Guidance: Air emissions risk assessment for your environmental permit

Environment Agency

May 21
11:28 2024

Before you start this risk assessment

Read the following guides before you start this risk assessment:

  • the risk assessment overview this explains the other steps to take in risk assessment and whether you need to do an air emissions risk assessment
  • best available techniques (BAT) from the European Commission you may need to apply, or in some cases exceed, BAT depending on how harmful your emissions could be to the environment

How this risk assessment works

You need to compare the impact of your emissions to air to the following environment standards:

  • Air Quality Standards Regulations 2010 Limit Values and Target Values
  • UK Air Quality Strategy Objectives
  • Environmental Assessment Levels

Find the environmental standards.

Steps to complete this risk assessment

To complete an air emissions risk assessment you need to follow these steps.

  1. Calculate the environmental concentration of each substance you release into the air known as the process contribution (PC).

  2. Identify PCs with insignificant environmental impact so that they can be screened out this means that you do not have to assess them any further.

  3. For substances not screened out in step 2, calculate the predicted environmental concentration (PEC) for each substance you release to air the PEC is the PC plus the concentration of the substance already present in the environment.

  4. Identify emissions that have insignificant environmental impact these can be screened out.

  5. Get detailed modelling (also known as detailed assessment or computer modelling) done for the emissions you cannot screen out.

  6. For each substance youve released to air, compare the PC and PEC with the relevant environmental standard and summarise your results.

  7. Check if you need to take further action.

  8. Check if you need to do any other risk assessments.

The Environment Agency sometimes refers to the following stages of air emissions risk assessment:

  • stage 1 this is steps 1 and 2
  • stage 2 this is steps 3 and 4

Risk assessment tool

You should use the Environment Agencys risk assessment tool to complete your risk assessment. The only stage you cannot use it for is to screen out PCs or PECs of substances in protected conservation areas.

The figures the tool gives you are worst case estimates. So the figures you get may be higher than if you calculate PCs or PECs using other methods, for example dispersion modelling software (which analyses how air pollutants disperse in the atmosphere).

This guide explains the steps to complete if youre not using the risk assessment tool.

Calculate PC to air

You must calculate both your short term and long term PC to air for each substance. PC to air is measured in micrograms per cubic metre.

To calculate the PC to air, multiply the dispersion factor, in micrograms per cubic metre per gram per second, by the release rate, in grams per second.

If you do not have existing data

Use estimates if you do not have existing data (for example if your activity is new).

Where possible, use estimates based on similar operations elsewhere or from trials. Otherwise, use worst-case estimates.

State what assumptions youve made for these estimates.

Grouping air emissions

If you release volatile organic compounds into the air you should provide details of all emissions. If you cannot identify what all the substances in them are, treat the unknowns as 100% benzene in your risk assessment. If you want to treat them as something else, youll need to explain why.

Nitrogen oxides (also known as oxides of nitrogen)

Emissions of nitrogen oxides should be recorded as nitrogen dioxide in your risk assessment (as nitrogen oxide converts to nitrogen dioxide over time):

  • for short term PCs and PECs, assume only 50% of emissions of oxides of nitrogen convert to nitrogen dioxide in the environment
  • for long term PCs and PECs, assume 100% of emissions of oxides of nitrogen convert to nitrogen dioxide

When using the risk assessment tool, and entering your nitrogen oxides emissions as nitrogen dioxide, it will do the conversion for you.

When your site does not operate all the time

Adjust your figures down, based on the percentage of the year that your site is not operating. For example, a site that only operates January to June should reduce its PC figures by 50%. This only applies to long term annual mean assessment. It does not apply to any other long term or short term assessment averaging periods.

When using the risk assessment tool, you can enter the percentage into operating mode (%) and it will do the calculation for you.

PC: dispersion factor

The risk assessment tool calculates intermediate dispersion factors where the effective height is between given values.

If youre not using the tool, this table shows the dispersion factors you can use. These factors are based on the point at which the substance is effectively released into the air. This is known as the effective height of release.

You must use different dispersion factors if your site has landfill gas engines, landfill gas flares or capped areas.

All dispersion factors are shown in micrograms per cubic metre per gram per second.

Effective height of release in metres Long term dispersion factor Monthly dispersion factor Hourly dispersion factor
0 148 529 3,900
10 32 33.7 580
20 4.6 6.2 161
30 1.7 2.3 77
50 0.52 0.68 31
70 0.24 0.31 16
100 0.11 0.13 8.6
150 0.048 0.052 4
200 0.023 0.026

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