Filtration and air cleaning

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EDIAQI TOPIC BRIEF · AIR CLEANING AND FILTRATION

Filtration and Air Cleaning: Technologies, Real-World Efficacy, and Operational Strategy

Filtration and air cleaning technologies physically capture airborne particles, sorb hazardous volatile chemicals, and reduce microbial loads in indoor spaces. While highly effective as supplementary engineering controls, air cleaners cannot replace outdoor air exchange, because mechanical filters do not remove carbon dioxide or generate oxygen.

Consult the EDIAQI Decision Tree → Explore action guidelines ↓

Quick Facts

Illustration representing air filtration and purification technologies

Core Types: Fibrous mechanical, Sorptive carbon, Hybrid
Main Targets: PM₁, PM_2.5, PM₁₀, bioaerosols, VOCs
Key Standards: EN 1822 (HEPA/EPA), ISO 16890
Sizing Metric: Clean Air Delivery Rate (CADR)
Gas Limitation: Particulate filters do not remove CO₂
Filter Life: Pre-filter 1-3 mo, HEPA 6-12 mo
Reviewed: 2026-09-30

Explore further
EDIAQI project website
EDIAQI Decision Tree
Sensor Technologies
Ventilation Principles
IAQ Simulation Tool

Jump to: Core Principles • Filter Standards • Air Cleaning Types • Real-World Evidence • Ventilation Synergy • Action Guidelines • Application Matrix • Technical Models • EDIAQI Research • Related Pages

≥ 99.95%

HEPA H13 Efficiency

Standard certified retention for particles at the Most Penetrating Particle Size (MPPS, 0.1 to 0.3 µm) under European standard EN 1822.

0 ppm CO2

Gas Removal Limitation

Mechanical particulate filters do not capture carbon dioxide or combustion gases. Fresh outdoor air exchange remains irreplaceable.

3 to 5 h⁻¹

Effective Clean Air Turns

Recommended equivalent clean air change rate (eACH) delivered by properly sized portable units in occupied public or educational rooms.

Why Filtration and Air Cleaning Matter

Indoor air contains a complex mixture of particulate and gaseous contaminants originating from indoor activities (cooking, cleaning, resuspension, occupant movement) and outdoor atmospheric infiltration (traffic emissions, industrial plumes, seasonal pollen). Because Europeans spend nearly 90% of their time indoors, reducing personal exposure to inhalable aerosols is vital for preventing respiratory and cardiovascular diseases.

While ventilation dilutes indoor contaminants by introducing outside air, ambient air in urban environments is frequently polluted with fine particulate matter (PM_2.5, PM₁₀), ultrafine particles, and black carbon. Under such conditions, increasing unconditioned natural ventilation can introduce significant outdoor pollution indoors. Filtration and air cleaning serve as targeted engineering controls that remove pollutants directly from the air stream without depending entirely on outdoor weather conditions.

However, filtration is often misunderstood:

• Filtration refers specifically to the mechanical separation of suspended solid or liquid particles from an air stream by passing it through a porous fibrous filter medium.
• Air cleaning is an umbrella term encompassing mechanical filtration as well as physical and chemical processes (such as gas-phase adsorption or chemisorption) that remove gaseous pollutants, vapors, and odors.
• The ventilation distinction: Air cleaners recirculate indoor air. Even high-efficiency particulate filters cannot remove carbon dioxide (CO₂), supply fresh oxygen, or control indoor humidity. Dedicated ventilation remains essential.

Mechanical Filtration Technologies and Standards

Mechanical fibrous filters capture airborne particles through four primary physical mechanisms acting simultaneously across different particle size ranges:

• Inertial Impaction: Governs larger particles (generally > 1.0 µm) whose inertia prevents them from following fluid streamlines around filter fibers, causing them to collide directly with the fiber surface.
• Interception: Occurs when a particle follows a streamline but passes within one particle radius of a fiber, making contact and adhering to it.
• Brownian Diffusion: Dominates ultrafine particles (< 0.1 µm), where erratic Brownian motion causes particles to deviate from streamlines and collide with fibers.
• Electrostatic Attraction: Utilizes natural or induced electrical charges on synthetic fibers to pull charged or polarized particles toward the fiber collectors.

Due to the physical transition between diffusion and interception, fibrous filters exhibit a dip in collection efficiency for particles between 0.1 µm and 0.3 µm, known as the Most Penetrating Particle Size (MPPS). Standard filter ratings are determined at this critical efficiency minimum.

