Air quality assessment or air quality neutral assessment

An air quality assessment asks whether your development makes pollutant concentrations worse where people breathe them. An air quality neutral assessment asks whether your development's own emissions exceed a published benchmark for its use and size. The first is an impact assessment against health-based objectives. The second is a policy test against a number. They use different inputs, produce different outputs, and a scheme in London commonly needs both.

Conflating them is the usual cause of a consultee asking for more work after validation, because an assessment that models concentrations says nothing about whether the benchmark has been met.

Two different questions

Set them side by side and the difference is obvious.

An air quality assessment can conclude that the effect on receptors is negligible while the scheme still fails the neutral test, because the neutral test is not about where the pollution lands. It is about how much the building and its traffic emit at all.

Construction dust sits outside both

Construction phase effects are assessed by their own method, and they belong in a separate chapter.

The Institute of Air Quality Management's construction dust guidance works on risk rather than on prediction. You classify the activities, demolition, earthworks, construction and trackout, assess the sensitivity of the surrounding area, combine the two to give a site risk category, and then apply the mitigation appropriate to that category. The output is a mitigation schedule, not a concentration.

That matters commercially because the mitigation is what ends up in the condition. Wheel washing, sheeted stockpiles, a dust monitoring regime, restrictions on cutting and grinding, and a complaints procedure all get secured through a construction environmental management plan. None of it depends on the operational assessment, and it should not wait for it.

Operational concentrations and dispersion modelling

The operational part of an Air Quality Assessment looks at two pathways and sometimes three.

Exposure of new occupants to existing pollution, which is the suitability question: will people living in your building be exposed to concentrations above the objectives, and if so what are you doing about layout, ventilation intake positions and habitable room locations.

The effect of your scheme's traffic on existing receptors, modelled from traffic data on the affected network, which is why the trip generation in the Transport Assessment has to be settled before the air quality modelling starts.

And where combustion plant is proposed, the effect of its emissions at the nearest sensitive windows, assessed by dispersion modelling with stack height and discharge velocity as the variables you can change.

Monitoring data from the local authority's own network, plus background maps, provide the baseline. State the monitoring sites used, their distance from the site and their classification, because a roadside site and an urban background site give very different baselines and the choice shifts the conclusion.

Benchmarks for building and transport emissions

The neutral test works the other way round. Two benchmarks apply, derived per unit of floorspace for the relevant use class: one for building emissions, covering heating and hot water plant, and one for transport emissions, covering the vehicle movements the development generates.

Calculate the scheme's annual emissions of nitrogen oxides and fine particulate matter for each, compare them with the benchmark for the floorspace and use proposed, and report the result. Exceed the benchmark and you mitigate, or offset the residual.

The building side is decided almost entirely by the energy strategy. An all-electric scheme with heat pumps has no combustion emissions at all, so the building benchmark is met without argument. A gas-fired scheme, or one with combined heat and power, will have emissions to compare and often to offset. Settle this in the Energy Statement first, because changing the heating strategy after the air quality work is complete invalidates the calculation.

The transport side is decided by trip generation and the assumed fleet. Reduce car movements, and the transport emissions figure falls with them.

This is the same structural logic as the other neutrality tests now applied in planning, where a scheme must demonstrate it adds nothing to an existing problem. The comparison with the catchment-based tests is drawn out in our note on nutrient neutrality or water neutrality.

Which schemes trigger which assessment

Outside London the neutral test applies only where a local plan or supplementary document has adopted it, and a number have. Check the adopted policy wording rather than assuming the London approach does or does not apply.

The thresholds for the traffic-based triggers come from the Institute of Air Quality Management's criteria rather than from statute, and they are expressed as changes in vehicle movements on affected links. Agree the scoping with the authority's environmental protection team before modelling, because it costs less than remodelling.

Mitigation and how each is secured

The mitigation hierarchy for both runs: avoid, reduce, then offset.

For concentrations: move habitable rooms and ventilation intakes away from the pollution source, increase stack heights, specify low emission plant with the emission rate stated in milligrams per kilowatt hour, and in the worst cases provide mechanical ventilation with filtration. These are design measures and they end up conditioned to the approved drawings.

For the neutral test: remove combustion, reduce parking, provide charging infrastructure, improve the walking and cycling offer. Where a residual remains, an offset payment is typically secured by a section 106 obligation, which must satisfy the statutory tests on necessity and relationship to the development. The distinction between a payment under an obligation and a levy liability for the same works is set out in our note on a levy payment or a section 106 contribution.

A point developers miss: mitigation for one test is not automatically mitigation for the other. Mechanical ventilation with filtration protects your occupants and does nothing for your emissions benchmark. Removing a gas boiler improves both.

Reporting both in one document

One report, clearly divided, works better than two reports that cross-refer.

A workable structure:

1. Baseline, with monitoring sites named and background concentrations stated.
2. Construction phase dust risk assessment and mitigation schedule.
3. Operational exposure of new occupants, with the design response.
4. Operational effects on existing receptors, with modelling assumptions and traffic data source.
5. Emissions benchmark calculation, building and transport, with the result stated plainly.
6. Mitigation, split between measures for concentrations and measures for emissions.
7. Residual effects, and any offset proposed with its basis.

State in the opening paragraph which tests the report addresses and under which policy. Officers and consultees read dozens of these, and a report that says at the outset that it covers both the impact assessment and the benchmark test, with sections numbered accordingly, avoids the request for clarification that otherwise costs a fortnight.

Keep every input traceable. The trip rates, the plant specification, the monitoring sites and the emissions factor year should each be cited to a source, because each one is a point at which a consultee can reopen the conclusion.

Settle the energy strategy and the trip generation before commissioning either assessment. Browse all 66 PlanScope report types to see how the air quality, energy and transport documents fit together.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.