Odour units and dynamic olfactometry explained

An odour unit is a dilution factor, not a mass. One European odour unit per cubic metre is the concentration at which a panel of screened assessors detects an odour half the time, so a sample reported at 20,000 ouE/m³ needed diluting twenty thousand times before it reached that threshold. Dynamic olfactometry under BS EN 13725 is the method that produces the number, and understanding what it does and does not tell you is the difference between a defensible odour assessment and an impressive-looking one.

The number says nothing about whether the odour is pleasant. Concentration and offensiveness are separate properties, and conflating them is the commonest error in review.

What the measurement actually is

The method works by presenting diluted sample to human assessors at an olfactometer and finding the dilution at which detection occurs. Each assessor gives a series of responses as the dilution reduces, the individual threshold estimates are combined, and the panel geometric mean becomes the reported concentration.

Panels are screened rather than assembled from volunteers. Candidates are tested against a reference gas, n-butanol, and admitted only if their individual threshold sits within a specified range and their responses are repeatable. That screening is what makes the method reproducible between laboratories, and it is why panel records matter when reviewing a report.

The output is a concentration in odour units per cubic metre. Multiply by the volumetric flow rate at the emission point and you get an emission rate in odour units per second, which is the input a dispersion model needs. Getting the flow measurement wrong is a more common source of error than the olfactometry itself.

Sampling, and why it constrains the programme

Sample collection is the stage where results are most often compromised.

Samples are drawn into inert bags using the lung principle, where the bag sits in a rigid vessel that is evacuated so the sample never contacts the pump. Materials matter because some compounds adsorb onto bag walls or permeate out. Hot or humid samples need pre-dilution at the point of collection, otherwise condensation inside the bag strips the compounds that matter.

The standard also limits storage time, commonly to the order of 30 hours, so sampling and analysis have to be programmed as one operation with laboratory capacity booked in advance. A sample collected on a Friday afternoon at a site three hours from the laboratory is a wasted visit.

Representativeness is the other half. A single grab sample from a process that varies through the day characterises that moment and nothing else. Reviewers should look for the number of samples, when they were taken relative to the process cycle, and whether the operating conditions during sampling were documented.

Benchmark levels and offensiveness

Modelled results are compared against benchmark exposure levels that vary with how offensive the odour is. The Environment Agency's odour management guidance sets indicative benchmarks expressed as the 98th percentile of hourly mean concentrations: in the region of 1.5 ouE/m³ for the most offensive odours, 3.0 ouE/m³ for moderately offensive ones, and 6.0 ouE/m³ for the least offensive.

Classifying the odour therefore does as much work as measuring it. Processes handling decaying animal material sit at the strict end. Food production and brewing sit at the tolerant end. A reviewer should check that the assigned category reflects the actual process rather than the one most convenient for the applicant, because moving a site from one benchmark to the next changes the acceptable concentration by a factor of two.

The qualitative framework behind this is usually summarised as frequency, intensity, duration, offensiveness and location. Olfactometry addresses intensity, in the sense of concentration, and contributes to duration through modelling. The rest comes from the process description and the receptor survey, and an assessment that reports only a concentration has answered one fifth of the question. The same balance between quantitative and qualitative evidence runs through our note on odour from waste transfer and composting facilities.

Feeding the results into a dispersion model

Once emission rates are established, a dispersion model predicts concentrations at receptors. The model needs the emission rate and its temporal variation, the source geometry including stack height, diameter, efflux velocity and temperature, building downwash parameters, terrain, surface roughness and several years of hourly meteorological data from a representative station.

Four modelling choices deserve scrutiny in review.

• The number of meteorological years used, since odour percentiles are sensitive to year-to-year variation and a single year can flatter a scheme.
• Whether emissions were modelled as continuous or with a realistic profile, because many odour sources are intermittent and an annual average smears the peaks that cause complaints.
• Treatment of fugitive and area sources, which on most waste and agricultural sites contribute more than the stack does.
• Receptor selection, including whether gardens and other outdoor amenity areas were assessed rather than only facades.

Results are reported as a percentile of hourly means rather than an annual average, because odour is a short-term nuisance. The 98th percentile leaves roughly 175 hours a year above the figure, which is worth stating plainly when explaining results to a planning committee.

Uncertainty, and how to present it

Olfactometry carries more uncertainty than most physical measurements, because the instrument is a group of people. The standard addresses this by setting accuracy and precision criteria that a laboratory must demonstrate and maintain, and by requiring the panel's performance to be monitored against the reference gas.

Even so, repeat measurements of the same source on different days can differ substantially, and that variation is real rather than a laboratory failing: the source itself varies. The honest way to present results is with the sampling conditions, the number of samples, the spread between them, and a sensitivity run in the model using a higher emission rate.

What should raise a flag in review is a report presenting a single sample result to three significant figures and modelling it as a fixed emission rate for a year. The arithmetic is fine. The implied precision is not.

When modelling is proportionate

Dispersion modelling is expensive and slow, and it is not the right response to every odour question.

A qualitative or separation-based assessment is usually proportionate for a commercial kitchen extract, a small bakery, a dry cleaner or a modest food unit, where the controls are well understood and the discharge arrangement does most of the work. The relevant evidence is the abatement specification, the discharge height and velocity, and the distance to receptors.

Modelling earns its place where the source is large, continuous or difficult to abate, where receptors are close, where there is a history of complaint, or where a permit is involved. Intensive livestock units, waste transfer and composting sites, wastewater treatment works, rendering and food processing at scale all fall into that group. Bioaerosol and separation-distance screening often run alongside it, as set out in our note on bioaerosols and separation distance from composting.

Field inspection by trained observers, under the European grid and plume methods, is a third route and is sometimes more persuasive than modelling where an existing site is already operating.

Reviewing an olfactometry report

A reviewer should be able to answer these questions from the document alone. Which laboratory carried out the analysis and against which accreditation. How many samples, taken when, and under what documented process conditions. How the volumetric flow was measured. What odour category was assigned and on what basis. Which model, which meteorological years, which receptors. And what the sensitivity analysis shows.

Where those answers are missing, the right response is to ask for them rather than to argue with the conclusion. An odour assessment that cannot evidence its inputs is not defensible at appeal, whatever the modelled contours show. The contrast with air quality monitoring, where method choice is also decisive, is covered in our note on diffusion tubes against continuous analysers.

Councils normally expect a suitably qualified author for odour work, and professional review before submission is strongly advised. PlanScope's drafts structure the document and the policy framework, and the measurement and modelling must come from a specialist.

Browse all 66 PlanScope report types and start a draft to see which environmental documents a scheme with an odour source 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.