Air quality assessor competence and the IAQM guidance

No statutory licence attaches to air quality assessment for planning. Nobody signs a declaration, nobody holds a register, and a council cannot refuse an application on the ground that the author lacked a particular letter after their name. What councils do instead is test the method against the published professional guidance, and an assessment that departs from it without explanation gets dismantled by an environmental health officer in a page and a half.

Air quality assessor competence is therefore about demonstrable method rather than credentials. The credentials are a proxy, and a useful one.

No licence, and what competence looks like instead

The Institute of Air Quality Management publishes the guidance that English practice runs on, including the construction dust method, the joint planning guidance produced with Environmental Protection UK, and guidance on odour and on effects at designated nature conservation sites. Its membership grades, and chartered environmentalist or chartered scientist status through the wider institution it sits within, are the usual markers of a competent author.

Three practical tests of competence are better than any of that.

Ask which version of each guidance document the assessor will apply, by name and date. Ask how they will verify the dispersion model and whether suitable local monitoring exists to do it. And ask what they will do if the result shows an exceedance, because an assessor who has never written that conclusion for a client is not one you want on a contested site.

An Air Quality Assessment is labelled as professional recommended rather than professional required, which reflects the legal position. Councils still expect a suitably qualified author, and the practical consequence of using an unqualified one is a consultation objection rather than an invalid application.

Construction dust against operational modelling

These are two different exercises with different methods, and conflating them is the commonest structural fault in a submitted report.

The construction method assesses the magnitude of each activity against the sensitivity of the surrounding area and produces a risk category, which then drives a proportionate mitigation package. It does not produce predicted concentrations, and a report that presents modelled construction dust concentrations has usually misunderstood the guidance.

Operational assessment is where the modelling sits, and most small and medium schemes do not need it. A scheme that generates a modest number of additional vehicle movements away from an air quality management area can often be dealt with by screening, with the reasoning stated.

Monitoring data and background maps

Three data sources do the work, and each has a trap.

Local authority monitoring, from diffusion tubes and automatic analysers, published in the authority's annual status report. Diffusion tube data needs the bias adjustment factor applied and the adjustment stated. Tubes sited at a kerbside do not represent a facade, and using them as though they do overstates the baseline.

National background maps, published by Defra on a one kilometre grid. Those backgrounds already contain a road contribution, so adding modelled road concentrations to an unadjusted background double counts, and the correction is a standard step that gets skipped.

Emission factors, from the published toolkit, with the version and the assessment year stated. Fleet composition changes year on year, so an assessment using a three year old factor set with a future year of assessment is internally inconsistent.

Model selection and verification

Verification is the part that separates a usable report from a decorative one. The modelled road contribution is compared against measured concentrations at monitoring locations with similar characteristics, a verification factor is derived, and that factor is applied to the modelled results.

Four things the report has to show. The monitoring sites used for verification and why they are comparable to the modelled receptors. The modelled and measured values at each. The derivation of the factor, with the arithmetic visible. And a statement of what happens to the conclusions if the factor is at the upper or lower end of a reasonable range.

Where no suitable local monitoring exists, say so and explain what was done instead. An unverified model with a confident conclusion is the single most common reason an environmental health officer asks for the assessment to be redone.

Receptor selection matters as much as the model. Facade heights for the relevant floor, gardens and balconies where amenity is being assessed, and the sensitive receptors that matter most, including schools, care homes and hospitals.

Damage costs and proportionate mitigation

Where residual effects remain, mitigation is expected to be proportionate to the effect rather than to the size of the scheme. The damage cost approach, using published values per tonne of pollutant, is the common way of monetising emissions for the purpose of sizing a contribution towards measures in the authority's air quality action plan.

Two cautions. The damage cost figures are revised, so cite the version used and the date, and never treat the calculated sum as a fixed tariff unless the authority has adopted it as one. And mitigation has to be secured by condition or obligation to count in the assessment, which means the report should name the mechanism rather than listing aspirations.

In London, policy requires development to be air quality neutral, with air quality positive expected for larger schemes, and the Greater London Authority publishes the method for both. That is a design requirement rather than an assessment outcome, so it has to be in the brief from the start.

Where environmental health pushes back

Six objections recur, and five of them are avoidable before submission.

1. No verification, or verification with the arithmetic hidden.
2. Background concentrations used without the road contribution removed.
3. Receptor heights at ground level where upper floors are the sensitive locations.
4. Committed development omitted from the cumulative case, where the authority holds a list of consented schemes.
5. Emission factor and assessment year combinations that do not match.
6. Mitigation listed but not secured.

The sixth is the one that survives into the decision notice, because an officer cannot recommend approval on the strength of measures nobody has committed to. The general points about a conflict of interest in a technical report apply here too, particularly where the same consultant is advising on both the transport inputs and the air quality outputs.

Briefing an assessor inside an air quality management area

Inside a declared area the brief changes. Air quality management areas are declared under Part IV of the Environment Act 1995 where the objectives are not being met, and the authority will have an action plan with measures it is trying to deliver.

Brief the assessor on five points. The declared area, the pollutants it was declared for and the latest status report. The action plan measures, because alignment with them is the strongest mitigation argument available. The committed development list from the authority. Whether the authority has adopted a damage cost or contribution approach. And the design decisions still open, since ventilation intake positions, car parking quantum and the location of any combustion plant cost far less to fix on a drawing than to mitigate afterwards.

Illustrative example: a 70 unit scheme on a main road inside an air quality management area declared for nitrogen dioxide. Screening is not available, so the package is a verified model of the road network with and without the development, facade concentrations at first and second floor level, a mechanical ventilation strategy with intakes on the rear elevation, cycle parking and car club provision as action plan measures, and a construction dust risk assessment with a dust management plan secured by condition.

Review before submission is the safeguard. PlanScope's professional directory is the route to a reviewer who can check the method and sign off a draft, and the parallel question of ground investigation contractor competence arises on many of the same brownfield sites. Browse the 66 report types to see which environmental documents your scheme needs.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.