The Indoor Air Crisis: Shocking Numbers

According to the World Health Organization, humans spend 90% of their time indoors, where air pollution can be 2-5 times higher than outdoors. This makes Indoor Air Quality (IAQ) a top priority in modern engineering design.

Sources of Indoor Air Pollution

  • PM2.5 and PM10: Fine particles from outdoor pollution and indoor fumes
  • VOCs: Volatile Organic Compounds from paints, furniture, and carpets
  • CO₂: Human respiration — high levels indicate insufficient ventilation
  • Germs and viruses: Airborne transmission, especially in enclosed spaces
  • Excess humidity: Encourages mold and fungi growth

Air Purification Technologies: Comprehensive Comparison

Technology What it removes Efficiency Best application
HEPA H13 Filters Particles ≥ 0.3 microns ≥ 99.95% Hospitals, offices
HEPA H14 Filters Particles ≥ 0.3 microns ≥ 99.995% Operating rooms
Activated Carbon (GAC) VOCs, odors, chemicals 85-95% Laboratories, kitchens
UV-C (254nm) Bacteria, viruses, mold 99.9% HVAC systems
Bipolar Ionization Particles, some germs Variable Supplementary for offices
Photo-Catalytic (PCO) VOCs, germs Moderate Limited areas

UV-C in HVAC Systems: Engineering Details

Microbes are killed by a UV-C dose measured in μW·s/cm². Required dose:

  • E.coli bacteria: 6,600 μW·s/cm²
  • Influenza A virus: 6,600 μW·s/cm²
  • SARS-CoV-2: 3,400 μW·s/cm² (CDC 2020 estimates)

Design of Make-Up Air Units (MAU)


Make-Up Air Unit (MAU) with thermal treatment and humidification — Administrative complex

A MAU conditions outdoor air before distributing it throughout the building. It should include:

  1. Pre-Filter (G4): Primary protection to remove large particles
  2. Cooling Coil: Cools and dehumidifies outdoor air
  3. Medium Filter (F7): Removes fine particles ≥ 1 micron
  4. Heating Coil: Reheats air to achieve required humidity
  5. HEPA H13 (optional): For clean room applications
  6. UV-C Lamps: Final disinfection before distribution

ASHRAE 62.1 Fresh Air Requirements

  • Offices: 8.5 L/s per person + 0.9 L/s/m²
  • Meeting halls: 10 L/s per person
  • Retail: 0.9 L/s/m² + 3.6 L/s per person
  • Academic spaces: 8 L/s per person