Checking air quality data before a planning application

Before anyone models dispersion or counts vehicle trips, an air quality data check answers three questions about your site: what the background concentrations are, whether the site sits inside or near an Air Quality Management Area, and what the council's own monitors have recorded nearby. All three are public, free and downloadable in an afternoon. Doing them first tells you whether the change of use you are planning raises an air quality issue at all.

For a restaurant, a gym, a nursery or a shop-to-flat conversion, that early check often decides whether the application needs an assessment or a paragraph.

National background concentration maps

Defra publishes modelled background concentrations for the whole of the United Kingdom on a 1km by 1km grid, as part of the local air quality management framework. The datasets cover nitrogen dioxide, oxides of nitrogen, PM10 and PM2.5, with a stated base year and projections forward.

Download the grid square containing your site and the squares around it. Record three things: the grid reference, the base year of the dataset, and the projection year you used. Those three pieces of information are what an environmental health officer will check first, and an assessment that omits them invites a request for clarification.

The limitation is resolution. A 1km square averages a motorway, a park and a residential street into one figure. On a street canyon site with heavy traffic the background map will understate the concentration at the facade, and on a suburban site set back from any road it will overstate it. Treat the background value as the contribution from everything other than the local road you are standing next to.

Local authority monitoring reports

Every English local authority reviews and assesses air quality in its area under the local air quality management regime and publishes an annual status report. That report is the most useful single document for a site-specific check.

It contains the location and results of every automatic analyser and diffusion tube the council operates, usually with grid references, the declaration details of any Air Quality Management Areas, progress against the action plan, and commentary on trends.

Find the report for the most recent year, then find the two or three monitoring points closest to your site. Note the site type recorded against each one: roadside, kerbside, urban background, and whether it sits at a location of relevant public exposure. A kerbside tube on a main road is not evidence of what a residential facade 40m away experiences.

Councils publish these reports on their own websites and submit them to Defra. Take the version from the council, because it is the one the environmental health officer will have on their desk.

Diffusion tubes and what they can support

Diffusion tubes are the workhorse of local monitoring and they are widely misused in planning submissions.

A tube is exposed for about a month, returned to a laboratory and analysed for nitrogen dioxide. Twelve monthly results give an annual mean. The technique is inexpensive, which is why councils run dozens of them, and it carries a known bias that has to be corrected using a factor derived from co-location studies.

Three consequences for anyone quoting tube data. Always state whether the figure is bias-adjusted and which factor was applied, because the raw and adjusted numbers differ materially. Never use tube data to assess a short-term objective, since a monthly average tells you nothing about hourly concentrations. And check for annualisation where a tube has fewer than nine valid months, because a part-year result adjusted to an annual mean carries extra uncertainty that should be stated.

Automatic analysers are a different matter. They give hourly data, they support both short-term and long-term objectives, and there are far fewer of them.

Checking the Air Quality Management Area

An Air Quality Management Area is declared where a council finds that an air quality objective is not being met, or is unlikely to be met, at locations of relevant public exposure. Declaration brings a duty to produce an action plan.

Two things to establish. Whether your site falls inside the boundary, which can be a single street, a town centre or an entire borough. And which pollutant and which objective the declaration relates to, because an area declared for the annual mean nitrogen dioxide objective raises different questions from one declared for the 24-hour PM10 objective.

Defra hosts an interactive map of declared areas alongside the underlying orders, and the council's own pages carry the declaration order and boundary plan. Take the boundary from the council's order rather than eyeballing the national map, since the orders are the legal instrument.

Being outside an area is not a clean bill of health. Boundaries are drawn where exceedances were found, and a site 100m outside one can still be a poor location for a receptor sensitive to pollution.

Turning the data into a screening assessment

The screening question is whether the proposal is likely to have a material effect, judged against the traffic generation and emissions it introduces and the sensitivity of the location.

Professional guidance from the Institute of Air Quality Management, used with guidance from Environmental Protection UK, sets indicative criteria for when a full assessment is needed based on changes in traffic flows, heavy vehicle movements and the introduction of new sensitive receptors. Most councils apply it, and several set their own thresholds in a supplementary planning document.

For a small commercial change of use the answer is often that a detailed assessment is not required, but that a statement dealing with the data, the thresholds and any mitigation is. Where combustion plant or a commercial kitchen is involved, the emissions point and its discharge become the issue instead, which is where a Ventilation and Extraction Statement does the work.

Where the answer is that a full assessment is needed, an Air Quality Assessment will take the same background and monitoring data as its input, so the checks are never wasted.

When monitoring at the site is worth it

Site monitoring is justified in a narrow set of cases: no representative council monitoring within a useful distance, a street canyon or other geometry the model handles badly, a site where the council has signalled scepticism about modelled values, or a scheme large enough that a year of baseline data is proportionate.

Diffusion tubes are the practical option, and they need planning. A meaningful survey runs for at least six months and preferably twelve, co-located with a council monitor where possible so a local bias adjustment can be derived. Tubes need secure mounting, a documented chain of custody and an accredited laboratory.

Start early. A twelve month survey commissioned after the council objects adds a year to the programme, which is the same trap that catches teams who leave other data gathering late, from ecology seasons to checking whether the strategic road network is engaged.

Referencing the data so it survives scrutiny

Write the provenance into the report rather than into a footnote nobody reads.

1. Background maps: grid reference, dataset base year, projection year and date of download.
2. Monitoring: council name, annual status report year, monitor identifier, site classification, bias adjustment factor and whether annualisation was applied.
3. Air Quality Management Area: declaration order reference, date, pollutant and objective.
4. Guidance: the version of the professional guidance used for screening.
5. Objectives: the regulations setting the objectives you are comparing against, with the values quoted from the current schedule.

Keep copies of everything you downloaded with the date. Datasets are revised, monitoring reports are superseded, and a reviewer six months later needs to reproduce what you did. The same discipline applies to other public datasets used in applications, including version control on the biodiversity metric.

Enter your postcode on PlanScope to see which environmental documents your change of use is likely to need.

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