Worked example: air quality damage cost calculation

The air quality damage cost calculation is how you turn a scheme's extra emissions into a pound figure that a council can ask for as a mitigation contribution. It multiplies the annual tonnes of each pollutant the development adds by a published damage cost per tonne, then totals that over a fixed number of years, conventionally five. It is a screening and valuation tool, not a dispersion model, and it answers a different question from a concentration assessment at a receptor. The worked figures below are illustrative only.

What the damage cost approach is used for

Councils use it in two situations. The first is where a scheme is not large enough to warrant detailed modelling but still adds traffic or combustion plant in or near an Air Quality Management Area, and the authority wants the emissions offset rather than assessed. The second is where modelling shows the concentration impact is not material but the local plan or an air quality supplementary planning document still requires emissions mitigation.

The approach comes from the Environmental Protection UK and Institute of Air Quality Management guidance on land-use planning and development control, which recommends valuing the development's emissions and spending an equivalent sum on measures that cut them. Defra publishes the damage cost values per tonne, differentiated by pollutant and by source sector, in its air quality appraisal material, and it revises them, so take the figures from the current Defra tables rather than from an older report.

What the method does not do is prove compliance with the objectives in the Air Quality Standards Regulations. If a receptor is at risk of exceeding an objective, you need concentrations, not a cheque.

Inputs the calculation needs

Six inputs, and five of them come from documents you are producing anyway.

The trip figure has to be net. Subtract the existing use's trips, and state whether you are using arrivals plus departures or a one-way count, because getting that wrong doubles the answer. The Transport Assessment or statement is the governing document and the air quality work should quote it rather than generate its own numbers.

Trip length is the input people argue about. A town-centre flatted scheme with a station 300m away does not generate the same vehicle kilometres as an edge-of-settlement estate, and if you use a national average for both you will overstate one and understate the other.

A worked estimate for a residential scheme

This example is illustrative. It uses assumed inputs to show the arithmetic, and none of the figures should be carried into a real submission.

Take a 100-dwelling scheme on a greenfield edge-of-town site, replacing agricultural land with no existing vehicle trips.

1. Assume the transport work gives 500 net new two-way vehicle trips per day.
2. Assume an average trip length of 10km, giving 5,000 vehicle-km per day.
3. Annualise: 5,000 x 365 = 1,825,000 vehicle-km per year.
4. Assume a fleet-average NOx emission factor of 0.25 g/km for the opening year, so annual NOx is 1,825,000 x 0.25 = 456,250 g, or 0.456 tonnes.
5. Assume, purely to carry the arithmetic through, a damage cost of £10,000 per tonne of NOx. That gives £4,560 per year.
6. Over a five-year appraisal period, £22,800.

Particulate matter is added the same way, using the PM2.5 and PM10 factors and their own damage costs, and the two totals are summed. Building emissions follow a parallel calculation: annual gas demand in kWh multiplied by the boiler NOx rating in mg/kWh gives mass of NOx, converted to tonnes and valued the same way. Specifying boilers at or below 40 mg/kWh, the standard the London Plan sets for new gas plant, cuts that element sharply, and an all-electric scheme removes it entirely. The Energy Statement is where the demand figure should come from.

Note that the arithmetic is sensitive to the trip length and the emission factor in a way it is not sensitive to the damage cost. Halve the assumed trip length and the contribution halves.

Applying it to a mitigation contribution

The output is a sum of money, and the planning system can only ask for it in a form that meets the tests in regulation 122 of the Community Infrastructure Levy Regulations 2010: necessary to make the development acceptable, directly related to the development, and fairly and reasonably related in scale and kind.

That matters for how you offer it. A contribution to a named measure, such as electric vehicle charge points beyond the Building Regulations minimum, a travel plan fund, cycle parking or a contribution towards the council's air quality action plan measures, is far easier to defend than an unallocated payment into a general fund. Where the measure can be built into the scheme, doing so and claiming the credit against the calculation is usually better value than paying the money.

Some authorities allow on-site measures to be netted off the emissions before the valuation. Check the local guidance, because the order of operations changes the figure.

Limits of the approach and where modelling is needed

The damage cost method averages. It attaches the same value to a tonne of NOx emitted on a bypass as to a tonne emitted in a street canyon with residential frontages, and that is precisely where it fails.

Dispersion modelling is needed where the scheme introduces or intensifies traffic on a road with sensitive receptors close to the kerb, where an exceedance of the annual mean nitrogen dioxide objective is plausible, where a biomass boiler or combined heat and power unit has a stack whose plume needs assessing, or where the authority's own guidance sets a modelling threshold. An Air Quality Assessment that uses the damage cost approach should say in terms why modelling was not required, with reference to the scale of the change and the distance to receptors.

Construction dust is a separate exercise again, assessed qualitatively under the Institute of Air Quality Management construction dust guidance and mitigated by a method statement. The damage cost calculation has nothing to say about it.

Presenting the figure to environmental health

Environmental health officers read these calculations closely, and they read them for one thing: whether the inputs are traceable. Put the arithmetic in a table with a row per step, cite the source document and page for every input, and give the emission factor version and the year you ran it for.

State the result as a range where an input is uncertain, with the central figure identified. A single number presented with false precision invites a challenge, and officers have seen enough of these to know which assumptions do the heavy lifting.

Offer the measures alongside the money. A short schedule listing what the contribution would buy, with indicative quantities, moves the conversation from whether the sum is right to whether the measures are the right ones.

Assumptions to state

Every damage cost calculation should carry an assumptions note covering the trip generation source and whether trips are net or gross, the trip length and its justification, the emission factor toolkit version and assessment year, the damage cost table edition and the sector used, the appraisal period and why, and the treatment of any on-site measures already counted. Label the example as illustrative wherever the figures are assumed.

PlanScope drafts an air quality assessment structured to these sections, with the policy framework and the assumptions schedule in place and a "TO BE CONFIRMED" marker wherever site data is missing, so the emission factors and the modelling judgement still come from a qualified air quality consultant. Councils expect a suitably qualified author for this report type and professional review is strongly advised. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. You can see where air quality work sits in a full application pack on the planning documents index.