Ventilation

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EDIAQI TOPIC BRIEF · BUILDING VENTILATION SYSTEMS

Ventilation Principles, Air Exchange, and IAQ Performance

Ventilation is the intentional exchange of indoor air with fresh outdoor air and the distribution of this air within enclosed spaces. It serves as the primary engineering control to dilute human bioeffluents, evacuate moisture, remove indoor chemical emissions, and lower airborne pathogen exposure.

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Quick Facts

Illustration representing ventilation pathways

Main Types: Mechanical, Natural, Hybrid
Base Fresh Air: 4 to 10 l/s per person
CO2 Benchmark: < 800 to 1000 ppm
Tracer Methods: Decay, constant injection
Filtration Match: ISO 16890 / EN 1822
Reviewed: 2026-09-30

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EDIAQI project website
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Jump to: Core Purpose • System Types • Air Distribution • Performance & CO2 • Filtration Interplay • Practical Actions • Application Matrix • Technical Models • EDIAQI Research • Related Pages

4 to 10 l/s

Supply Air per Person

Standard fresh air baseline required to dilute human bioeffluents and maintain acceptable perceived air quality in occupied spaces.

< 1000 ppm

Indoor CO2 Threshold

Operational surrogate threshold for adequate air exchange: levels exceeding 1000 to 1200 ppm indicate insufficient fresh air intake.

0.5 h⁻¹

Minimum Air Exchange Rate

Recommended baseline air changes per hour (ACH) in residential buildings to prevent moisture accumulation and mold risk.

Why Ventilation Matters

Ventilation is the physical process of introducing outdoor air into a building while simultaneously exhausting stale indoor air. Because Europeans spend roughly 90% of their daily lives indoors, the performance of ventilation systems directly dictates exposure to airborne hazards, cognitive performance, and general wellbeing.

Properly designed and balanced ventilation achieves three critical objectives:

• Dilution of Internally Generated Pollutants: Sourced from occupant respiration, volatile organic compounds (VOCs) off-gassing from furniture and building products, cleaning agents, and indoor combustion.
• Moisture and Biological Control: Evacuation of excess humidity generated by breathing, cooking, and showering, which prevents structural condensation and fungal/mold proliferation.
• Risk Mitigation for Airborne Transmission: Lowering the steady-state concentration of infectious respiratory droplets and viral pathogens in shared spaces.

However, ventilation acts as a double-edged sword: in urban environments with heavy traffic or industrial emissions, drawing untreated outdoor air inside without appropriate filtration increases indoor concentrations of fine particulate matter (PM_2.5, PM₁₀), black carbon, and nitrogen dioxide (NO₂).

Classification of Ventilation Systems

Ventilation air movement is governed by pressure differences between the interior and exterior of a building envelope, driven by mechanical fans, natural buoyancy forces, or wind pressure.

Mechanical Ventilation

Mechanical ventilation uses electric fans to actively deliver and exhaust air, providing steady, predictable airflow rates regardless of outdoor weather fluctuations.

• Balanced Mechanical Systems with Heat Recovery (MVHR): The gold standard for modern energy-efficient construction. Outdoor air is mechanically drawn through high-efficiency filters, tempered via a heat exchanger using heat recovered from the exhaust air stream, and delivered directly to occupied rooms.
• Mechanical Exhaust Only: Relies on central or local extract fans in humid rooms (kitchens, bathrooms) to induce a slight negative pressure, pulling replacement air through window trickle vents or structural envelope gaps. While common in older retrofits, incoming air cannot be centrally filtered or thermally conditioned.
• Advantages: Controllable flow rates, automated demand-controlled modulation (DCV) based on CO₂ or occupancy sensors, integration of certified air filtration (ISO 16890), and structural noise mitigation.

Natural Ventilation

Natural ventilation moves air through openable windows, doors, louvers, and vertical architectural stacks without mechanical fans, driven entirely by natural physical forces:

• Wind Pressure: Creates positive pressure on the windward facade and negative pressure on the leeward facade, driving cross-ventilation.
• Stack Effect (Thermal Buoyancy): Temperature differences cause warm indoor air to rise and escape through high vents, drawing colder, denser outdoor air through lower envelope openings.
• Limitations: Highly unpredictable flow rates that vary with outside temperatures and wind velocity. During cold weather or extreme heat, occupants tend to keep windows closed, leading to acute pollutant accumulation. In polluted urban centers, natural ventilation permits unfiltered ingress of outdoor aerosols.

Hybrid (Mixed-Mode) Ventilation

Hybrid ventilation combines natural and mechanical strategies to optimize energy use and indoor air quality. During mild weather with low ambient pollution, natural cross-ventilation is utilized. When external conditions are unfavorable (extreme temperatures, calm windless days, or elevated ambient PM spikes), the building automatically engages mechanical fans and filtration systems.

