Odour Assessment: source, pathway and receptor sections

An odour assessment report structure follows the same three-part chain every time: source, pathway, receptor. Describe what smells and how strongly, work out what happens to the plume between the discharge point and the nearest window, then judge how sensitive the people at that window are. The Institute of Air Quality Management's guidance on the assessment of odour for planning sets that sequence out, and environmental health officers read reports in that order. Get one link wrong and the conclusion collapses, however careful the modelling was.

Most refusals on odour grounds are not caused by bad arithmetic. They are caused by a report that describes a carbon filter without saying what it removes, or that names a neighbouring flat without saying how far away the window is or which way it faces.

Describing the source and how it is released

The source section has to be specific enough that someone else could rebuild the calculation. Name the process, the ingredients or materials, the operating hours and the throughput. A wood-fired pizza oven, a Chinese wok range and a sandwich bar produce different odours at different rates from the same floor area, and the report should say which of those it is describing.

Then describe the release. Efflux velocity, discharge height above the ridge or parapet, the direction the cowl faces, and whether the discharge is vertical or capped. A vertically discharging flue at 15 m/s behaves nothing like the same volume of air pushed sideways out of a louvre at 3 m/s, and a Chinese hat cowl that deflects the plume downwards undoes much of the height you paid for.

Set out the emission rate if you have one. Dynamic olfactometry to BS EN 13725 gives an odour concentration in European odour units per cubic metre, which combined with the volume flow gives an emission rate in odour units per second. For a new kitchen there is nothing to sample, so the assessment works from the qualitative risk route instead, and the report should say plainly which route it is using.

Odour potential and how it is banded

The IAQM approach bands the source by its odour potential as small, medium or large, taking account of the intrinsic offensiveness of the odour, the scale of the operation and the release conditions. A domestic-scale extract serving a bakery sits at the bottom. A rendering plant sits at the top. Most restaurant and takeaway applications land in the middle, which is exactly where the argument happens.

Offensiveness matters as much as quantity. The Environment Agency's benchmark exposure levels reflect that, using a lower threshold for the most offensive odours than for the least offensive ones, expressed as the 98th percentile of hourly mean concentrations. If your report quotes a benchmark, say which category the odour falls into and why, because that choice moves the answer by a factor of four.

The pathway between the discharge and the window

Pathway analysis is where reports get thin. Distance alone tells you very little. What matters is the combination of separation distance, the height of the discharge relative to nearby buildings, local topography, prevailing wind direction and the presence of anything that creates a downwash cavity.

Three questions carry most of the weight. Is the discharge above the roof level of every building within a plume's reach, usually taken as at least 1 metre above the ridge? Does the prevailing wind carry the plume towards the sensitive receptor or away from it? Is there a taller building, a parapet or a lightwell that will pull the plume down into a courtyard?

Where a scheme is close to housing and the geometry is awkward, dispersion modelling earns its fee. Where the flue clears everything by several metres and the nearest window is 40 metres downwind of the prevailing direction, a reasoned qualitative pathway assessment is usually accepted. The judgement between those two routes belongs to the assessor, and the report should justify it rather than assert it.

Receptor sensitivity and why land use decides it

Sensitivity is about the receptor, not the smell. IAQM grades receptors as low, medium or high sensitivity by reference to how people use the location and how long they are exposed.

A residential window 20 metres away is a high sensitivity receptor. The same distance to a self-storage unit is not, and an assessment that treats them identically will be sent back. Identify each receptor individually, with its distance, orientation, floor level and land use, and mark them on a plan. Where a proposal sits near schools or care settings, the population health angle can also surface separately, which is why odour sometimes appears in the evidence base alongside a Health Impact Assessment on larger schemes.

Abatement effectiveness and what has to be evidenced

This is the section councils reject most often. Naming equipment is not evidence of performance.

For a commercial kitchen, a specification should cover the grease filtration stage, the odour abatement stage, the fan duty, the duct route and the termination detail. Carbon filtration is only as good as its residence time and its replacement interval. Ultraviolet and ozone systems need a stated contact time and a safety interlock. Electrostatic precipitators need a cleaning regime or they stop working within weeks.

State the claimed removal efficiency, state where that figure comes from, and state the conditions under which it holds. Defra's guidance on odour control from commercial kitchen exhaust systems links the level of abatement to a risk score built from the type of cooking, the volume and the distance to receptors, and most environmental health officers will expect to see that scoring shown rather than summarised. If a figure comes from a manufacturer's datasheet, say so.

Details of the flue route also have to match the drawings. A specification describing a discharge 1 metre above the ridge, sitting alongside an elevation showing a terminal at first-floor level, fails on internal consistency before anyone considers the odour itself. The same discipline applies to the Ventilation and Extraction Statement that usually accompanies it, and to the fan noise figures in any Noise Impact Assessment for the same plant.

Bringing it together as a risk of odour exposure

The conclusion combines the odour exposure at each receptor with that receptor's sensitivity to give a risk of odour exposure, then translates it into an effect descriptor running from negligible through slight and moderate adverse to substantial adverse. National policy asks whether the development would give rise to an unacceptable adverse effect on living conditions, so the report has to land its conclusion in those terms rather than stopping at a technical category.

Illustrative example: a 60-cover restaurant with a gas range, extract at 1.2 m³/s, flue terminating 1 metre above a two-storey ridge, nearest residential window 18 metres away and cross-wind of the prevailing direction. Medium odour potential, an effective pathway broken by height, high sensitivity receptor. With ultraviolet and carbon abatement and a maintenance schedule, that combination usually reads as a slight adverse effect and is controllable by condition. Remove the abatement and the same geometry reads as moderate adverse.

Odour work is a specialist field, and councils generally expect a suitably qualified author. An Odour Assessment drafted by PlanScope gives you the structure, the policy framing and the section content, and professional review before submission is strongly advised for anything beyond the simplest case.

Monitoring, maintenance and the condition that follows

Expect the permission to carry conditions requiring the extract system to be installed as approved, maintained in accordance with a schedule, and kept operational whenever cooking takes place. Some authorities go further and require a written odour management plan, with filter change records kept on site and produced on request.

Write those commitments into the assessment rather than waiting for the condition. A report that already specifies a six-monthly service interval, a carbon replacement trigger and a complaints log gives the officer the wording to use, and it removes the most common reason for a deferred decision. Failure to maintain the system also exposes the operator to statutory nuisance action under section 79 of the Environmental Protection Act 1990, which is a separate regime with its own teeth.

If you are scoping a change of use and want to see what else the site is likely to need, enter your postcode on PlanScope and work through the guided questions.

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