“Class 100” and “ISO 5” describe the same cleanroom, but they come from two different standards, and both phrases are still in active use—on equipment nameplates, in older facility drawings, and in casual conversation among people who trained before 2001. Cleanroom classification is the system for stating, numerically, how many airborne particles of a given size a controlled environment is permitted to contain. The current governing standard is ISO 14644-1; the legacy U.S. federal standard it replaced, FED-STD-209E, is officially withdrawn but its class numbers (Class 100, Class 1,000, Class 10,000) remain common shorthand, especially around semiconductor fabs, older pharmaceutical facilities, and secondhand cleanroom equipment.
This guide covers the ISO 14644-1 classification table in full, how it maps to the retired FED-STD-209E classes, and—because a classification number on a spec sheet is meaningless without knowing how it’s actually achieved and confirmed—what maintains a class in daily operation and how classification is verified.
What a Cleanroom Classification Actually Measures
ISO 14644-1 (Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration) classifies a space by the maximum permitted concentration of airborne particles, expressed as particles per cubic meter of air, at one or more particle-size thresholds between 0.1 µm and 5 µm. It says nothing directly about temperature, humidity, chemical contamination, or microbial (viable) counts—those are addressed separately, by facility design specifications and, in pharmaceutical and biologics manufacturing, by frameworks like EU GMP Annex 1 that layer viable-particle (microbial) grades on top of the ISO particle classes.
Classification is assigned a single ISO class number, N, ranging from ISO Class 1 (the cleanest defined class) to ISO Class 9 (the least clean). The maximum concentration limit for each particle size is derived from a standard formula:
Cn = 10N × (0.1 / D)2.08
where Cn is the maximum permitted concentration (particles/m³) of particles equal to or larger than the size D (in micrometers), and N is the ISO class number. Every value in the classification table below is generated from this single equation—which is why the table has a consistent internal logic rather than being an arbitrary lookup list.
ISO 14644-1 Classification Table (ISO Class 1–9)
The table below gives the maximum permitted particle concentration, in particles per cubic meter, at each specified particle size. A dash indicates the standard does not define a limit at that size for that class—either because the expected concentration is not statistically meaningful to measure (very low counts of large particles in ultra-clean classes) or because, in practice, coarser classes are specified only from 0.5 µm upward.
| ISO Class | Nearest FED-STD-209E equivalent | ≥0.1 µm | ≥0.2 µm | ≥0.3 µm | ≥0.5 µm | ≥1 µm | ≥5 µm |
|---|---|---|---|---|---|---|---|
| ISO Class 1 | — | 10 | 2 | — | — | — | — |
| ISO Class 2 | — | 100 | 24 | 10 | 4 | — | — |
| ISO Class 3 | Class 1 | 1,000 | 237 | 102 | 35 | 8 | — |
| ISO Class 4 | Class 10 | 10,000 | 2,370 | 1,020 | 352 | 83 | — |
| ISO Class 5 | Class 100 | 100,000 | 23,700 | 10,200 | 3,520 | 832 | 29 |
| ISO Class 6 | Class 1,000 | 1,000,000 | 237,000 | 102,000 | 35,200 | 8,320 | 293 |
| ISO Class 7 | Class 10,000 | — | — | — | 352,000 | 83,200 | 2,930 |
| ISO Class 8 | Class 100,000 | — | — | — | 3,520,000 | 832,000 | 29,300 |
| ISO Class 9 | — | — | — | — | 35,200,000 | 8,320,000 | 293,000 |
Values per ISO 14644-1:2015, Table 1, rounded to three significant figures. ISO Class 9 is approximately equivalent to ordinary room air and is rarely specified as a design target; it exists mainly as the outer boundary of the classification system.
FED-STD-209E: Why “Class 100” Is Still Said Out Loud
FED-STD-209E was a U.S. General Services Administration federal standard that classified cleanrooms by the number of particles ≥0.5 µm per cubic foot of air—Class 1, Class 10, Class 100, Class 1,000, Class 10,000, and Class 100,000, with the class number literally equal to the maximum particle count per cubic foot at that threshold. ISO 14644-1 superseded it internationally in 1999, and the U.S. General Services Administration formally cancelled FED-STD-209E in 2001 in favor of the ISO standard.
The commonly cited equivalences are:
- FED-STD-209E Class 1 ≈ ISO Class 3
- FED-STD-209E Class 10 ≈ ISO Class 4
- FED-STD-209E Class 100 ≈ ISO Class 5
- FED-STD-209E Class 1,000 ≈ ISO Class 6
- FED-STD-209E Class 10,000 ≈ ISO Class 7
- FED-STD-209E Class 100,000 ≈ ISO Class 8
These are approximate because the two standards use different base units (particles per cubic foot vs. particles per cubic meter) and, in some cases, different reference particle sizes—the mapping is a widely used practical correspondence, not an exact unit conversion. Despite being withdrawn for over two decades, the FED-STD-209E class numbers persist because they are shorter to say, appear on the nameplates of equipment still in service, and are embedded in the design documentation of facilities built before 2001. When a legacy number appears in a spec sheet or purchase order, treat it as informal shorthand and confirm the actual required ISO class against current design documentation rather than assuming the mapping above applies exactly to that specific facility.