Ventilation Rates and Air Distribution

Ventilation Rate Requirements

The ventilation rate represents the volumetric flow of outdoor air supplied to a space per unit time, expressed as litres per second per person (l/s per person), litres per second per square metre (l/s per m²), or air changes per hour (ACH, h⁻¹).

• For human bioeffluent dilution alone, health-based studies recommend a baseline outdoor airflow rate of at least 4 l/s per person in spaces without additional chemical sources.
• In school classrooms and office environments with complex indoor materials, international engineering standards (such as EN 16798-1 and ASHRAE 62.1) specify baseline design rates between 7 and 10 l/s per person to sustain cognitive performance and prevent sick building syndrome (SBS) symptoms.

Air Distribution Patterns

Supplying adequate air volume is insufficient if fresh air fails to reach the breathing zone:

• Mixing Ventilation: Dilutes room air uniformly by injecting fresh air at high velocity. It is standard in commercial spaces but can disperse localized contaminants throughout the entire room.
• Displacement Ventilation: Introduces cool air at floor level with low velocity. Warm plumes generated by occupants and electronic devices cause the fresh air to rise past the breathing zone, carrying heat and contaminants toward ceiling exhaust grilles with minimal lateral mixing.

Performance Assessment: CO2 and Tracer Techniques

Evaluating whether an occupied room is ventilated adequately cannot rely on visual inspection or human olfactory perception, because habituation desensitizes occupants to stale air.

Carbon Dioxide (CO2) as a Ventilation Surrogate

Because humans exhale carbon dioxide at a rate proportional to metabolic activity, indoor CO₂ concentrations serve as an accessible proxy for fresh air delivery per occupant:

• Outdoor background air sits at approximately 420 ppm.
• In an occupied room, steady-state CO₂ concentrations below 800 to 1000 ppm indicate adequate ventilation rates.
• Levels rising beyond 1200 to 1500 ppm indicate stuffiness, degraded air exchange, and heightened risk of airborne cross-infection.
• Sensor Placement: CO₂ monitors must be installed at seated breathing height (1.0 to 1.5 m above floor level), at least 1.0 m away from walls and windows, and away from direct occupant exhalation plumes or supply diffusers.

Tracer Gas Experimental Methods

In scientific evaluations and building commissioning, air exchange rates and airflow distribution are determined via standardized tracer gas techniques:

• Concentration Decay Method: An inert tracer (such as CO₂ or SF₆) is injected and mixed thoroughly. The injection is stopped, and the exponential decline in concentration is logged over time to calculate the exact air change rate (n, h⁻¹).
• Ventilation Effectiveness: Evaluates the ratio between pollutant concentration in the exhaust grille and concentration in the breathing zone, verifying whether stagnant pockets exist.

The Interplay Between Ventilation and Filtration

A frequent operational misconception is confusing air filtration with ventilation:

• What Filtration Does: Mechanical filters (ePM1, HEPA) and recirculating portable air purifiers capture airborne particles, lint, pollen, bacteria, and viral aerosols.
• What Filtration Cannot Do: Standard particulate filters do NOT remove carbon dioxide, carbon monoxide, or gaseous chemical contaminants.
• The Synergistic Approach: In areas impacted by outdoor vehicular exhaust or seasonal biomass smoke, increasing raw natural ventilation introduces harmful ambient aerosols. The optimal solution couples controlled mechanical air exchange with multi-stage filtration: pre-filters to capture coarse dust, ePM1 filters to remove submicron traffic exhaust, and activated carbon beds where gaseous outdoor odors or VOCs must be controlled.

Practical Recommendations for Building Operators

1. Prioritize Source Control Before Increasing Airflow

Do not attempt to solve excessive chemical emissions solely through higher ventilation. Eliminate unvented indoor combustion, substitute high-VOC cleaning detergents with certified low-emission products, and isolate indoor emission processes at the source.

2. Implement Continuous CO2 Monitoring to Guide Actions

Install NDIR CO₂ screening sensors at breathing height (1.0 to 1.5 m) in classrooms, meeting areas, and living spaces. If CO₂ crosses 1000 ppm, initiate immediate cross-window airing or increase mechanical air exchange rates.

3. Time Natural Window Airing Away from Rush-Hour Traffic

In naturally ventilated buildings located near busy roads, avoid opening windows during peak traffic hours (morning and evening commutes) to minimize PM_2.5 and NO₂ infiltration. Ventilate deeply during midday or off-peak periods using cross-ventilation configurations.

4. Maintain Filter Media and Air Handling Hygiene

Regularly inspect and replace mechanical HVAC filters (at least twice annually, or as dictated by pressure drop sensors). Ensure heat recovery exchanger surfaces and condensate trays are clean to prevent secondary microbial growth.

