Scientific blog · Design and verification
Smoke escaping the hood: why capture must be checked before treatment
Artur Cokaj ·
Air cleaner performance concerns the stream passing through it. Distinguishing capture, separation and air control in a professional kitchen.
Smoke escaping the hood is not conveyed directly to the air cleaner connected to that duct. This is the first point to clarify when smoke escapes into the kitchen during service despite treatment being installed. Before attributing the problem to the separator, trace the effluent's route from its source.
Two efficiencies, two measurement boundaries
The HSE guide to local exhaust ventilation distinguishes the hood, where the contaminant enters, from the device treating extracted air. These are successive functions. Performance measured between the air cleaner's inlet and outlet does not automatically describe the proportion captured at the cooking source. HSE, HSG258, third edition 2017, page 6.
We can define them using a mass balance model. The captured fraction is the mass of contaminant conveyed to treatment divided by the mass generated by the source over the same interval. The separated fraction is the mass removed by treatment divided by the mass entering it. Both fractions must refer to the same contaminant and operating conditions.
An example: 90% at the air cleaner is not 90% of the source
Suppose, purely for calculation, that a source generates 100 grams of a contaminant per hour and the hood conveys 80 to the air cleaner. If the cleaner separates 90% of what it receives, it removes 72 grams per hour. Another 8 remain in the outlet stream, while 20 were not captured. The fraction removed relative to the source is therefore 72%.
Here, the calculation is 0.80 × 0.90 = 0.72. The model assumes a single treatment path, without duct losses, deposits, transformations, accumulation or recapture of the fraction that initially escaped. The 28 grams per hour not removed follow two different paths: they are not a workplace concentration and cannot predict people's exposure.
These figures are hypothetical, not ETC performance data. The balance concerns the mass of a defined contaminant: it must not be transferred directly to odour intensity or a neighbour's perception. These aspects are discussed separately in the article on odours from professional kitchens.
The hood must intercept the actual plume
In hot processes, an overhead hood receives a plume that widens as it rises. The HSE guide considers hood position, opening dimensions and cross-draughts; extraction must provide sufficient flow to evacuate the contaminated stream entering the hood. These principles require assessment of the actual geometry, rather than deriving a universal flow rate solely from hood length. HSG258, pages 39 and 42.
Air entering the kitchen is also part of the design
The HSE catering information sheet recommends considering replacement of extracted air and the source of incoming air, avoiding contaminated areas. It also addresses cooking load and appliance layout. Asking how many cubic metres per hour the fan extracts is therefore insufficient: the source of replacement air and its path through the room must be known. HSE, CAIS10 rev. 3, July 2017, pages 2–3.
To prepare for an assessment, the operator can record which appliances were in use when the escape occurred, which doors were open and which supply-air systems were active. This information is for the technician, not proof of compliance. It describes a reproducible event rather than a general impression of inadequate extraction.
Observe airflow and measure the installation
The HSE guide distinguishes tracer observations from flow rate and pressure measurements. The former help reveal air movement; the latter provide data that can be compared over time. The technician must choose the method for the particular installation. A visual demonstration alone does not establish a capture percentage. HSG258, pages 76–77.
When requesting an assessment, specify:
- the processes and appliances being tested;
- measurement locations and supply-air conditions;
- the criterion used to assess capture;
- the data to retain for subsequent inspections.
ETC treatment selection must also start with this installation description. A supply sized for the conveyed stream does not, on its own, document source capture. The relationship between fan and circuit is discussed in the article on motors and fans for fume extraction.
Sources consulted on 16 October 2026. HSE guides are used for the technical principles of extraction: their references to British legislation are not Italian requirements. The numerical example is an original balance with explicit assumptions.