air quality benchmarks that model results will assess against.
  • Background concentrations to be used.
  • Emission parameters, to include:
    • Location of release point(s) (Grid reference).
    • Height of release point(s) (Metres).
    • Diameter of release point(s) (Metres).
    • Exit temperature (Kelvin, degrees Celsius), % oxygen, % moisture and pressure (Kpa).
    • Efflux velocity or flow rate (actual) (Metres per second or Cubic metres per hour).
    • Emission concentrations (Milligrams per cubic metre).
    • Calculated emission rate (grams per second).
  • Meteorology to be used (including years to be modelled, percentage of calm periods in data and where it has been sourced from).
  • Buildings to be included in model.
  • Terrain to be included in model.
  • Grid domain, resolution and locations of sensitive receptors.
  • Scenarios to be modelled.
  • Model output formats to be presented.
  • Any other special treatments which may be required to be assessed.
  • Stack height assessment (if relevant) including the range of heights and parameters assessed.

If a method statement is not agreed with SEPA in advance, this may delay determination of the application and ultimately mean that the modelling report is rejected. 

Stack Height Assessment

Whenever detailed modelling is required a stack height assessment should always be carried out. Justify that the chosen stack height represents Best Available Techniques (BAT) as part of these assessments.

A stack height assessment can determine an appropriate stack height (where relevant). This should include all averaging periods (long and short-term) relevant to the pollutants which are being assessed and a graph showing the impact of different stack heights. The range of stack heights used should be agreed with SEPA prior to making an application.

Modelling Report

The report must explain:

  • The purpose of the study.
  • The site and proposed operation.
  • The modelled scenarios and how they represent the proposed operations at the site.

Please include all the information identified below, in the detailed modelling report. If some information is not included, please provide a justification. There needs to be enough information in the report to make an exact copy of the model.

Include a location map and site layout plan. These must:

  • Show the location of the proposed site and surrounding land-use.
  • Show the size of the modelled area domain.
  • Use National Grid referencing and indicates terrain contours, such as the Ordnance Survey Landranger series (1:50,000).

List Emissions and Environmental Benchmarks for Air

A list of the emissions modelled, including their chemical specification must be provided. For example, oxides of nitrogen and halogenated compounds. The report must also identify the relevant environmental benchmark for air emissions for those substances modelled.

Ambient and Background Concentration Levels

The report must include:

  • How a representative value for the background concentration has been worked out.
  • An explanation of how you’re the background concentrations are representative of the local environment.
  • How ambient background levels are applied to different averaging periods (e.g. long and short-term).

This information can be obtained from:

  • Local authority ambient air quality monitoring data.
  • Background concentration maps produced by Defra/Scottish Government (found on Scottish Air Quality website). For ecological receptors use the background values obtained from the APIS website (Air Pollution Information System | Air Pollution Information System (
  • Data from the Scottish Air Quality Database monitoring networks.
  • If a different source for this information has been used, please explain why.

Model Software

The report must include the:

  • Type of dispersion modelling software used.
  • Software name, including version number.
  • Type of model, for example Gaussian or new-generation.
  • Supplier and details of who carried out the work

The model used must be fit for the intended purpose, based on established science, and be validated and independently reviewed.

Explain Emission Parameters

The report must include a table to include the following information, including relevant units:

Parameter Units
Release point(s) location Grid reference
Release point(s) height Metres
Pollutant emission rate* Grammes per second
Exit diameter Metres
Exit temperature, % oxygen, moisture, and pressure Kelvin, degrees celsius, % oxygen, % moisture and Kpa

* To calculate this, you would require the concentration in mg per cubic metres, stack, and gas velocity (usually in m/s) and the stack area at exit. Guidance on how to calculate pollutant emission rates is here: Monitoring stack emissions: guidance for selecting a monitoring approach. Go to the “Calculating concentration and mass emissions” section. In most cases, the Emission Limit Values (ELVs) should be used as this is the maximum potential concentration that can be emitted under the permit.

If relevant to the application, information about all of the following parameters must be provided.