What Actually Maintains a Class in Practice
A classification number describes an outcome, not a mechanism. Achieving and holding it depends on a small number of engineering controls working together continuously:
HEPA and ULPA Filtration
High-efficiency particulate air (HEPA) filters remove at least 99.97% of particles at 0.3 µm (the standard’s “most penetrating particle size” test point); ultra-low particulate air (ULPA) filters remove at least 99.999% at 0.12 µm. Cleaner ISO classes (ISO 5 and below) typically require ULPA filtration or very high-coverage HEPA ceiling arrays; less stringent classes (ISO 7–8) can usually be held with HEPA filtration at partial ceiling coverage.
Air Changes Per Hour (ACH)
Air changes per hour—the number of times the room’s full air volume is filtered and replaced each hour—is the primary dilution mechanism for particles generated by people, equipment, and processes inside the room. ISO 14644-1 itself does not mandate specific ACH values (that is a design and engineering decision, addressed by companion standards such as ISO 14644-4 on design and construction), but commonly cited industry design ranges are approximately 5–48 ACH for ISO 8, 60–90 ACH for ISO 7, 90–180 ACH for ISO 6, and 240–480 ACH (often approaching full unidirectional, “laminar” airflow) for ISO 5 and cleaner. Higher-classified rooms increasingly rely on unidirectional airflow across the entire work zone rather than simple air-change dilution.
Pressure Cascade
Adjacent rooms of different classifications are held at different static pressures, with the cleaner room at higher pressure, so that air only ever flows from clean to less-clean spaces through door gaps and other small openings—never the reverse. This cascade is what keeps a contaminant excursion in an adjacent corridor or gowning room from migrating into the classified space, and it is verified independently of particle counting, typically with a differential-pressure gauge and a documented alarm threshold.
Gowning and Personnel Protocol
People are the dominant particle source in most classified environments—shedding skin cells, fibers, and cosmetics at rates that vary enormously with movement and garment coverage. Gowning requirements (coveralls, hoods, gloves, and in the cleanest classes, full-coverage suits with limited skin exposure) scale directly with class: an ISO 8 space may require only a lab coat and hairnet, while ISO 5 and cleaner typically require a full cleanroom suit donned in a dedicated, sequenced gowning room. Gowning procedure, not just garment selection, is what determines its actual effectiveness—an approved suit put on incorrectly contributes almost nothing.
How Classification Is Verified: As-Built, At-Rest, and In-Operation
ISO 14644-1 explicitly defines three distinct occupancy states in which a room can be tested, because particle counts differ substantially between them:
- As-built — the facility is complete, with all services connected and functioning, but no production equipment, materials, or personnel are present.
- At-rest — the facility is complete with equipment installed and running (or in an agreed idle mode), but no personnel are present.
- Operational — the facility is functioning in its normal production mode, with personnel present and working as specified.
A room’s particle count rises substantially, sometimes by an order of magnitude or more, between at-rest and operational states, because personnel movement is the largest variable particle source. A facility’s classification certificate should always state which occupancy state it was tested in—an “ISO 7 at-rest” certification is not evidence of ISO 7 performance during active production, and comparing a vendor’s at-rest spec against a competitor’s operational spec is a common, misleading error when evaluating cleanroom equipment or facilities.
Formal classification testing follows the sampling and statistical procedures set out in ISO 14644-1 Annex A: a minimum number of sample locations (scaled to room area), a minimum sample volume per location and particle size, and a statistical confidence calculation (the Upper Confidence Limit, or UCL) applied to the mean of the sample results, not simply the raw average. Ongoing monitoring between formal recertifications is typically continuous or scheduled particle counting at fewer, fixed locations—a lighter-weight check meant to catch drift, not a substitute for full re-classification testing, which is generally repeated on a periodic schedule (commonly every six to twelve months, though the applicable GxP or accreditation framework governing the facility may set a specific interval).
Frequently Asked Questions
Is FED-STD-209E still a valid standard?
No. It was officially cancelled by the U.S. General Services Administration in 2001, with ISO 14644-1 as its replacement. The class numbers remain in informal, conversational use, but no current facility or equipment specification should cite FED-STD-209E as its governing standard.
What is the cleanest ISO class actually built in practice?
ISO Class 1 through ISO Class 3 environments are extremely rare and technically demanding—seen mainly in leading-edge semiconductor lithography and a handful of specialized research applications. Most pharmaceutical sterile manufacturing operates at ISO 5 (critical zones) through ISO 8 (background areas); most semiconductor fabrication operates in the ISO 3–6 range depending on the process step.
Does a higher ISO class number mean a cleaner room?
No—the opposite. A lower ISO class number means fewer permitted particles and a cleaner room. ISO Class 1 is the cleanest defined class; ISO Class 9 is the least clean (roughly equivalent to typical indoor room air).
Can a room be classified for one particle size but not another?
Yes. ISO 14644-1 allows classification against a single specified particle size threshold rather than the full set, provided at least one size is chosen from the standard’s defined range and the classification is reported alongside the size(s) actually tested. A room’s designation should always specify which threshold(s) the classification applies to when it isn’t the full standard table.
Does ISO 14644-1 cover microbial or viable particle limits?
No. ISO 14644-1 addresses only non-viable (total) particle counts. Viable (microbial) contamination limits for pharmaceutical and biologics manufacturing are addressed separately, most notably by EU GMP Annex 1, which assigns its own Grade A–D system with both viable and non-viable limits for at-rest and in-operation states.
Related CASRAI Resources
- Laboratory Compliance & Quality — cluster hub for GxP, accreditation, biosafety, and facility compliance
- Good Manufacturing Practice (GMP): A Guide for Research Institutions
