Cleanroom classification rates air quality by maximum particle count per volume, with lower numbers indicating cleaner environments. Standards: ISO 14644-1 (international) and Federal Standard 209E (legacy but still referenced). ISO classes: ISO 1 (10 particles/m3 at 0.1um) to ISO 9 (ambient air). Semiconductor uses ISO 1-5. Federal classes: Class 1, 10, 100, etc. Numbers indicate particles per cubic foot at 0.5 microns. Typical semiconductor requirements: Lithography: Class 1 (ISO 3). General fab: Class 10-100 (ISO 4-5). Backend/packaging: Class 1000+. Particle sizes measured: 0.1, 0.2, 0.3, 0.5, 1.0, 5.0 microns. Smaller particles harder to filter, more damaging at advanced nodes. Monitoring: Continuous particle counters throughout cleanroom. Validate classification regularly. Achieving classification: HEPA/ULPA filters, air changes per hour, positive pressure, gowning protocols, material controls. Cost correlation: Cleaner classification = exponentially more expensive. Restrict highest class to critical areas. Evolution: As chip features shrink, cleaner environments needed. Sub-10nm requires extreme cleanliness.