Damage cost calculations in an air quality assessment

An air quality damage cost calculation turns a development's extra emissions into a sum of money. It is not a prediction of concentrations at anyone's front door, and it does not tell you whether an objective will be exceeded. It estimates the health and environmental cost that the emissions impose, using published values per tonne of pollutant, and converts that into a contribution the applicant pays towards measures in the authority's air quality action plan.

Officers reach for it when dispersion modelling shows effects that are real but small, which is most of the time on a residential scheme.

What the approach measures and what it does not

Dispersion modelling answers a spatial question: what is the annual mean nitrogen dioxide concentration at this receptor, and does it breach the objective. The damage cost approach answers a different one: what is the cost to society of the extra tonnes this development emits, wherever they end up.

The two are complementary rather than alternatives. A scheme can produce concentrations well within the objectives and still emit enough additional nitrogen oxides to justify a contribution, particularly inside an air quality management area where the authority has an action plan to fund.

What the approach cannot do is substitute for modelling where a receptor is close to an exceedance. If the modelling shows a problem, mitigation has to address the problem, and a payment does not.

Calculating the emissions

The calculation has two sources on most schemes: road traffic and combustion plant.

For traffic, the inputs are the trip generation from the transport work, the distribution across the network, the link lengths over which those trips travel, the assumed speed on each link, and the fleet composition for the assessment year. Government publishes an emissions factors toolkit that converts vehicle kilometres by fleet and speed into tonnes of nitrogen oxides and particulate matter per year. Use the current version, state which version, and state the assessment year, because factors fall over time as the fleet changes and an older toolkit produces a higher number.

For plant, the inputs are the rated thermal input, the annual operating hours, the fuel and the emission concentration from the manufacturer or from the relevant emission standard. Gas boilers, standby generators and any combined heat and power unit all count. Standby generators are often forgotten because they rarely run, and their testing regime still produces hours.

Net the figures. The calculation is of additional emissions, so emissions associated with the existing lawful use of the site come off, provided the existing use is still operating rather than one abandoned years ago.

Applying the published values

Government publishes damage cost values per tonne for the main pollutants, differentiated by source sector and by whether the emissions occur in an urban or rural setting. The values are revised, so take the current published set and say in the report which set you used and when it was published.

The arithmetic is then simple. Tonnes of each pollutant per year, multiplied by the value per tonne for the relevant sector and location, gives an annual damage cost. That annual figure is then multiplied by a number of years to produce the one-off contribution.

The multiplier is where authorities differ. Published guidance from the air quality practitioner bodies has commonly used five years, and some authorities adopt a different period in their own supplementary planning document. Find the authority's stated position before you calculate, because recalculating on a different multiplier after the officer's report has gone out wastes a committee cycle.

Where the authority has no adopted position, say which multiplier you used and why, and expect it to be negotiated.

What the contribution funds

A damage cost contribution is not a fine and it is not general revenue. It funds measures that improve air quality, normally those already identified in the authority's air quality action plan.

The usual list: electric vehicle charging infrastructure, cycle parking and route improvements beyond the site, bus service contributions or stop upgrades, car club provision, and monitoring equipment. Some authorities apply the money to a wider fund rather than to named measures.

That matters for the obligation tests. A contribution secured by a planning obligation has to be necessary to make the development acceptable, directly related to the development, and fairly and reasonably related in scale and kind, under the Community Infrastructure Levy Regulations 2010. A payment into an unidentified fund with no stated purpose is vulnerable on the first and second of those.

Testing a council's figure

Applicants accept these contributions more readily than they should, partly because the sums are usually modest relative to the scheme.

Five checks are worth making every time.

• Which emissions factors toolkit version and which assessment year were used. An assessment year set too early inflates the result.
• Whether existing site emissions have been netted off. On a redevelopment of an existing commercial use this can be most of the answer.
• Whether the trip generation matches the transport evidence rather than a separate assumption.
• What multiplier has been applied and where it comes from.
• Whether the measures named are in the adopted action plan, and whether any of them is already funded by the Community Infrastructure Levy, which would make a separate contribution double counting.

The last one is worth real attention. Where an authority charges the levy and also seeks air quality contributions for measures listed on its infrastructure list, the overlap has to be explained.

Recording the calculation so it can be audited

Put the whole calculation in the Air Quality Assessment rather than reporting a conclusion. The reader should be able to reproduce the number.

That means a table of annual emissions by source and pollutant, with the inputs beside them. The toolkit version and assessment year stated. The damage cost values with their publication date. The multiplier with its source. And the netting-off calculation shown, not just applied.

Where the contribution is being offered rather than demanded, set out the alternative in-kind measures and their equivalent value, because many authorities prefer delivered infrastructure to cash. Charging points installed at the development, or a travel plan with funded incentives, can satisfy the same policy objective, and a Travel Plan with monitoring and a default payment attached is often a better outcome for both sides than a lump sum.

An air quality assessment is a report type where councils expect a suitably qualified author and professional review is strongly advised. The structure, the policy narrative and the presentation of the calculation can be drafted efficiently. The emissions work and the judgement about what the result means belong to the practitioner.

One honest limitation: damage cost values carry wide uncertainty, and a contribution calculated to the nearest pound implies a precision the method does not have. Round it, and say why.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Browse the 66 report types to see which air quality and transport evidence your scheme is likely to need.

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