European and International Filter Standards

Certified air filters are tested and classified under two primary standard frameworks:

• General Ventilation Filters (EN ISO 16890): Replaced the older EN 779 system (G1-F9) and classifies filters based on mass removal efficiency across realistic particle size fractions:
  • ISO ePM1: High-efficiency filters capturing fine combustion aerosols and viruses (tested against particles 0.3 to 1.0 µm; e.g., ePM1 80% retains at least 80% of PM₁).
  • ISO ePM2.5: Certified for ambient fine particles (0.3 to 2.5 µm).
  • ISO ePM10: Retains coarse dust, pollen, and large fungal spores (0.3 to 10 µm).
  • ISO Coarse: Captures lint, sand, and visible hair; primarily used as protective pre-filters.

• High-Efficiency Particulate Air Filters (EN 1822 / ISO 29463): Classifies filters by single-pass retention at their individual MPPS:
  • EPA (Efficient Particulate Air): Classes E10 (≥ 85%), E11 (≥ 95%), and E12 (≥ 99.5%).
  • HEPA (High Efficiency Particulate Air): Classes H13 (≥ 99.95%) and H14 (≥ 99.995%). Widely utilized in medical facilities, cleanrooms, and high-performance portable air purifiers.
  • ULPA (Ultra Low Penetration Air): Classes U15 (≥ 99.9995%) to U17 (≥ 99.999995%), applied in microelectronics and pharmaceutical production.

The Multi-Stage Filtration Cascade

High-performing portable purifiers and balanced ventilation handling units deploy a sequential three-stage cascade:

1. Pre-Filter (G4 / ISO Coarse): Captures coarse hair, fibers, and insects, protecting downstream media from premature clogging.
2. Activated Carbon Layer: Adsorbs chemical vapors, cooking odors, and ozone.
3. Main High-Efficiency Filter (ISO ePM1 or EN 1822 H13): Captures submicron combustion exhaust, bacteria, fungal spores, and respiratory aerosol droplets.

Gas-Phase Air Cleaning and Emerging Methods

Because standard fibrous filters cannot capture gaseous molecules or volatile organic compounds (VOCs), alternative air cleaning mechanisms must be utilized.

Adsorption on Porous Media

• Activated Carbon: High-surface-area porous carbon (500 to 1500 m²/g) physically adsorbs non-polar gaseous hydrocarbons, aromatic VOCs (such as benzene and toluene), and offensive odors through van der Waals forces.
• Chemisorption Media: Chemically impregnated sorbents (such as alumina pellets treated with potassium permanganate) neutralize reactive acidic gases, formaldehyde, or hydrogen sulfide via chemical bonding.
• Limitations: Adsorption beds reach saturation over time. Once saturated, changes in room temperature or humidity can cause desorptive re-emission of captured chemicals back into indoor air. Regular replacement is required.

Electronic and Reactive Cleaning Technologies

A wide variety of active air cleaners exist on the market, but their performance and safety require careful evaluation:

• Electrostatic Precipitators (ESP): Electrically charge incoming airborne particles and collect them on oppositely charged metal collector plates. While effective with low airflow resistance, poorly maintained or high-voltage corona electrodes can generate hazardous ozone (O₃) as an unintended byproduct.
• Ozone Generators: Marketed for odor and microbial remediation, but intentionally emitting ozone into occupied spaces presents serious health risks, causing acute airway irritation, asthma exacerbation, and secondary particle formation through reactions with terpenes. Ozone generators are not recommended for occupied spaces.
• Photocatalytic Oxidation (PCO) and Plasma Systems: Utilize ultraviolet light and catalysts (e.g., TiO₂) or ionization fields to decompose VOCs. Independent studies demonstrate that incomplete catalytic degradation often produces harmful secondary byproducts, including formaldehyde and acetaldehyde.

EDIAQI guidelines emphasize that mechanical fibrous filtration (ISO ePM1 / HEPA) combined with deep activated carbon represents the safest, evidence-based approach for occupied buildings.

Real-World Efficacy and Operational Evidence

Air cleaner performance is conventionally rated in sealed laboratory test chambers using the Clean Air Delivery Rate (CADR), defined as the volumetric flow of particle-free air delivered by the device (expressed in m³/h). However, extensive experimental work within the EDIAQI project demonstrates that laboratory CADR ratings do not automatically translate to equivalent real-world exposure reductions.

Key Findings from EDIAQI Field Studies (Pilot P4)

Within EDIAQI Pilot P4, commercial multi-stage portable air purifiers were deployed in occupied office environments (workstations and shared kitchens) under continuous operation, paired with dense low-cost sensor networks, gravimetric samplers, and chemical speciation:

• The Efficacy Gap in Occupied Spaces: When purifiers operated continuously, bulk indoor mass concentrations (PM₁₀, PM_2.5) did not exhibit constant, systematic reductions. In active, occupied rooms, localized particle generation, walking, and door opening generated episodic emission spikes that temporarily outweighed the purification removal rate.
• Secondary Resuspension: High-velocity fan discharge and daily manual filter handling caused minor resuspension of coarse settled dust from surrounding floor surfaces, temporarily raising PM₁₀ spikes near the device.
• Chemical Speciation Insights (PAHs): Chemical analysis of collected filters revealed a clear divergence between particle mass and chemical toxicity:
  • High-molecular-weight polycyclic aromatic hydrocarbons (PAHs) bound to the submicron PM₁ fraction decreased by approximately 30% during purifier active periods.
  • In contrast, lighter, volatile PAHs in total suspended particles (TSP) showed little reduction, indicating that semi-volatile organic partitioning is governed by room temperature and ventilation dynamics rather than mechanical filtration alone.

