Local capture of pollutants

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EDIAQI TOPIC BRIEF · LOCAL EXTRACTION & SOURCE CAPTURE

Local Capture of Pollutants: Principles, Capture Efficiency, and System Design

Local capture is a frontline indoor air quality engineering control designed to intercept high-intensity airborne contaminants and excess moisture directly at the point of origin, exhausting them outdoors before they disperse into the occupied breathing zone.

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Quick Facts

Illustration representing local exhaust extraction hood

Control Tier: Point-source engineering control
Capture Efficiency: 70% to > 90% (when ducted)
Key Targets: Cooking PM, NO₂, VOCs, steam, odors
Face Velocity: 0.25 to 0.5 m/s at boundary
Exhaust Type: Ducted outdoor discharge required
Pressure Risk: Depressurization / backdrafting
Reviewed: 2026-09-30

Explore further
EDIAQI project website
Source Control
Ventilation Principles
Filtration Systems
EDIAQI Decision Tree
IAQ Simulation Tool

Jump to: Core Purpose • Aerodynamics • Ducted vs Recirculating • Applications • Make-Up Air • Action Guidelines • Application Matrix • Technical Models • EDIAQI Research • Related Pages

70% to 90%

Cooking Particle Capture

Average reduction in dispersed particulate matter achievable when operating an outdoor-venting extractor hood over rear cooking hobs.

0.25 to 0.5 m/s

Minimum Capture Velocity

Standard capture air velocity required at the boundary of a thermal or chemical plume to overcome cross-drafts and room turbulence.

100% Outdoor Exhaust

True Gas & Moisture Removal

Dedicated ducted extraction is the only configuration capable of removing combustion gases (NO₂, CO) and water vapor from indoor air.

Why Local Capture Matters

Indoor human activities generate acute, concentrated contaminant plumes that far exceed regular background concentrations. High-temperature cooking, domestic combustion appliances, sanitary moisture loads, office printing equipment, and craft or cleaning chemicals introduce large quantities of fine and ultrafine particles (PM_2.5, UFP), nitrogen dioxide (NO₂), carbon monoxide (CO), aldehydes (e.g. acrolein, formaldehyde), and volatile organic compounds (VOCs) directly into living spaces.

Attempting to control these concentrated emissions through general room dilution ventilation alone requires excessive air exchange rates, leading to substantial energy waste and draft discomfort. Local capture operates on the physical principle of point-source containment: by intercepting the pollutant plume at or near its origin, local exhaust ventilation (LEV) evacuates the contaminants before they mix into the broader room volume. This preserves indoor environmental quality and lowers occupant respiratory exposure while minimizing total building airflow requirements.

Working Principles and Aerodynamics

Local capture systems function by creating a localized negative pressure field that induces an air stream toward an exhaust opening or hood. The aerodynamic success of a local capture system depends on four interdependent physical factors:

• Thermal Buoyancy and Plume Convection: Heated processes (such as stovetop cooking or industrial soldering) generate upward convective thermal plumes. An effective capture hood must be positioned directly in the trajectory of the rising plume, encompassing the entire expanding thermal cone.
• Capture Velocity: Capture velocity (V_x) represents the minimum air speed required at the farthest point of emission to overcome competing ambient air currents and pull the contaminants into the hood face. For thermal domestic cooking plumes, capture velocities between 0.25 m/s and 0.5 m/s are typically required at the stove perimeter.
• Distance from Source (The Inverse-Square Penalty): Exhaust air velocity drops precipitously as distance from the hood face increases, diminishing proportionally with the square of the distance (x²). Doubling the distance between a cooking pot and an extractor hood reduces the centerline suction velocity by roughly 75%, allowing lateral air currents to divert contaminants into the room.
• Enclosure and Baffles: Physical side panels, rear walls, or flanged edges channel the induced airflow, eliminating inactive air drawn from behind the hood and substantially lowering the required fan exhaust volumetric flow rate (Q).

Ducted Exhaust vs Recirculating Hoods

A fundamental engineering distinction exists between ducted outdoor exhaust hoods and recirculating kitchen cooker units:

Feature / Mechanism	Ducted Local Exhaust (Vented Outdoors)	Recirculating Filter Hood (Non-Vented)
Moisture & Water Vapor	Completely expels cooking and shower steam outside; prevents mold and structural dampness.	Zero removal. Returns 100% of generated water vapor into the living space, heightening condensation risks.
Combustion Gases (NO2, CO)	Safely vents toxic combustion byproducts from gas hobs directly to the exterior atmosphere.	Zero removal. Sorbent pads do not capture NO2 or CO, allowing dangerous toxic accumulation.
Particulate Matter (PM2.5, UFP)	Highly effective (70% to > 90% capture), extracting aerosol plumes outside before inhalation.	Variable capture (30% to 60%). Coarse mesh captures grease; submicron combustion aerosols largely penetrate back into the room.
Odor & VOC Control	Continuously extracts pungent aroma compounds, acrolein, and volatile lipids outside.	Partially sorbed by activated carbon beds, but media saturate rapidly and re-emit VOCs if not regularly replaced.

