Air Cleaning Technologies
Learning Objectives
- Compare mechanical filtration, electrostatic, UV, and photocatalytic technologies
- Interpret filter efficiency ratings (MERV, HEPA, ULPA)
- Calculate Clean Air Delivery Rate (CADR) and equivalent air changes
- Analyze tradeoffs between efficiency, airflow, and pressure drop
- Evaluate commercial air cleaner claims and certifications
Air Cleaning Technology Comparison
| Technology | Mechanism | Targets | Concerns |
|---|---|---|---|
| Mechanical filtration | Physical interception, impaction, diffusion | Particles of all sizes | Pressure drop, filter replacement |
| Electrostatic | Particle charging + collection | Particles | Ozone generation, efficiency decline |
| UV-C germicidal | DNA/RNA damage to pathogens | Bacteria, viruses, mold | No particle removal, exposure risk |
| Photocatalytic oxidation | OH radical generation | VOCs, pathogens | Byproduct formation, limited data |
| Activated carbon | Adsorption to porous media | VOCs, odors | Saturation, no particle removal |
| Ionizers | Particle charging + deposition | Particles | Ozone, redeposition concerns |
Mechanical Filtration
Filter Efficiency Ratings
| Rating | Test Standard | Particle Size | Efficiency Range |
|---|---|---|---|
| MERV 1-4 | ASHRAE 52.2 | 3-10 um | <20% |
| MERV 8 | ASHRAE 52.2 | 1-3 um | 70-85% |
| MERV 13 | ASHRAE 52.2 | 0.3-1 um | 50-85% |
| MERV 16 | ASHRAE 52.2 | 0.3-1 um | >95% |
| HEPA | DOE STD-3020 | 0.3 um (MPPS) | >99.97% |
| ULPA | IEST-RP-CC001 | 0.12 um | >99.999% |
Key insight: 0.3 um is the Most Penetrating Particle Size (MPPS) where filtration is least efficient. Smaller and larger particles are actually captured more effectively.
Filtration Mechanisms
Interception
Particle follows airstream but contacts fiber when passing within one radius. Dominates for medium particles (0.1-1 um).
Impaction
Large particles cannot follow curved airstream around fiber and impact directly. Dominates for particles >1 um.
Diffusion
Small particles undergo Brownian motion and wander into fibers. Dominates for ultrafine particles <0.1 um.
Clean Air Delivery Rate (CADR)
CADR = Airflow (CFM) x Efficiency
CADR represents the volume of clean air delivered per unit time. AHAM certifies CADR for three particle types:
- Smoke: 0.09-1.0 um particles
- Dust: 0.5-3.0 um particles
- Pollen: 5.0-11.0 um particles
Room sizing rule (AHAM): CADR (CFM) should be at least 2/3 of room area (sq ft)
Example: 300 sq ft room needs CADR ≥ 200 CFM for adequate air cleaning.
Equivalent Air Changes
eACH = (CADR x 60) / Room Volume
Example: A cleaner with CADR = 200 CFM in a room 20 x 15 x 9 ft (2700 cu ft):
eACH = (200 x 60) / 2700 = 4.4 air changes per hour
CDC recommendation for airborne infection control: 5+ eACH combined from ventilation and filtration.
The Corsi-Rosenthal Box
A DIY air cleaner design that emerged during COVID-19, combining a box fan with MERV-13 filters:
- Design: 4-5 MERV-13 filters arranged as a cube with box fan on top
- Performance: CADR typically 400-600 CFM depending on fan and filter selection
- Cost: $50-100 in materials
- Advantages: High CADR for cost, easy to build, no proprietary filters
- Limitations: Larger footprint, aesthetics, no AHAM certification
Research finding (Segalman et al., 2023): Well-built CR boxes achieve CADR comparable to commercial units at fraction of cost.
Activity: Technology Evaluation
Evaluate three commercial air cleaners for your design scenario:
- Find AHAM-certified units with published CADR values
- Calculate cost per CFM of CADR
- Determine energy efficiency (watts per CFM)
- Research noise levels at different speeds
- Calculate filter replacement costs per year
- Identify any claims without scientific support (ionizers, "plasma," etc.)
Decision matrix: Create a weighted scoring matrix with criteria including CADR, cost, noise, energy, and maintenance.
Key Takeaway
Mechanical filtration with HEPA or high-MERV filters remains the most reliable, evidence-based air cleaning technology. CADR provides the key metric for comparing devices - higher CADR means more clean air delivered. While newer technologies like UV and PCO have applications, they should complement rather than replace filtration. Engineering decisions require evaluating tradeoffs among performance, cost, noise, and energy.