Cleanroom Basics
Cleanrooms are controlled environments where airborne particles are kept below defined limits through high air change rates, HEPA filtration, and pressurization. This guide introduces the fundamental concepts — classification, airflow, filtration, and standards — without commercial intent.
📝 Educational scope
What Is a Cleanroom?
A cleanroom is a room or series of rooms designed to maintain very low concentrations of airborne particles (and sometimes viable organisms). They are used in:
- Semiconductor and microelectronics manufacturing
- Pharmaceutical and biotechnology manufacturing
- Medical device manufacturing
- Aerospace component assembly
- Hospital operating rooms and isolation rooms
- Research laboratories
The defining characteristic of a cleanroom is that its particle count is controlled through a combination of high air change rates, HEPA or ULPA filtration, pressurization relative to adjacent areas, and material/personnel protocols.
ISO Classification
ISO 14644-1 defines cleanroom classes by maximum airborne particle concentration. Each ISO class allows no more than a defined number of particles per cubic meter:
| ISO Class | Old Class (209E) | Max particles ≥0.5 µm/m³ | Typical Applications |
|---|---|---|---|
| ISO 3 | Class 1 | 1,000 | Leading-edge semiconductor fabs |
| ISO 4 | Class 10 | 10,000 | Advanced semiconductor, disk drives |
| ISO 5 | Class 100 | 100,000 | Semiconductor, sterile pharma fill/finish |
| ISO 6 | Class 1,000 | 1,000,000 | Pharma, medical devices |
| ISO 7 | Class 10,000 | 10,000,000 | Pharma bulk manufacturing, medical devices |
| ISO 8 | Class 100,000 | 100,000,000 | Packaging, less critical pharma areas |
Airflow Principles
Two airflow patterns are used in cleanrooms:
Unidirectional (Laminar) Flow
Air moves in a single, parallel direction — typically downward from ceiling HEPA filters through the work zone to floor-level returns. This airflow pattern sweeps particles away from the work area rather than mixing them into the room air. ISO 5 and cleaner cleanrooms require unidirectional flow.
Unidirectional flow requires large areas of ceiling HEPA coverage (50–100% of ceiling area) and high air change rates (typically 240–600+ ACH for ISO 5).
Non-Unidirectional (Turbulent) Flow
Air is introduced through ceiling diffusers and returned at low wall or floor level, creating turbulent mixing that dilutes particles. Less stringent classes (ISO 6–8) may use non-unidirectional flow with significantly lower air change rates (20–90 ACH).
HEPA and ULPA Filtration
All cleanrooms use high-efficiency particulate air (HEPA) or ultra-low penetration air (ULPA) filtration:
- HEPA — removes ≥99.97% of particles ≥0.3 µm (MERV 17). The 0.3 µm size is the most penetrating particle size (MPPS).
- ULPA — removes ≥99.9995% of particles ≥0.12 µm. Required for the most critical semiconductor and pharma applications.
HEPA filter integrity is tested using a PAO (polyalphaolefin) aerosol challenge — scanning the filter face and frame for leaks. Any leak point above the specified penetration limit requires filter replacement or repair.
Pressurization
Cleanrooms are maintained at positive pressure relative to adjacent areas to prevent particle infiltration through door gaps and penetrations. When a door opens, air flows outward (from cleanroom to corridor) rather than inward.
Cascading pressure schemes use a hierarchy of cleanliness:
- ISO 5 production area: highest pressure
- ISO 7 gowning/support area: intermediate pressure
- Unclassified corridor: lowest pressure (relative)
Minimum pressure differentials of 0.03–0.05 in. w.g. between adjacent cleanroom areas are typical. Higher differentials are harder to maintain through door cycling.
ACH in Cleanrooms
Air change rates in cleanrooms are far higher than in conventional HVAC — not for ventilation, but to continuously dilute and remove particles generated by equipment and personnel:
- ISO 8: 10–25 ACH (ASHRAE 170 hospital general area minimum is 6 ACH for comparison)
- ISO 7: 30–60 ACH
- ISO 6: 60–90 ACH
- ISO 5 (non-unidirectional): 150–240 ACH
- ISO 5 (unidirectional): 240–600+ ACH (velocity-based: 60–90 fpm face velocity across filter coverage area)
These high ACH rates require enormous amounts of conditioned air and cooling energy. Cleanroom HVAC systems are typically the largest energy consumers in cleanroom facilities.