Target Audience Application Matrix

Target Audience	Practical Value and Action
Homeowners and Residents	Air rooms through short, intensive cross-ventilation periods rather than continuous window tilting in winter. Time window airing away from rush-hour traffic and maintain cooker hood exhaust.
Schools and Kindergartens	Deploy NDIR CO2 monitors to govern window airing protocols between and during classes, aiming for levels below 800 to 1000 ppm to maintain pupil attention and lower infection risks.
Facility and Building Managers	Balance mechanical ventilation systems to deliver at least 7 to 10 l/s per person in occupied zones. Implement demand-controlled ventilation (DCV) and inspect heat recovery units regularly.
Municipalities and Public Health Bodies	Include ventilation performance and continuous CO2 monitoring mandates in public building refurbishment plans, following European Energy Performance of Buildings Directive (EPBD) standards.

Simulate ventilation and infiltration dynamics in your building
Use the EDIAQI IAQ Simulation Tool → to explore how envelope airtightness, mechanical ventilation rates, and outdoor pollution interact across European building archetypes.

[+] Click here to view technical and mathematical formulations

Indoor Aerosol Mass-Balance Model

The dynamic behavior of indoor particulate matter under the combined influence of ventilation, envelope penetration, surface deposition, and air filtration is described by the single-zone mass-balance differential equation:

$${\frac {dC_{\text{in}}(t)}{dt}}=P\cdot n\cdot C_{\text{out}}(t)-(n+\lambda _{d}+\lambda _{f})C_{\text{in}}(t)+{\frac {S}{V}}$$

Where:

• C_in(t) is the indoor particle concentration at time t (µg/m³ or #/cm³)
• C_out(t) is the outdoor ambient concentration (µg/m³ or #/cm³)
• P is the dimensionless penetration factor through the building envelope (typically 0.6 to 0.9 for fine particles)
• n is the ventilation air change rate (h⁻¹), where n = Q / V (with Q as the outdoor airflow rate in m³/h)
• λ_d is the indoor surface deposition rate (h⁻¹)
• λ_f is the equivalent removal rate provided by mechanical filtration or portable air cleaning (h⁻¹), defined as λ_f = CADR / V
• S is the effective indoor source emission rate (µg/h or #/h)
• V is the internal room volume (m³)

Under steady-state conditions (dC_in / dt = 0), the equilibrium concentration simplifies to:

$$C_{\text{in}}={\frac {P\cdot n\cdot C_{\text{out}}+{\frac {S}{V}}}{n+\lambda _{d}+\lambda _{f}}}$$

Tracer Concentration Decay Equation

When determining the air change rate using an inert tracer gas (or metabolic CO₂ after occupants have vacated the room and the outdoor baseline is stable), the concentration decay over time is expressed as:

$$C(t)=C_{\text{out}}+(C_{0}-C_{\text{out}})e^{-nt}$$

Where:

• C(t) is the tracer concentration at time t (ppm)
• C₀ is the initial tracer concentration at time t = 0 (ppm)
• C_out is the ambient outdoor concentration (ppm)
• n is the ventilation air change rate (h⁻¹)
• t is the elapsed decay time (hours)

Rearranging the equation yields the operational formula used in building commissioning to calculate the actual air change rate from time-series monitoring data:

$$n=-{\frac {1}{t}}\ln \left({\frac {C(t)-C_{\text{out}}}{C_{0}-C_{\text{out}}}}\right)$$

How EDIAQI Investigates Ventilation

Within the EDIAQI project, ventilation performance is evaluated through extensive experimental campaigns and digital twins:

• Large-Scale Building Seire in Estonia (Pilot P2): Led by Tallinn University of Technology (TalTech), Pilot P2 instruments 51 municipal buildings with 107 continuous multi-sensor stations across administrative facilities, schools, and kindergartens. The pilot investigates real-world ventilation patterns, occupant-driven CO₂ dynamics, and building performance metrics to optimize indoor air hygiene and energy consumption.
• Filtration Dynamics (Pilot P4): Pilot P4, led by ASC with consortium partners, evaluates real-world air filtration and purification systems under varying airflow regimes, quantifying the distinct removal capacities for particles versus chemical vapors.
• School and Transport Campaigns: Field studies in Ferrara (Pilot P1) and campaign deployments in schools evaluate window opening protocols, mechanical ventilation retrofits, and their net impacts on occupant exposure.
• Digital Twins and CFD Modeling (WP4): Computational Fluid Dynamics (CFD) simulations and machine learning state-space models simulate indoor airflow distribution, stagnant pocket formation, and filtration placement to support evidence-based space management.

Related wiki pages

The EDIAQI Decision Tree
Four-step diagnostic and remediation roadmap.

Sensors
CO2, PM, and VOC monitoring instrumentation.

Indoor Air Pollutants
Physical and chemical determinants in indoor air.

References