Parameter Units
Efflux velocity Metres per second
Volumetric flow rate (actual) Cubic metres per second
Volumetric flow rate (at reference conditions) Normal cubic metres per second

This information must be provided for each stack which has been part of the modelling process. An explanation of how you have worked out the emission rates used in the model must also be provided.

These rates can be calculated using release point exit concentration values from the appropriate ELVs or BAT conclusion document or by using a measured concentration value where justified. Where measured concentrations are used, raw monitoring data and details of the monitoring methodology needs to be included.

You must also include other relevant data to calculate emission rates, including:

  • Gas release temperature.
  • Actual oxygen and moisture levels used to adjust volumetric flows.
  • Emissions and source terms that vary with time, if appropriate.

An explanation of how each emission source has been defined is required. For example, point source, area, line, volume or other. Include all relevant co-ordinates and their emission parameters in a table.

The emission locations should be included on a site layout plan.

Explain the Modelled Domain and Receptors

The resolution of the model receptor grid, the model domain size (including co-ordinates) and receptor height (above ground level) needs to be justified. Details of any discrete or sensitive receptors (including grid references) and heights above ground level which are used to assess impact at sensitive locations must be included.

The resolution of the model grid and the location of discrete ecological receptor points must be sufficient to adequately capture the variability in pollutant process contribution in relation to the spatial variation in habitat across the extent of the site (as shown in the habitat mapping).

Explain Meteorological Data and Surface Characteristics

The choice of meteorological data and why it is representative of the dispersion site must be explained along with the surface characteristics that have been chosen to represent land use. For example, surface roughness, Monin-Obhukov length, albedo, Bowen ratio or Priestley-Taylor parameter.

The report must include:

  • The location of the chosen Met Office weather station in relation to the modelled domain.
  • The number of years covered by the data – a minimum of 5 years is recommended. The source of the data, for example, the UK Met Office, or other data supplier.
  • A description of the data quality and uncertainties relating to any alternative meteorological data (e.g. on-site data).
  • The format of the meteorological data – hourly sequential (preferred) or long-term statistical.
  • A wind rose showing the distribution of wind speed and direction for each year used in the model.

Explain Terrain and Building Treatments

The report must justify and explain whether or not terrain and building treatments are included in the assessment.

The report should:

  • Justify why building treatments are included in the assessment – if not, explain why.
  • Justify why terrain is included in the assessment – if not, explain why.
  • Explain the source, format and processing of digital terrain data used in the model.
  • Show the location and dimensions of all buildings included in the model, including National Grid reference, height, width and rotation.
  • Show the location and relative orientation of buildings and their dimensions on a site plan.

Explain Model Uncertainty

The estimated level of uncertainty in your predictions must be shown. Validation documents and reports have examples of the differences between measured values and those estimated by models. Where the validation documents indicate that levels of uncertainty may be large and would affect the conclusions, then this should be considered in the overall risk assessment.

On some occasions, it may be appropriate to consider running the data through another validated model to check the differences between models and compare conclusions.

Carry Out Sensitivity Analysis

The sensitivity of model results to model inputs must be explored through sensitivity analysis. For example, by varying the following input parameters:

  • Meteorological data, such as different Met Office weather stations, data sources, inter-annual variation and surface characteristics.
  • Emission parameters, such as stack parameters, pollutant release rates and different plant operating scenarios.
  • Treatment of terrain and buildings.
  • The receptor grid resolution.

Special Treatments

Describe the input parameters and any assumptions that have been made on specialised model treatments, such as:

  • Coastal models.
  • Fluctuations.
  • Photochemistry.
  • Wet or dry deposition*
  • Flare releases.
  • Short-term (puff) releases.
  • Effects of wind turbines.
  • Post-processing or short-term statistical analysis – to calculate the likelihood of intermittent emissions coinciding with meteorological conditions that could increase pollution.

*Note: SEPA does not accept the use of the plume depletion modules within the model alone; any assessment considering deposition impacts should present a scenario which does not use the plume depletion modules as a minimum). The deposition must be calculated from the process contribution concentration at the receptor, using the conversions in “Technical guidance on detailed modelling approach for an appropriate assessment for emissions to air”, AQTAG06, 2014.

