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Air emissions risk assessments

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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 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.

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).

View the environmental benchmarks by substance document 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.

Permit application requirements

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
   • Note: Regardless of the activity, there is a minimum stack height requirement — no discharge stack should be less than 3 metres above either the roof ridge height of the building it is on, or the ground if it is freestanding.
3. Identification of all sensitive receptors, including people and protected ecological sites, located within the relevant screening distance from your site. Use the Screening Distance Table to determine the appropriate radius.
4. For combustion activities, use the Proximity Screening Tool (hosted on the APIS website) to assess ecological receptors based on fuel type, thermal capacity, and distance from the emission source. If no relevant ecological receptors are identified using this tool, the habitats impact assessment is considered complete and no further ecological assessment is required.
5. 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).
6. Submit your air emissions risk assessment as part of your permit application/variation.

Step-by-step assessment method

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

1. Initial screening
   • 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).
   • These tools will identify PCs with insignificant environmental impact so that they can be ‘screened out’ meaning that no further assessment is needed.
2. Assess remaining substances
   • For substances not screened out, calculate the predicted environmental concentration (PEC) for each substance released to air.
   • The PEC is the sum of the PC and the background concentration of the substance already present in the environment at the receptor. More guidance on background concentration is below.
3. Determine significance
   • Identify any substances that still have insignificant impact, these can also be screened out at this stage.
4. Detailed modelling
   • For substances that cannot be screened out, carry out detailed dispersion modelling to assess their environmental impact more accurately.
5. Reporting and mitigation
   • Compare the PC and PEC for each substance against the relevant environmental standards (e.g. EALs, critical levels).
   • If any substance shows an unacceptable impact, consider mitigation measures and consult SEPA for guidance.

Make sure to summarise all findings clearly, including which substances were screened out and why, modelling results for substances that were not screened out, and any proposed mitigation.

Special case: Energy from waste applications

For energy from waste facilities, detailed dispersion modelling is required for all parameters. Therefore, Steps 1–4 (screening and PEC calculations) are not applicable in these cases.

Background concentration

To calculate PECs, you’ll need reliable background concentration data. You can obtain this from:

• Your local council
• Government background concentration maps, or
• The Air Pollution Information System (APIS) especially for SPAs, SACs, and SSSIs.

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

Important considerations

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.

Submission and compliance

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Step-by-step process

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

1. Initial screening: Using screening tools to calculate the concentration substances released into air to identify potential environmental impact
2. Assess remaining substances: Calculate the predicted environmental concentration for each substance released to air.
3. Determine significance: Identify any substances that can be screened out at this stage.
4. Detailed modelling: Carry out detailed modelling to assess their environmental impact more accurately.
5. Reporting and mitigation: Compare results against the relevant environmental standards and consider mitigation measures for substances with an unacceptable impact