Odour assessment for a hot food premises using FIDOL

Odour from a commercial kitchen is assessed qualitatively, using five factors that together decide whether an odour amounts to a problem: frequency, intensity, duration, offensiveness and location. The initials spell FIDOL, and the framework comes from Environment Agency odour guidance, with the planning application of it set out in guidance from the Institute of Air Quality Management and in the long-standing guidance on control of odour and noise from commercial kitchen exhaust systems.

For a hot food premises, the practical consequence is that you cannot answer an odour objection with a filter specification alone. The officer wants to know how often the cooking happens, how strong the smell is, how long it lasts, how unpleasant that type of cooking smells and how close the nearest window is. Filtration is the response, not the assessment.

What the FIDOL factors are

Each factor does a different job, and they interact rather than adding up.

Frequency is how often exposure occurs. A premises open seven evenings a week presents a different case from a lunchtime-only cafe.

Intensity is the perceived strength of the odour at the receptor, not at the source.

Duration is how long each exposure lasts. Continuous extraction during a six-hour service period is a different exposure from 20 minutes of frying.

Offensiveness is about the character of the odour and its acceptability. Guidance groups odour types into broad bands of offensiveness, and cooking odours sit across them: baking and coffee roasting at the less offensive end, and fish frying, chargrilling and heavily spiced cooking at the more offensive end.

Location covers the sensitivity of receptors and their proximity, the layout of buildings, the presence of enclosed courtyards or light wells, and prevailing wind. A flat above the premises with an openable window three metres from the flue is the most sensitive location there is, and it is also the most common one.

Frequency, intensity and duration in practice

These three are established from the operation rather than from the plant, which is why an odour assessment should begin with the business, not the ductwork.

State four things in the assessment. The opening hours and the cooking hours within them, which are not the same. The days of the week. The cooking methods used, by equipment type. And the seasonal pattern where one exists. A premises that chargrills at weekends and reheats during the week has two exposure cases and should present both.

Intensity is where honest assessments part company with optimistic ones. Intensity at a receptor depends on the source strength, the dilution achieved between the discharge and the receptor, and the presence of anything that stops dispersion. Discharging at low level into an enclosed rear courtyard produces high intensity at short range whatever the filtration, because there is nowhere for the plume to go.

Offensiveness and the type of cooking

Cooking method drives odour more than cuisine labels do. Deep frying, chargrilling over an open flame, wok cooking at high temperature and roasting fatty meats all produce strong, persistent odour with a high grease load. Baking, steaming and boiling produce much less.

This matters for two practical reasons. The odour control level required follows the risk, and the risk follows the cooking method and the number of meals served. And a condition restricting the cooking method is a genuine option where the risk cannot otherwise be managed, so a business that wants to keep its options open should apply for the cooking it actually intends rather than understating it.

Grease is the mechanism behind most failures in service. A high grease load blinds a carbon filter, coats duct walls and reduces fan performance, and the odour complaint that arrives 18 months after opening is usually a maintenance story rather than a design one.

Location and receptor sensitivity

Sensitivity is not a matter of opinion. Residential windows, particularly bedrooms, schools, hospitals and hotel rooms are high sensitivity. Offices and shops are lower. A commercial yard is lower again.

The features that make a location difficult:

• habitable windows above or immediately adjacent to the premises
• an enclosed rear courtyard, light well or narrow alley where the plume cannot disperse
• a terminal below the roofline of a taller adjoining building, which causes downwash
• a flue on the same elevation as openable windows, at a similar height
• roof terraces and balconies, which are now common and are frequently forgotten

Where the site has any of these, the design has to respond before the assessment is written. Increasingly the answer is a flue taken up the rear elevation to above the ridge, which brings its own planning issues on a listed building or in a conservation area, and those need addressing in the same application rather than left for later.

Filtration selection against the risk level

Odour control for kitchen exhaust is specified in escalating levels, matched to the assessed risk. In broad terms:

Two points about carbon. It has a finite life that depends on loading, so the assessment should state the assumed change interval and the maintenance arrangement. And carbon needs contact time, so an undersized bed in a high-velocity duct does little regardless of the tonnage of media.

Electrostatic precipitators need cleaning to work, and ultraviolet and ozone systems need residence time in the duct and careful handling of ozone. All of these are legitimate technologies. All of them fail without maintenance, which is why a maintenance contract is worth offering as part of the submission.

Discharge height and termination

Termination is the one thing that improves dispersion for every risk level, and it is usually the least costly part of the design to get right.

The guidance for commercial kitchen exhaust recommends discharging vertically, at high level, terminating at least a metre above the roof ridge or the eaves where a ridge discharge is not practical, and at an efflux velocity high enough to give the plume momentum, commonly quoted in the region of 10 to 15 metres per second. Verify the current figures in the guidance rather than taking them from a previous job.

Other termination rules that matter: no cowl or fitting that deflects the plume downwards, adequate separation from openable windows and from air intakes, and no discharge into a courtyard or under a balcony. A terminal that satisfies the odour case may create a noise problem, so coordinate it with the noise design and with the background measurements described in our note on unattended noise monitoring duration, position and validity. The same dispersion logic applies to combustion flues, covered in chimney heights, biomass boilers and the Clean Air Act.

An illustrative case. A three-storey terrace with flats above proposes a takeaway with a chargrill and a fryer at ground floor. Cooking hours are 16:00 to 23:00, seven days. The nearest openable bedroom window is on the rear elevation, 4 metres from the proposed flue position. Risk assessed as high on cooking method and receptor proximity. The design response is grease filtration plus electrostatic precipitation plus carbon, a duct taken up the rear elevation to 1 metre above the ridge, efflux velocity at the upper end of the recommended range, and a quarterly maintenance regime with records. Without the flue extension, no amount of filtration answers the objection.

Recording the assessment for the officer

An odour submission that an environmental health officer can approve contains, in order:

1. The operation: hours, cooking hours, methods, equipment schedule and meals served.
2. The receptors: a plan and elevation showing the nearest sensitive windows with distances and levels.
3. The FIDOL assessment, factor by factor, with the risk conclusion stated.
4. The control system: filtration train, with manufacturer performance data, contact times and airflow.
5. The termination: height above ridge, direction, efflux velocity, shown on an elevation.
6. The maintenance regime: what is cleaned or replaced, how often, who does it, and where the records are kept.
7. The proposed conditions, drafted by you.

Put the same figures in the Odour Assessment and the Ventilation and Extraction Statement so the two do not contradict each other. Odour work of this kind is normally expected from a suitably qualified author, and councils will usually want professional review before they rely on it.

One honest uncertainty. Odour assessment is qualitative, and two competent assessors can reach different risk conclusions on the same premises. That is why the design margin matters: a scheme that just scrapes through on assumptions has no room left when the officer takes a more cautious view.

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

Draft your ventilation and odour documents on PlanScope, then send them for professional review. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.