Influence of Device Placement and Air Mixing

Computational Fluid Dynamics (CFD) simulations and digital twin experiments conducted within EDIAQI WP4 demonstrate that physical room geometry and purifier placement dictate actual performance:

• Placement Height: Positioning air cleaners at breathing height (approximately 1.0 to 1.5 m) or slightly elevated (0.5 m off the floor) significantly improves clean air delivery to seated occupants compared to placing units flush against corner baseboards.
• Avoiding Short-Circuiting: Placing a purifier directly beneath an open window or adjacent to a mechanical supply diffuser causes clean air to be expelled prematurely or creates localized loops, leaving the breathing zone uncleaned.
• Dead Zones: In L-shaped rooms or compartmentalized offices, a single centralized purifier leaves stagnant pockets where pollutant residence times remain elevated.

Synergy with Ventilation and Source Control

Filtration and ventilation are complementary engineering disciplines that must be coordinated within an integrated indoor air management framework:

Parameter / Function	Mechanical Filtration / Purifiers	Fresh Air Ventilation (Natural/Mechanical)
Fine Particles (PM1, PM2.5)	Highly effective (ePM1 / HEPA removes > 90-99%) without thermal energy penalties.	Dilutes indoor particles, but risks importing outdoor combustion soot and traffic PM if unfiltered.
Carbon Dioxide (CO2)	No effect. Cannot reduce CO2 or replenish oxygen.	Primary mechanism. Evacuates human exhalation and restores baseline 420 ppm levels.
Relative Humidity & Mold	Captures airborne fungal spores; does not remove water vapor or stop condensation.	Evacuates moisture generated by occupant respiration, cooking, and hygiene.
Chemical Vapors & VOCs	Effective only if equipped with substantial, unsaturated activated carbon.	Continuously vents background off-gassing from building materials and furniture.

Neither filtration nor ventilation can compensate for intense internal pollution sources. Eliminating indoor smoking, replacing solvent-based cleaning agents with certified low-emission products, and venting cooking hobs directly outside remain the essential first tier of indoor air protection.

Practical Recommendations for Building Operators

1. Prioritize Source Control Before Air Purifier Deployment

Do not rely on portable air purifiers to overcome unvented combustion or heavy chemical use. Isolate printing hubs, eliminate synthetic fragrances, and enforce kitchen exhaust range hoods before calculating filtration requirements.

2. Size Air Cleaners to Match Room Volume and Occupancy

Select air purifiers based on verified Clean Air Delivery Rate (CADR), not advertised floor area. Sizing should deliver an equivalent clean air change rate (eACH) of at least 3 to 5 air turnovers per hour. Calculate required CADR by multiplying room volume (m³) by target eACH.

3. Optimize Device Placement in the Breathing Zone

Position portable purifiers in central, unobstructed room locations, elevated 0.5 m to 1.5 m above the floor. Avoid tucking devices behind furniture, in tight corners, or directly below open windows where clean air short-circuits.

4. Enforce Strict Multi-Stage Filter Maintenance Schedules

Filter media require structured maintenance to avoid pressure drop penalties and secondary re-emission:

• Pre-filters: Inspect and vacuum monthly; replace every 3 months.
• Activated carbon beds: Replace every 3 to 6 months to prevent VOC saturation and odor breakthrough.
• HEPA / ePM1 filters: Replace every 6 to 12 months, or immediately if static pressure indicates clogging.
• Ensure tight gasket seals during filter replacement to prevent air bypass.

5. Avoid Byproduct-Emitting Electronic Cleaners

Avoid uncertified electronic air purifiers, ionizers, and ozone generators in occupied classrooms, daycare centers, and homes. Stick to passive mechanical filtration (HEPA) and sorptive media that do not generate secondary chemical byproducts.

Target Audience Application Matrix

Target Audience	Practical Value and Action
Residents and Homeowners	Deploy HEPA-equipped units in bedrooms and living rooms during wildfire smoke or high urban traffic episodes. Place purifiers away from walls, operate on continuous low-noise modes, and pair with regular cross-airing when outdoor air is clean.
Schools and Kindergartens	Install low-noise HEPA filtration units to supplement window ventilation, especially during cold seasons when window opening is restricted. Monitor CO2 continuously to guide manual airing schedules while filtration controls particulate exposure.
Commercial Building Managers	Upgrade centralized HVAC intake filters to ISO ePM1 ≥ 70% or ePM1 ≥ 80% to protect occupants from ambient urban particulate ingress. Monitor differential filter pressure to schedule timely filter changeouts before airflow rates drop.
Municipalities and Public Health Bodies	Incorporate filtration standards into public procurement guidelines for public buildings, daycare centers, and senior care homes. Avoid procurement of uncertified electronic or ozone-generating cleaners.

