Odour assessment methods and the dose response question

An odour assessment is trying to predict annoyance, and annoyance does not track concentration in a straight line. That is the whole difficulty. A dispersion model produces hourly mean concentrations, while people notice short sharp peaks, so the assessment has to bridge between what the model outputs and what a resident would complain about. The industry answer is to express the prediction as a high percentile of hourly means over a year and compare it against benchmark concentrations that differ according to how offensive the odour is.

Get the offensiveness category and the receptor sensitivity right and the rest is modelling. Get them wrong and the numbers are irrelevant.

What an odour assessment is trying to predict

The output that matters is exposure at a receptor, not emission at a source. So the assessment has three parts: characterise what leaves the site, predict how it disperses, and judge the effect on the people exposed.

The planning question behind it is whether the proposal gives rise to an unacceptable loss of amenity. That is a lower threshold than statutory nuisance, which is why a scheme can be refused on odour grounds even though the operator would not be committing an offence. It also means an Odour Assessment has to address amenity in planning terms rather than reporting compliance with a permit.

Two scenarios arise and they are assessed differently. A new odour source near existing receptors, where the question is the effect of the proposal. And new sensitive development near an existing source, where the question is whether future occupiers would be exposed to unacceptable conditions and whether the existing operator's position would be prejudiced.

Source characterisation and emission rates

Nothing downstream is better than the emission rate, and there are three ways to get one.

Measurement by dynamic olfactometry to the European standard, where a sample is presented to a panel and the concentration at the detection threshold is determined in European odour units per cubic metre. This is the most reliable route and it needs an existing, operating source to sample.

Derivation from an analogous site, using published emission rates for comparable processes, scaled by throughput or area. This is normal for a proposed facility that does not yet exist, and the comparability of the analogue is the weak point. Say which site or dataset the rate came from.

Calculation from a mass balance where the chemistry is understood, which works for some processes and not for most.

Point sources through a stack are straightforward to model. Area sources, such as a lagoon, a windrow or an open building, are harder, and fugitive emissions through doors, vents and failed seals are harder still. Many assessments underestimate the fugitive contribution because it is the hardest to quantify, and an honest report flags that rather than omitting it.

Dispersion modelling and benchmark concentrations

A standard atmospheric dispersion model run with several years of hourly meteorological data, terrain where relevant and building effects where the source is near structures, produces a concentration field. The reported metric is the 98th percentile of hourly mean concentrations over the year, which is the convention the sector uses as a proxy for the peaks that drive annoyance.

That percentile is then compared against benchmark concentrations that vary by offensiveness. The recognised approach, originating in environmental regulator guidance and carried into the industry planning guidance, uses a graded set: the most offensive odours, such as those from processes handling animal or putrescible waste, carry the lowest benchmark, moderately offensive odours a middle value, and the least offensive, such as some food processing or brewing odours, the highest. Take the current figures from the published guidance rather than from an earlier report, and state which category you have assigned and why.

Two technical points decide a lot of assessments. Peak to mean ratios, which convert hourly means into the short duration concentrations people actually perceive, and which depend on source type and distance. And model sensitivity to building downwash, which on a site where the stack is lower than an adjacent building can change predicted ground level concentrations substantially.

Receptor sensitivity and the exposure context

Benchmarks are a starting point rather than a pass mark. The effect on a receptor depends on how sensitive it is and how it is used.

High sensitivity receptors include housing, hospitals, schools and outdoor areas used for extended periods. Medium sensitivity covers places of work and commercial premises where people are present for long periods but not living. Low sensitivity covers industrial areas, footpaths and roads where exposure is brief.

Combining exposure with sensitivity produces an effect descriptor, and the planning judgement follows from the descriptor rather than from the raw concentration. A predicted exposure at the benchmark is a different matter at a nursery garden from the same figure at a factory car park.

Context matters too, and this is where the professional judgement sits. The existing odour environment, the duration and frequency of exposure, the time of day, and whether the odour is of a kind the area already experiences all affect how an effect should be characterised. None of that licenses an assessment to argue a result away, and the report should separate the modelled prediction from the judgement applied to it.

Qualitative assessment where modelling is disproportionate

For a small source with a short pathway, modelling is often unjustifiable, and the recognised framework is a structured qualitative assessment.

The factors are frequency, intensity, duration, offensiveness and location. Work through each with evidence rather than assertion: how often the odour arises from the operation as proposed, how strong it is at source, how long each episode lasts, what the substance is, and the geometry between source and receptor.

Add two things a model cannot provide. A site visit with a recorded sniff survey at the receptor positions, under conditions noted at the time. And the complaint history, obtained from the local authority's environmental protection team, which is the only direct evidence of how an existing source actually affects people. A documented absence of complaints over several years is weak evidence of acceptability on its own, and it is still worth having.

Qualitative work is also the right approach to an extract system on a commercial kitchen, where the design and discharge arrangements do the work rather than a model. That exercise sits inside a Ventilation and Extraction Statement rather than a dispersion study.

Mitigation hierarchy for odour sources

Work down the hierarchy and say in the report which steps were considered and rejected.

1. Avoid at source, by process selection, feedstock control or operating practice. The most effective odour mitigation is not generating the odour.
2. Contain, by enclosing the activity in a building under negative pressure with fast acting doors.
3. Capture and treat, using biofilters, chemical scrubbers, carbon adsorption or thermal oxidation according to the compound and the loading.
4. Disperse, by stack height and efflux velocity, which moves exposure rather than reducing emission and so sits low in the hierarchy.
5. Separate, by layout, buffer distance and the position of sensitive uses within a mixed scheme.
6. Manage, through an odour management plan with monitoring, maintenance and a complaint response procedure.

Treatment plant has to be designed for the loading and maintained, and an assessment that assumes a 95 per cent removal efficiency without specifying the plant and its maintenance regime is assuming the conclusion.

Conditions and complaint response procedures

Where the activity needs an environmental permit, planning should not duplicate pollution control. National guidance is clear that the planning system should assume the separate regulatory regime will operate effectively, so a condition that restates a permit requirement is both unnecessary and, if the permit changes, unhelpful.

What planning conditions can properly do is secure the things permits do not reach: the layout, the stack position and height, the enclosure, the hours of the odour generating activity, an approved odour management plan where no permit exists, and a review mechanism.

An odour management plan worth conditioning contains the source inventory, the control measures with maintenance intervals, monitoring arrangements including routine sniff testing, trigger levels for action, the complaint recording and response procedure with timescales, and a named responsible person. Keep it operable. A plan nobody on site can follow at 22:00 on a Friday achieves nothing.

One honest uncertainty to carry into any report. Two competent assessors can assign the same source to different offensiveness categories and reach different conclusions on the same modelled output, and there is no appeal-proof answer to that. The way to handle it is to state the category, state the alternative, and show what the result would be under each. Where neighbouring effects on light or noise arise from the same plant, the same even handedness applies, and the approach to what counts in light nuisance objections carries across. Odour work also frequently sits beside an Air Quality Assessment and the two should use the same source and meteorological assumptions.

This is general guidance, not legal advice. Odour assessment is a specialist discipline and councils usually expect a suitably qualified author, so professional review is strongly advised before submission.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.