Odour assessment competence and olfactometry explained
Odour assessment uses a human panel, not an instrument. Here is how sampling and olfactometry work, what dispersion modelling adds, and how to judge whether the person writing your report is competent to do it.
Odour assessment competence is judged differently from acoustic competence, because odour has no instrument. There is no sound level meter for smell. The measuring device is a screened human panel, the unit is defined by how far a sample has to be diluted before half the panel can just detect it, and everything downstream of that, the modelling, the conditions, the mitigation, rests on how well the sampling was done.
That single fact explains most of what goes wrong. An air quality consultant who models nitrogen dioxide competently is not automatically competent to sample a fugitive odour source from a waste transfer building, and case officers who have been through a contested odour application know it.
Why odour behaves differently from noise
Noise is continuous, measurable in real time, and broadly linear in the way people respond to it. Odour is none of those things.
It arrives in plumes rather than steadily, so a receptor may experience nothing for six hours and then a strong episode lasting four minutes. It is chemically complex, with mixtures behaving unpredictably compared with their components. Response depends on character as much as concentration, which is why the same concentration of roasting coffee and rendering waste produce entirely different complaint rates. And people habituate to their own odours while remaining acutely sensitive to someone else's.
A practitioner who treats odour as a variant of air quality dispersion, with a threshold and an annual mean, will produce a report that reads well and predicts the wrong thing.
Sampling and olfactometry
The measurement standard is BS EN 13725, which sets out dynamic dilution olfactometry. A sample is collected into an inert bag, taken to a laboratory within a short holding time, and presented to panel members through an olfactometer at a series of dilutions. The concentration reported is the dilution factor at which half the panel detects the sample, expressed as European odour units per cubic metre.
Panel quality is the whole method. Members are screened against n-butanol as a reference gas and only those falling within a defined sensitivity range are used, with rescreening at set intervals. A laboratory holding accreditation for the method will have that documented, and asking to see it is a fair question.
Sampling is where competence shows most clearly:
- A stack or duct is straightforward, sampled isokinetically from a defined point with the flow rate measured at the same time.
- An area source such as a lagoon or an open bay needs a flux hood or wind tunnel, and the choice of device changes the answer.
- Fugitive sources, which is to say doors, gaps and yard activities, are the hardest, and are frequently estimated rather than measured.
- Holding time matters, because some compounds degrade in the bag within hours.
Ask which sources were sampled, which were estimated, and on what basis. A report that quietly estimates the dominant source has not measured the thing that will generate complaints.
Modelling and the inputs that move the answer
Where modelling is used, the odour emission rate in odour units per second feeds a dispersion model alongside meteorological data, terrain, building geometry and receptor locations. The output is usually presented as a high percentile of hourly mean concentrations at each receptor, compared against benchmark levels that vary with how offensive the odour character is. Check the current guidance for those benchmarks, because they are revised.
Four inputs dominate the result. The emission rate, which comes straight from the sampling. The release height and efflux velocity, because a stack terminating below the ridge line behaves completely differently from one that clears it. Building downwash, which pulls a plume into the lee of a structure. And the meteorological dataset, since a single unrepresentative year can move a percentile result noticeably.
If you are reviewing an odour assessment, those four are where to look first. Sensitivity runs showing how the result changes with a different met year or a lower efflux velocity tell you whether the conclusion is stable or balanced on an assumption.
When a qualitative assessment is enough
For a single commercial kitchen extract serving a restaurant or takeaway, full olfactometry is usually disproportionate. The accepted route is a risk-based qualitative assessment working through source, pathway and receptor, describing the cooking type, the extract design, the separation distances and the sensitivity of nearby occupiers.
That assessment normally sits alongside a ventilation and extraction statement describing the equipment schedule, the filtration stages and the discharge arrangement. Together they are what an environmental health officer expects for a food business, and they are proportionate to the scale of the risk.
Modelling earns its place when the operation is continuous, when the source is large, or when there is an existing complaint history that needs explaining rather than predicting.
Reading the report as an operator
You do not need to be an acoustician's equivalent to interrogate an odour report. Five questions get you most of the way.
- Which sources were measured and which were estimated.
- What the assumed operating hours and throughput were, and whether they match your business plan rather than a typical case.
- What efflux velocity and discharge height the conclusion depends on.
- Whether the mitigation specified is maintainable by your staff, or assumes a service contract nobody has priced.
- Whether the author has done this source type before.
The fifth matters more than people expect, and it is worth understanding the membership grades used in air quality work before you compare quotes. Councils can and do reject a report because of who wrote it, which turns a saving on fees into months of delay.
Mitigation the assessor should be specifying
A conclusion of no likely odour impact is worth very little without the design that delivers it. A usable report specifies the abatement train, the maintenance regime and the verification.
For kitchen extract that typically means grease removal, then the odour abatement stage, whether carbon, electrostatic or ultraviolet treatment, then a discharge point above roof level with an unobstructed vertical release and a high efflux velocity. For industrial sources it may be enclosure, negative pressure and a treated exhaust rather than dilution at all.
The maintenance schedule is not an afterthought. Carbon media exhausts, filters load up, and an odour control system that is not serviced becomes an odour source. A condition requiring a maintenance log is common, and it is easier to comply with when the report already sets out the intervals.
What the council does with it
Two separate regimes apply. In planning, the authority conditions the approved odour control scheme, its installation before first use, and its ongoing maintenance, and a breach is enforceable as a breach of condition.
Alongside that, odour can be pursued as a statutory nuisance under Part III of the Environmental Protection Act 1990, which is a different test applied by a different team and does not depend on any planning condition. An operation that complies with its planning condition can still be served with an abatement notice, which is why getting the design right matters more than getting the wording right.
If you want the draft checked before it goes in, PlanScope's directory lists a reviewer who can check the assessment and sign it off.
This is general guidance, not legal advice; 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.
Frequently asked questions
Who is qualified to write an odour assessment?
There is no statutory licence for odour assessors in England. Councils look for demonstrable experience in odour work, membership of a relevant institute at an appropriate grade, and, where olfactometry is used, a laboratory holding accreditation for testing to the published standard. Experience with your specific source type matters more than a general air quality background.
What is an odour unit?
The European odour unit per cubic metre is the concentration at which half a screened panel can just detect the odour. It is defined by dilution rather than by chemistry, so it describes perception rather than the amount of any particular compound. That is why two samples with identical chemical composition can produce different odour concentrations.
Do I always need dispersion modelling?
No. For smaller sources such as a single kitchen extract, a qualitative risk-based assessment following recognised guidance is usually accepted, focusing on source, pathway and receptor. Modelling becomes appropriate where the source is large or continuous, where sensitive receptors are close, or where an existing operation has generated complaints that need to be explained.
How is a panel for olfactometry selected?
Panel members are screened against a reference gas, n-butanol, and only those whose sensitivity falls within a defined range are used. The standard also sets requirements for how often screening is repeated. This is what makes the result reproducible between laboratories, and it is the part of the method an operator should check has been followed.
What happens if the council does not accept my assessment?
The usual outcome is a request for further information rather than a refusal, so the application stalls while the point is argued. Where the objection is to the author's competence or to the method used, a fresh assessment by a suitably experienced practitioner is normally faster than defending the original. Allow for that risk rather than picking on price alone.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.