Integrate air cleaning into your building strategy
Consult the EDIAQI Decision Tree → to evaluate whether indoor particulate or chemical issues are best resolved through source control, ventilation adjustments, or certified air filtration.

[+] Click here to view technical and mathematical formulations

Clean Air Delivery Rate (CADR) and Equivalent Air Change Rate

The Clean Air Delivery Rate (CADR) defines the effective volumetric flow rate of clean air delivered by an air cleaner, incorporating filter capture efficiency and device airflow:

$${\text{CADR}}=\eta \cdot Q_{f}$$

Where:

• CADR is the clean air delivery rate (m³/h)
• η is the fractional single-pass filter efficiency for a specific contaminant size fraction (dimensionless, 0 to 1)
• Q_f is the volumetric airflow through the device (m³/h)

The equivalent air change rate contributed by the filtration device (λ_f) in a well-mixed room volume is calculated as:

$$\lambda _{f}={\frac {\text{CADR}}{V}}={\frac {\eta \cdot Q_{f}}{V}}$$

Where:

• λ_f is the filtration air change rate (h⁻¹)
• V is the room volume (m³)

Indoor Aerosol Mass-Balance Model with Air Cleaning

In an enclosed space equipped with both ventilation and air cleaning, the dynamic concentration of particulate matter is governed by the mass-balance differential equation:

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

Where:

• C_in(t) is the indoor particle concentration at time t (µg/m³ or #/cm³)
• C_out(t) is the outdoor concentration (µg/m³ or #/cm³)
• P is the penetration factor through the building envelope (dimensionless)
• λ is the ventilation air change rate (h⁻¹)
• λ_d is the particle surface deposition rate (h⁻¹)
• λ_f is the effective removal rate by filtration or portable air cleaning (h⁻¹)
• S is the indoor source emission rate (µg/h or #/h)
• V is the room volume (m³)

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

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

This formulation demonstrates that air cleaning (λ_f) operates additively with ventilation (λ) and deposition (λ_d) to suppress indoor concentrations, offering significant protection when outdoor concentrations (C_out) are elevated.

Single-Pass Fractional Efficiency

The single-pass capture efficiency of a filter medium as a function of particle diameter (d_p) is defined by comparing upstream and downstream concentrations:

$$\eta (d_{p})=1-{\frac {C_{\text{down}}(d_{p})}{C_{\text{up}}(d_{p})}}$$

Where:

• C_up(d_p) is the particle number concentration upstream of the filter medium
• C_down(d_p) is the particle number concentration downstream of the filter medium

How EDIAQI Investigates Filtration

Within the EDIAQI project, filtration performance and air cleaning technologies are investigated across laboratory chambers, field pilots, and computational simulations:

• Filtration Dynamics Field Trial (Pilot P4): Led by ASC with support from IMROH, TROPOS, and consortium partners, Pilot P4 evaluates multi-stage commercial filtration units in occupied office rooms and common areas. The pilot evaluates the difference between bulk particle mass metrics (PM₁₀, PM_2.5) and chemical toxicological constituents (PM₁-bound PAHs), documenting how daily office routines, resuspension, and placement affect actual exposure.
• CFD Simulation and Digital Twins (WP4): Computational Fluid Dynamics (CFD) modeling developed within Task 4.4 simulates airflow velocity vectors and particle dispersion profiles under varying purifier placements (e.g., 50 cm vs 150 cm height) and room configurations. These simulations identify stagnant pockets and provide algorithmic guidelines for optimizing purifier positioning.
• Laboratory Sensor and Filter Intercomparisons (WP3 & Campaign C1): Controlled aerosol chamber evaluations conducted by TROPOS and IMROH test low-cost optical monitors alongside scientific reference instrumentation (SMPS, MPSS, CPC) to quantify filter removal efficiency across particle size distributions from 10 nm to 20 µm.
• Building Performance and Ventilation Monitoring in Estonia (Pilot P2): Led by TalTech, Pilot P2 continuously monitors indoor environmental parameters and CO₂ dynamics across 51 municipal buildings (107 stations), establishing baseline airflow regimes necessary to integrate filtration effectively into energy-efficient HVAC retrofits.

Related wiki pages

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

Sensors
Sensor technologies, accuracy, and monitoring setups.

Ventilation
Air exchange mechanisms, rates, and IAQ performance.

Indoor Air Pollutants
Health determinants, thresholds, and emission sources.

References