When dynamic olfactometry is needed for an odour case

Dynamic olfactometry under BS EN 13725 is the only method that produces a measured odour concentration, and it is needed when a planning case turns on predicted exposure at a named receptor rather than on a qualitative judgement. If the authority or an objector is asking what concentration will occur at the nearest dwelling, you need a number, and the number has to come from a panel of human assessors diluting a real sample until half of them can just detect it.

That is the whole method in one sentence. Everything else is about sampling discipline and what you do with the result.

What the method measures

The output is an odour concentration in European odour units per cubic metre. One unit is defined as the amount of odorant that, in one cubic metre of neutral gas, produces a response in half of a calibrated panel. It is a detection threshold, not an intensity scale and not an offensiveness scale.

Panels are screened against a reference gas so that individual sensitivity falls within a defined range, and the standard sets out the statistical treatment of panel responses. This is why olfactometry results are reproducible in a way that field sniffing is not, and why they survive cross-examination.

What the number does not tell you is whether people will object. An odour can be detectable and inoffensive, or faint and deeply unpleasant. The offensiveness judgement sits outside the measurement and is applied later, when the results are compared against benchmarks.

Sampling the source correctly

Sampling is where most assessments go wrong, and a badly taken sample cannot be rescued by good analysis.

Point sources such as a stack, a biofilter outlet or an extraction duct are sampled directly, with the volumetric flow measured at the same time so that a mass emission rate can be calculated. Take the sample under representative operating conditions, which usually means several samples across the operating cycle rather than one.

Area sources are harder. A lagoon surface, an open windrow or a silage clamp has no defined flow, so the sample is taken using a hood or wind tunnel placed on the surface, and the result is expressed as a specific emission rate per square metre. The hood design affects the answer, so state which was used.

Two constraints govern the logistics. Samples are collected into inert bags and have to reach the laboratory quickly, because the standard limits the interval between collection and analysis to well under two days. And the laboratory has to be accredited for the method, which means booking panel time in advance rather than sending samples on spec.

Turning concentrations into an emission rate

The arithmetic is straightforward once the sampling is right. For a point source, multiply the measured concentration by the volumetric flow rate to get an emission rate in odour units per second. For an area source, multiply the specific emission rate by the source area.

Where the source varies, and almost all of them do, build an emission profile rather than a single figure. A composting operation emits differently during turning than during maturation. A rendering plant emits differently on a hot afternoon than on a cold morning. The profile is what makes the subsequent modelling defensible.

Where no facility exists to sample, as on a new-build proposal, you are working from literature emission factors or from a comparable operating site. Say which, and say how comparable the proxy is. An objector's consultant will go straight to that assumption.

Feeding the results into dispersion modelling

The emission rate becomes an input to an atmospheric dispersion model run with several years of hourly meteorological data from a representative station. The output is a concentration field, usually reported as the 98th percentile of hourly mean concentrations over a year at each receptor.

That percentile convention exists because odour is an intermittent phenomenon. Reporting an annual mean would hide exactly the short high-concentration episodes that generate complaints.

Model inputs that reviewers check first: the meteorological station and years used, surface roughness, building downwash where structures sit near the source, terrain where the ground is not flat, and receptor heights. Getting building downwash wrong is the single most common technical error, because a plume caught in the wake of a building behaves nothing like a free plume.

Comparing against benchmark exposure levels

The Environment Agency publishes indicative benchmark concentrations for planning and permitting, expressed at the same percentile, and they vary with how offensive the odour character is. The most offensive categories, including sludge, rendering and untreated animal waste, attract the lowest benchmark. Less offensive categories such as brewing or confectionery attract a higher one.

Check the current figures at the time of writing rather than lifting them from an older report, and state the category you have applied and why. The offensiveness category is a judgement, and it is where a reviewer will push, because moving a source from one category to another changes the conclusion without changing a single measurement.

Receptor sensitivity matters alongside the benchmark. Housing, schools and hospitals are treated differently from an industrial estate or a farmyard, and the reasoning follows the assessment framework used for odour assessment work generally.

Cost, programme and proportionality

Olfactometry is an expensive line item, and the cost scales with the number of samples rather than the size of the scheme. A meaningful emission profile needs samples across operating conditions, and each one carries laboratory panel costs.

The programme implication is usually more painful than the cost. Sampling has to happen when the source is operating representatively, the laboratory needs booking, and the modelling follows afterwards. On an application with a determination clock already running, that sequence can take longer than the consultation period.

Scope it early. Where a ventilation and extraction statement for a commercial kitchen is the real requirement, commissioning olfactometry is a costly answer to a question nobody asked.

When a qualitative assessment is enough

For most planning applications it is. A qualitative assessment working through frequency, intensity, duration, offensiveness and location, supported by a standard extraction or abatement specification, satisfies the great majority of cases.

The situations that justify measurement are narrower: a large waste, agricultural or industrial source close to housing, an existing site with a complaint history where the operator needs to demonstrate the effect of an upgrade, an appeal where the odour evidence is contested, or a permit variation where the regulator has asked for it.

There is one more case worth naming. Where an applicant has already been refused once on odour grounds, a qualitative reassessment rarely changes an officer's mind, because it is the same kind of evidence that failed before. Measured data, from the actual source or from a properly justified proxy, is the thing that shifts the position, and the cost of it is small against a second refusal.

Intensive livestock cases sit on the boundary, and the way distance and modelling interact there is set out in odour from a poultry unit. Where dust or bioaerosols are also at issue, scope the air quality assessment alongside rather than afterwards, because the sampling visits can often be combined.

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This is general guidance, not legal advice. Councils usually expect a suitably qualified author for odour assessment work, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.