Carry Out An Impact Assessment

The risk assessment must present comparison of the modelled process contributions, and where relevant predicted environmental concentrations, against the environmental benchmarks. For human health impact assessments, it may also be useful to include additional percentiles in addition to the air quality benchmarks to help assess risk Assumptions you have made relating to pollutant conversion processes (such as nitric oxide and nitrogen dioxide photochemistry) for different averaging times need to be justified. For combustion processes where no more than 10% of oxides of nitrogen are emitted as nitrogen dioxide, worst case conversion ratios to nitrogen dioxide of can be assumed:

  • 35% for short-term average concentrations.
  • 70% for long-term average concentrations.

Breaches of environmental benchmarks for air: Comments on any potential breaches of environmental benchmarks and objectives for air emissions must be included in the report. This must take into account model uncertainty and provide an assessment of different:

  • Stack heights.
  • Emission characteristics.
  • Process operation scenarios.
  • Other special features in the model (e.g. terrain).

Contour plots: Contour plots must be provided for each air quality standard and objective you assess. These must:

  • Overlay suitable OS maps showing release point(s)/site boundary and relevant receptors.
  • Include the pollutant name and modelling scenario.
  • Provide the averaging time, and for human health the appropriate percentile plotted, clearly indicating areas of exceedance.
  • Use the same colour scale for all contour plots relating to a particular air quality standard or objective.
  • For ecological receptors, where data availability allows, contour plots of modelling results should overlay habitat mapping. Spatial habitat data is available to download from the NatureScot Data Hub National Vegetation Classification | NatureScot Spatial Data Hub.

Present your results: Results must be shown for:

  • The locations of maximum air quality impacts.
  • Discrete sensitive receptor locations.
  • Cumulative impacts.

you must present them in a table showing:

  • The process contribution (PC).
  • The predicted environmental concentration (PEC) – which is the PC plus appropriate ambient background values.
  • Appropriate ambient background values.
  • PCs and PECs as a percentage of the relevant environmental standard for air.

The report must include a discussion of results (what they mean and their significance) before making final conclusions.

It must explain how you have judged the risk and base this on the site-specific circumstances and model scenarios/inputs.

Include Input Files and Input Parameters

Model input files for the air dispersion model that has been used must be provided in addition to the report.

A separate annex must be provided with a table of all the input parameters used within the model that supports any further assessments such as:

Enough data must be provided in the report so that the model configuration and parameters can be audited, such as emission sources, meteorological inputs and any special treatments used.

Detailed Human Health Risk Assessments

Certain activities regulated under PPC and located or to be located near residential areas will require a more detailed Human Health Risk Assessment (HHRA). This assesses pollutant releases to the atmosphere using appropriate software such as IRAP-H VIEW.

The pollutants, also called Compounds Of Potential Concern (COPC) in IRAP-H VIEW should be identified and included in the air emissions impact assessment. H1 and SCAIL tools mainly consider exposure via inhalation for a limited number of substances. The IRAP-H VIEW software considers all pathways for assessing a more comprehensive list of substances. This includes the inhalation and oral exposure pathways for dioxins/furans and PCBs.

Contact Us

If you require any further help or advice, please contact:


ADMS: Air Dispersion Modelling System
AERMOD: AMS/EPA Regulatory Model
APIS: Air Pollution Information System
BAT: Best Available Techniques
CERC: Cambridge Environmental Research Consultants
COPC: Compounds of Potential Concern
EAL: Environmental Assessment Levels
ELV: Emission Limit Value
HHRA: Human Health Risk Assessment
MCP: Medium Combustion Plant
PC: Process Contribution
PEC: Predicted Environmental Contribution
PPC: Pollution Prevention and Control
SAC: Special Area of Conservation
SCAIL: Simple Calculation of Atmospheric Impact Limits
SSSI: Sites of Special Scientific Interest
USEPA: United States Environmental Protection Agency

Published May 2024.