FIDOL factors and odour effect descriptors explained

Odour is assessed by description rather than by a single measured number, and the FIDOL factors odour practitioners use are the reason why. Frequency, intensity, duration, offensiveness and location together decide whether an odour is a planning problem, because the same smell can be trivial at one receptor and unacceptable at another 30 m away. That is why an environmental health officer will not accept "the extraction system meets the relevant standard" as an answer.

The five factors come from long-standing regulatory practice and sit at the centre of the Institute of Air Quality Management approach to odour in planning.

What each of the five factors covers

Frequency is how often the odour is detectable at the receptor. A fryer running two evenings a week is a different proposition from a continuous process.

Intensity is the perceived strength when it is detectable, which is not the same as concentration at the stack. Duration is how long each episode lasts. A brief pulse at the start of a service is tolerated in a way that a three-hour plume is not.

Offensiveness, sometimes written as offensiveness or character, is the hedonic tone: how unpleasant people find that particular smell. Frying fat, rendering and sewage sit at one end. Bakery and coffee roasting sit at the other, though even pleasant odours generate complaints when they are constant.

Location is the sensitivity of the receptor and its surroundings. Homes, schools, hospitals and care homes are high sensitivity. Offices and shops are medium. Industrial yards and open farmland are low.

Why odour is described rather than measured

Concentration can be measured. Dynamic olfactometry to BS EN 13725 produces an odour concentration in European odour units per cubic metre for a sampled source, and dispersion modelling converts that into predicted concentrations at receptors, usually expressed as the 98th percentile of hourly means over a year.

Environment Agency guidance sets indicative benchmarks that vary with how offensive the odour is, with the tightest figure applied to the most offensive odours and progressively higher figures for moderately and less offensive ones. Check the current version of that guidance before quoting a number, because it has been revised.

For most planning cases, none of this happens. Olfactometry and modelling are proportionate for a rendering plant or a waste facility. For a takeaway below flats, the assessment is a qualitative one built on the FIDOL factors, the separation distance, the abatement proposed and the assessor's judgement.

From risk to effect descriptor

The method combines the odour exposure risk at a receptor with the sensitivity of that receptor to produce an effect descriptor. Descriptors run from negligible through slight adverse and moderate adverse to substantial adverse.

A moderate adverse descriptor at a residential receptor is normally enough to attract a condition. A substantial adverse descriptor is normally a refusal reason unless the abatement case is rewritten. Say which descriptor you have reached, and say it in the words the method uses, because officers compare your conclusion against the ones they have seen before.

Uncertainty deserves a sentence of its own. Where the process is unusual or the surroundings are sheltered, an honest assessment says so rather than reaching for the descriptor that suits the applicant.

Abatement, dispersion and separation

Three levers, applied in order.

1. Reduce the odour at source, by changing the process, the menu or the throughput.
2. Abate what remains, using grease filtration, then electrostatic precipitation or ultraviolet treatment, then activated carbon as the final stage where the odour is offensive.
3. Disperse what survives abatement, by terminating the discharge high and fast, well clear of nearby openings and of any zone where the plume gets caught in the lee of a taller building.

Carbon becomes hard to avoid where the discharge cannot be taken above the ridge of the building, or where windows of residential accommodation sit within a short distance of the termination point. It also carries a maintenance obligation, since spent carbon does nothing, and councils condition maintenance records for exactly that reason.

Separation distance does more work than any single piece of equipment. A unit 60 m from the nearest window with a clear discharge path needs far less than the same kitchen with a flat roof outlet 4 m below a bedroom.

Topography and shelter belong in the same paragraph. A discharge into a narrow enclosed yard surrounded by taller buildings behaves nothing like the same discharge on an open frontage, because the plume gets trapped in the recirculation zone behind the obstruction instead of dispersing. Assessors who have walked the site pick this up. Assessors working from a plan often do not, and the first anyone hears about it is a complaint in the second month of trading.

What environmental health officers actually ask for

Expect a request for the schematic of the whole system, the specification and rating of each filtration stage, the fan duty and the discharge velocity, a drawing showing the termination point relative to the ridge and to neighbouring openings, and the proposed maintenance regime with frequencies. Noise data for the fan comes in the same request, because plant noise and odour are usually handled by the same officer.

Where the case is finely balanced, some councils ask for a commitment to an odour management plan and a complaints procedure. Offering both in the application usually costs less than negotiating them after an objection has landed.

An illustrative example. A hot food unit occupying the ground floor of a converted terrace, with flats above and a rear yard, proposes a duct on the rear elevation terminating 1 m above the ridge, with grease filtration, an electrostatic stage and carbon, discharging vertically at high velocity without a cowl. On a qualitative FIDOL assessment the residual effect would typically be slight adverse at the flats above and negligible at the properties opposite. Change one variable, such as terminating at first-floor level because the freeholder refuses a riser, and the descriptor moves. The example is illustrative only.

Where odour work overlaps with other reports

An Odour Assessment answers the effect question. The Ventilation and Extraction Statement answers the design question, setting out what is being installed and how it will be maintained. Many applications need both, and they must describe the same system.

If the site also sits inside an air quality management area, the environmental health team will read the odour case alongside the emissions case, and combustion plant serving the kitchen may attract separate comment. Where the complaint history involves both smell and plant noise, the agent of change principle decides who pays to mitigate, which matters when new flats are proposed above an existing kitchen rather than the other way round.

Councils expect odour assessments from a suitably qualified author and professional review is strongly advised. You can see which documents your change of use is likely to need before instructing anyone. 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.

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