In residential renovations and new building construction, EDIAQI guidelines strongly recommend ducted outdoor exhaust systems over recirculating hoods wherever structurally feasible.

Primary Application Domains

Residential and Commercial Kitchens

Cooking activities represent the single largest episodic source of indoor fine particles, ultrafine particles, and nitrogen dioxide in domestic homes.

• High-temperature cooking (frying, grilling, oil heating) triggers the thermal breakdown of fats, emitting acrolein, formaldehyde, and carcinogenic polycyclic aromatic hydrocarbons (PAHs).
• Gas hobs emit significant quantities of NO₂ and ultrafine particles. In kitchens without active extraction, NO₂ levels regularly breach the WHO 1-hour guideline (200 µg/m³).
• Cooking on rear burners increases the capture efficiency of overhead range hoods by 20% to 40% compared to front burners, as the wall serves as an aerodynamic baffle.

Wet Rooms and Sanitary Facilities

Bathrooms, laundries, and indoor swimming facilities generate acute relative humidity spikes:

• High moisture loads cause surface condensation on cold envelope surfaces, promoting toxic mould colonization and dust mite propagation.
• Dedicated intermittent or continuous mechanical exhaust fans (sized to at least 15 to 30 l/s per room per EN 16798-1) evacuate saturated air before humidity diffuses into bedrooms and living spaces.

Office Printing Hubs and Photocopier Areas

Commercial laser printers and photocopiers emit toner micro-dust, volatile organic compounds, and reactive ozone during heavy print jobs. Isolating printing equipment in dedicated copy rooms equipped with point exhaust grilles prevents ozone and particle dispersion across open-plan workspaces.

Salons, Workshops, and Vocational Classrooms

Art rooms, chemical labs, nail salons, and woodworking shops generate concentrated solvent vapors (toluene, ethyl acetate, acetone) and respirable dust. Flexible articulated LEV arms or downdraft tables capture chemical fumes directly at the work bench, protecting operators and pupils from occupational respiratory irritation.

Pressure Balance and Make-Up Air Dynamics

A critical, often overlooked hazard of local exhaust capture in modern energy-efficient buildings is envelope depressurization.

• Modern European homes constructed under nearly zero-energy building (nZEB) standards possess high airtightness.
• Operating a high-capacity kitchen extractor hood (expelling 300 to 800 m³/h of air) in a tight envelope without dedicated air compensation creates an immediate negative pressure drop (-10 to -30 Pa).
• The Backdrafting Risk: Negative pressure can reverse the natural draft in atmospheric chimneys, wood stoves, or gas water heaters, pulling lethal carbon monoxide (CO) and combustion gases down the flue and into the living area.
• Solution: Building codes require interlocking make-up air dampers or trickle vents that automatically introduce tempered outside air whenever high-volume local exhaust fans operate.

Practical Recommendations for Building Operators

1. Always Specify Ducted Outdoor Extraction

Ensure kitchen cooker hoods and wet-room exhaust systems are ducted directly to the building exterior. Avoid relying on ductless recirculating hoods in kitchens equipped with gas hobs or during frequent frying.

2. Initiate Exhaust Operation Before Emission Begins

Turn on the kitchen range hood 1 to 2 minutes prior to firing cooking burners to establish a stable upward airflow column. Continue running the exhaust for 5 to 10 minutes post-cooking to evacuate lingering submicron particles and VOCs.

3. Cook on Back Burners and Use Fitted Lids

Position high-emission pots and pans on the rear burners of the stovetop. Back burners achieve significantly higher capture efficiency because the kitchen backsplash acts as an aerodynamic suction baffle.

4. Enforce Routine Degreasing and Duct Maintenance

Clean or wash metal mesh grease filters monthly in hot soapy water or a dishwasher to prevent static pressure buildup and fire hazards. Replace activated carbon secondary filters every 3 to 6 months if recirculating configurations are unavoidable.

5. Ensure Adequate Replacement Air

In airtight apartments, crack open a window in an adjacent room or engage dedicated fresh air dampers while operating high-flow kitchen exhaust fans to prevent room depressurization and backdrafting of combustion chimneys.

Target Audience Application Matrix

Target Audience	Practical Value and Action
Homeowners and Residents	Operate range hoods on medium-to-high speeds whenever frying or boiling. Clean metal mesh filters regularly to maintain airflow. If retrofitting a kitchen, prioritize installing outdoor-vented ductwork over recirculating hoods.
Schools and Kindergartens	Equip institutional school kitchens, science chemistry workstations, and craft rooms with dedicated LEV hoods. Ensure exhaust rates meet EN 16798 standards and isolate commercial dishwashing steam from student dining spaces.
Commercial Facility Managers	Isolate high-throughput photocopiers and 3D printing equipment in dedicated rooms with continuous negative-pressure extract fans. Maintain grease extract ducts in cafeteria kitchens to meet fire safety and air hygiene standards.
Municipalities and Housing Planners	Mandate external kitchen and bathroom exhaust ductwork in municipal housing renovation tenders. Prohibit completely unvented gas cooking stoves in new social housing developments.

