Olfactometry panels and odour measurement standards

Olfactometry odour measurement is the only way to put a number on how strong an odour is, and in planning it matters because dispersion modelling needs an emission rate to work from. The method is dynamic dilution olfactometry to BS EN 13725, which uses a screened human panel rather than an instrument. That makes the panel, the sampling and the laboratory accreditation the three things to check before you believe any odour figure in a planning report.

What olfactometry measures

The output is odour concentration, expressed in European odour units per cubic metre. One odour unit per cubic metre is the concentration at which half a standard panel can just detect the odour, so the number is a measure of detectability rather than of offensiveness or of chemical content.

That distinction matters in planning arguments. Two sources with the same measured concentration can produce very different levels of complaint, because one smells of baking bread and the other of rendering waste. Assessment benchmarks deal with this by applying a different criterion according to how offensive the odour is judged to be.

Nor does olfactometry tell you what the odour is. Chemical speciation is a separate analysis, and it is the one you need if the question is about a specific substance with a health-based limit rather than about amenity.

Panel selection and screening

A panel is a measuring instrument that has to be calibrated. The standard requires panel members to be screened against a reference gas, n-butanol, and only admits people whose individual detection threshold falls inside the band it specifies, with acceptable repeatability across repeated tests.

Screening has to be ongoing rather than a one-off qualification, because sensitivity drifts. Panel members with a cold, a recent cigarette or strong perfume are excluded on the day. Panel size is prescribed, and results from a panel that fell below the minimum during a session should be discarded rather than reported with a caveat.

Ask for the panel records. A laboratory operating properly will produce them without difficulty, and their absence from a report is a reason to question every figure in it.

Sampling method and holding times

Samples are drawn into bags made from a low-odour film, pre-flushed with the sample gas so the bag does not absorb or contribute odour. For stack and duct sources a heated lance and a dilution probe may be needed where the gas is hot or wet, because condensation inside the bag changes the result.

Area sources need a different technique. A flux hood or wind tunnel placed on the surface of a lagoon, a compost windrow or a landfill cap measures the emission per unit area, which is then scaled across the whole source. The number of placements and their positions are judgement calls, and they should be recorded on a plan.

Holding time is the detail that ruins surveys. The standard limits the interval between sampling and analysis, commonly cited as 30 hours, so the laboratory slot has to be booked before anybody goes to site. A Friday afternoon sample with a Monday analysis is outside the limit and the results are not usable.

Sample at representative operating conditions, and say what those were. A survey taken while the odour abatement plant was running at full capacity on a cool, still morning tells you very little about a summer afternoon with the doors open.

Laboratory accreditation

Look for accreditation by the United Kingdom Accreditation Service to the international laboratory competence standard, with the olfactometry method named on the schedule of accreditation. Accreditation of the laboratory in general is not the same as accreditation for this method.

Three further checks are worth making. Whether the laboratory participates in inter-laboratory comparison exercises and what its performance was. Whether the olfactometer is calibrated against the reference gas at the stated frequency. And who took the samples, because a well accredited laboratory analysing badly collected samples produces precise answers to the wrong question.

Converting results to emission rates

The concentration on its own cannot be modelled. Multiply the measured concentration by the volumetric flow rate of the source to give an odour emission rate in odour units per second, and that is the figure a dispersion model takes as an input.

For a stack, the flow rate comes from a duct traverse measured at the same time as the sample. For an area source, the emission per unit area from the flux measurement is multiplied by the total area, which means the area estimate needs to be defensible. For a building with natural ventilation, the flow has to be estimated, and that estimate becomes the largest single uncertainty in the whole assessment.

Modelled results are then compared against the indicative benchmarks the Environment Agency publishes for odour exposure, which are expressed as a percentile of hourly mean concentrations and vary with how offensive the odour is. The benchmark selection is itself a point of argument, and it should be justified rather than asserted.

Uncertainty and how to report it

Olfactometry carries wide confidence intervals even when the standard is followed to the letter. A single sample from a single day is a snapshot, and no honest report presents one as a characterisation of a source.

The way to handle it is replication and transparency. Take duplicate samples, sample on more than one day, report the spread alongside the mean, and carry the uncertainty through to the modelled result by running the model at the upper end of the measured range.

One honest uncertainty worth stating in any report: the relationship between modelled annoyance thresholds and actual complaint behaviour is weak, because complaints depend on expectation, local history and the presence of an existing source. An assessment that predicts compliance with a benchmark has not predicted an absence of complaints, and saying so in the report is better than being asked about it at committee.

Commissioning a survey properly

Write the specification rather than accepting a quotation for "an odour survey".

1. Name the sources to be sampled, with the number of samples at each and the number of survey days.
2. Specify the operating conditions required during sampling, including worst-case scenarios such as full production or waste turning.
3. Require the laboratory to be accredited for dynamic olfactometry and name the standard.
4. Require duplicate samples, with the spread reported alongside the mean.
5. Require the volumetric flow measurement to be taken at the same time as each sample.
6. Specify the dispersion modelling to follow, including the receptor set, the met data and the benchmark to be applied.

Councils usually expect a suitably qualified author for odour work, and professional review is strongly advised before submission. A draft structured to the right methodology is a sound starting point for the specialist who signs it off, and the scope of the report type is set out on the Odour Assessment page. Where the source is kitchen extract rather than an industrial process, the relevant document is usually a Ventilation and Extraction Statement instead.

Reviewers who can check this kind of work can be found through the professional directory, and every version of the report should carry a named author and a date for the reasons set out in why every planning report needs a named author and date.

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, and for specialist technical reports, qualified professional input may be required.