Odour assessment and the FIDOL factors

An odour is not assessed by whether it smells. It is assessed against five characteristics that together describe exposure: frequency, intensity, duration, offensiveness and location. The FIDOL factors are the framework an environmental health officer will use to judge your extraction scheme, your waste transfer station or your restaurant, and understanding them changes what you put in the application.

The point of the framework is that no single factor decides anything. A strong smell for two minutes a year is not a problem. A weak smell for sixteen hours a day next to a nursery might be.

What the five factors describe

Frequency is how often the odour is detectable at a receptor, usually expressed as a percentage of hours in a year. Intensity is the perceived strength, judged on a standardised scale running from not perceptible to extremely strong. Duration is how long each episode lasts, which is separate from how often episodes happen.

Offensiveness, sometimes called hedonic tone, is the character of the odour rather than its strength. Assessors group processes into broad bands, with sewage sludge, rendering and intensive livestock at the most offensive end, food frying and brewing in the middle, and bakery or coffee roasting at the least offensive. The same odour concentration from a rendering plant and a bakery will be judged very differently.

Location covers the sensitivity of the receptor and the nature of the surroundings. Housing, schools, hospitals and care homes are high sensitivity. Offices and workplaces sit lower. A public footpath crossing an agricultural holding is lower again.

Location also carries an expectation element. People living beside a working farm anticipate farm smells in a way that occupiers of a new town centre flat do not, and assessors record that context rather than pretending every receptor is identical.

Field assessment and sniff testing

Human noses remain the primary instrument, because no chemical measurement captures offensiveness.

A field assessment involves trained assessors visiting predetermined points on a grid or downwind of a plume on a schedule, recording detectability and intensity against a reference scale, with wind speed, direction and temperature logged at the same time. Enough visits across enough weather conditions are needed for the result to mean anything. One afternoon in June tells you nothing about a still, damp October evening, which is when complaints arrive.

The weakness is obvious. Assessors differ, noses fatigue, and the method is expensive to run properly. The strength is that it measures the thing people actually complain about.

Olfactometry in a laboratory measures odour concentration in European odour units per cubic metre by diluting a sample until a trained panel can just detect it. That gives a number for the source, which can then be fed into a dispersion model.

Modelling and where it stops being useful

Dispersion modelling predicts concentration at receptors from a source term, a release height and meteorological data. It produces contour plots that look authoritative, and it is the only way to test a scheme that has not been built.

Treat the source term with suspicion. Model output is only as good as the emission rate put into it, and emission rates for a proposed kitchen, a planned composting operation or a new poultry unit are often taken from literature values for a broadly similar process. That is reasonable practice. It is also the largest uncertainty in the whole exercise, and an assessment that does not discuss it is incomplete. Our note on poultry unit odour modelling and separation distance goes into how sensitive those results are to inputs.

Modelling also handles intermittent, short-duration releases badly, and those are exactly the episodes that generate complaints.

Use it for what it does well: comparing options, sizing a stack, and showing the relative effect of moving a discharge point. Treat a single contour line as a screening result rather than a finding of fact.

Benchmark criteria and how they are applied

Regulators publish indicative benchmark concentrations expressed as a high percentile of hourly averages over a year, with a tighter benchmark for the most offensive odours and a looser one for the least offensive. The benchmark is a screening tool. Meeting it does not prove there will be no complaints, and exceeding it does not prove a refusal.

What the benchmark does is convert the offensiveness factor into a number a planning officer can work with. It also forces the assessor to state which offensiveness band the process falls into, which is where arguments usually start. An industrial bakery producing a cooked-fat smell rather than a bread smell should not be assessed as a bakery.

Receptor sensitivity then shapes the judgement. The same predicted exposure can be acceptable next to a distribution warehouse and unacceptable next to housing, and a proposal that introduces new housing near an existing odour source has the agent of change principle to deal with as well.

Designing out the source

The best assessment is a short one, because the source has been dealt with.

Containment first: enclose the process, negatively pressurise the building, and keep doors closed with fast-acting shutters or air curtains. Then abatement sized for the risk, whether that is grease filtration and carbon for a kitchen, biofiltration for composting, or wet scrubbing for a rendering process. Then dispersion, meaning a discharge point high enough and fast enough to carry the plume away from receptors.

Dispersion comes last, always. A tall stack on an uncontrolled source moves a problem rather than solving it, and the planning system has become much less tolerant of that approach. For commercial kitchens specifically, the specification of the discharge point sits alongside the abatement choice, and both belong in a Ventilation and Extraction Statement rather than being left to a building control drawing.

Conditions, management plans and what happens after consent

An Odour Assessment is a report type where councils expect a suitably qualified author, and professional review before submission is strongly advised. The reason shows up at condition stage.

A well-drafted odour condition does three things. It requires the abatement described in the application to be installed before the use starts. It requires an odour management plan covering maintenance intervals, filter replacement, records and a complaints procedure. And it requires those records to be available to the authority on request.

Weak conditions ask for an odour management plan to be submitted later, with no specification of what the plant must be. An officer reading the application months afterwards cannot tell whether the two-stage system in the quotation is the one that was assessed. By the time the plan arrives the equipment is installed and the authority has nothing left to bargain with.

Planning permission does not immunise a business from statutory nuisance action under the Environmental Protection Act 1990. That point deserves emphasis with any occupier who assumes consent is the end of the matter, and we cover it in statutory nuisance after planning permission is granted.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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