Simulate cooking emissions and extraction dynamics in your building
Use the EDIAQI IAQ Simulation Tool → to explore how point-source cooking emissions, cooker hood capture efficiency, and background ventilation interact to influence indoor air quality across European building typologies.

[+] Click here to view technical and aerodynamic formulations

Capture Velocity and Volumetric Flow Rate (DallaValle Equation)

For an exterior, unflanged rectangular or round exhaust hood, the volumetric airflow rate (Q) required to produce a design capture velocity (V_x) at a distance (x) along the hood centerline is calculated using DallaValle's classic aerodynamic formulation:

$$Q=V_{x}(10x^{2}+A)$$

Where:

• Q is the exhaust airflow volume (m³/s)
• V_x is the target capture velocity at distance x (m/s, typically 0.25 to 0.5 m/s)
• x is the centerline distance from the hood opening to the emission source (m)
• A is the surface area of the hood opening (m²)

When a physical flange or wall baffle is added around the hood perimeter, air intake from inactive zones behind the hood is suppressed, reducing the required airflow volume by approximately 25%:

$$Q_{\text{flanged}}=0.75\cdot V_{x}(10x^{2}+A)$$

Hood Capture Efficiency in the Indoor Mass-Balance Equation

The physical performance of local capture is defined by the fractional capture efficiency (η_cap), representing the proportion of generated pollutant mass intercepted directly by the hood before escaping into the room:

$$\eta _{\text{cap}}={\frac {E_{\text{captured}}}{E_{\text{total}}}}=1-{\frac {E_{\text{escaped}}}{E_{\text{total}}}}$$

When integrating local capture into the dynamic indoor single-zone mass-balance differential equation, the effective indoor source generation term (S) is modified directly:

$${\frac {dC_{\text{in}}(t)}{dt}}=P\lambda C_{\text{out}}(t)-(\lambda +\lambda _{d})C_{\text{in}}(t)+{\frac {S_{\text{indoor}}(1-\eta _{\text{cap}})}{V}}$$

Where:

• C_in(t) is the indoor pollutant concentration (µg/m³ or ppm)
• C_out(t) is the ambient outdoor concentration (µg/m³ or ppm)
• P is the building envelope penetration factor (dimensionless)
• λ is the room air exchange rate (h⁻¹)
• λ_d is the particle deposition rate onto surfaces (h⁻¹)
• S_indoor is the gross emission rate of the source (µg/h or mg/h)
• η_cap is the local capture efficiency of the LEV system (dimensionless, 0 to 1)
• V is the room volume (m³)

When a ducted hood operates with high capture efficiency (η_cap ≥ 0.85), the net pollutant emission injected into the indoor breathing volume is suppressed by 85%, eliminating concentration peaks without demanding extreme room-wide air change rates.

Duct Aerodynamic Pressure Loss

The total pressure drop (ΔP_t) that the LEV exhaust fan must overcome across the ducting system comprises frictional losses and dynamic fitting losses:

$$\Delta P_{t}=\left(f{\frac {L}{D_{h}}}+\sum K\right){\frac {\rho v^{2}}{2}}$$

Where:

• f is the Darcy-Weisbach friction factor (dimensionless)
• L is the total duct length (m)
• D_h is the hydraulic duct diameter (m)
• K is the local resistance coefficient for elbows, grilles, and dampers (dimensionless)
• ρ is air density (kg/m³, approximately 1.2 kg/m³ at 20 °C)
• v is the mean airflow velocity inside the duct (m/s)

How EDIAQI Investigates Local Capture

Within the EDIAQI project, localized emission dynamics and point-source extraction efficiency are evaluated through multi-pilot field campaigns and computational simulations:

• Pilot P1 Field Studies in Ferrara (Schools and Restaurants): Pilot P1 monitors indoor-outdoor air quality across public facilities, recording real-time particle and VOC spikes during kitchen operations and meal preparation, documenting how kitchen exhaust maintenance impacts adjacent dining spaces.
• Filtration and Extraction Dynamics (Pilot P4): Pilot P4 assesses the interaction between episodic domestic/office kitchen cooking emissions, mechanical filtration units, and localized extraction systems, confirming that unvented cooking creates sharp submicron aerosol peaks that can overwhelm recirculating air cleaners.
• Office Equipment Emission Characterization (WP3): Testing campaigns evaluate localized VOC and ultrafine particle plumes generated by commercial printing equipment, benchmarking isolation and exhaust strategies.
• CFD Simulation and Digital Twin Modeling (WP4): Computational Fluid Dynamics (CFD) modeling developed within Task 4.4 simulates thermal plume rise, velocity streamlines, and capture zones, evaluating how room geometry and purifier/exhaust placement optimize contaminant removal.

Related wiki pages

Source Control
Eliminating indoor emissions before generation.

Ventilation
Air exchange mechanisms, rates, and distribution.

Filtration Systems
Air cleaners, filter media, and capture efficiency.

The EDIAQI Decision Tree
Diagnostic and intervention roadmap.

References