TL;DR: ISO 14644-1 classifies a cleanroom by how many airborne particles are allowed per cubic meter of air. The lower the ISO class number, the cleaner the room, and each step is ten times stricter: at 0.5 µm, ISO 5 allows 3,520 particles per cubic meter and ISO 8 allows 3,520,000. That class drives the HVAC design, filters, airflow, gowning and construction rules, which is why cleanroom knowledge is so valuable on fab and plant projects.
When I recruit for U.S. fab and plant projects, “cleanroom experience” shows up in job posts for mechanical engineers, project engineers, superintendents, QA staff and even safety roles. Many candidates have walked through a cleanroom but cannot explain what “ISO 6” or “ISO 7” actually means. This post is my plain-English guide to cleanroom classes ISO 14644 explained for job seekers, so you can talk about it confidently in an interview.
I am summarizing, not reproducing, the ISO standards, which are sold by ISO. Where I give numbers, they come from public government documents that cite ISO 14644-1. Everything is as of late October 2026.
What ISO 14644-1 is, and what it covers
ISO 14644-1:2015 is the international standard for classifying air cleanliness in cleanrooms and clean zones by the concentration of airborne particles. A few basics from ISO’s own page:
- It is the second edition, published in December 2015, last reviewed and confirmed in 2021, and still current.
- Classification considers particle sizes from 0.1 µm to 5 µm.
- Ultrafine particles below 0.1 µm are outside its scope, and larger “macroparticles” can be described separately with an M descriptor.
ISO 14644 is a family of standards, not one document. For job seekers, three parts matter most:
| Part | What it covers | Who uses it most |
|---|---|---|
| ISO 14644-1:2015 | Classification of air cleanliness by particle concentration | Owners, designers, certifiers |
| ISO 14644-4:2022 | Design, construction and start-up of cleanrooms, with checklists for each stage and energy-efficiency guidance | A&E firms, contractors, commissioning teams |
| ISO 14644-5:2025 | Operations: an operations control program covering personnel, gowning, cleaning, training, material transfer, maintenance and monitoring | Plant operations, facilities, EHS |
Cleanroom classes ISO 14644 explained: what the number means
The ISO class number is a cleanliness grade. A lower number means fewer particles allowed. Each step down is ten times cleaner. The easiest public source showing this is the FDA’s aseptic processing guidance, which lists the ISO 14644-1 designations used in drug manufacturing next to the old U.S. class names:
| ISO class | Max particles ≥0.5 µm per m³ | Old U.S. name (particles per ft³) |
|---|---|---|
| ISO 5 | 3,520 | Class 100 |
| ISO 6 | 35,200 | Class 1,000 |
| ISO 7 | 352,000 | Class 10,000 |
| ISO 8 | 3,520,000 | Class 100,000 |
Source: FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, Table 1. The FDA notes that an ISO 5 particle concentration equals Class 100. This table shows only the four classes common in drug manufacturing. ISO 14644-1 covers a wider range (the NASA standard below, for example, refers to spaces cleaner than Class 5) and sets limits at other particle sizes between 0.1 and 5 µm; for those exact values, read the standard.
Two things to remember for interviews:
- Old names still float around. You will hear “Class 100” or “Class 10,000” on sites. Those come from the old U.S. system that counted particles per cubic foot. Know how they line up with ISO classes.
- The class depends on the occupancy state. A NASA facility standard, MSFC-STD-246E, uses the common terms as-built (no equipment or people), at-rest (equipment installed and running, no people) and operational (equipment and people present). A room that passes as-built may not pass in operation, so always ask which state the requirement refers to.
How HVAC and airflow keep a room in class
People and processes create particles all day. The HVAC system’s job is to remove them faster than they build up and to keep dirty air from coming in. The main tools:
- HEPA (and ULPA) filters: MSFC-STD-246E defines a HEPA filter as at least 99.97% efficient for particles 0.3 µm and larger, and allows ULPA filters for spaces cleaner than Class 5.
- Airflow pattern: unidirectional (often called laminar) flow pushes filtered air in one direction across the work area; conventional (non-unidirectional) flow mixes and dilutes. Cleaner classes generally need unidirectional flow and much more filter coverage.
- Air change rate: how many times per hour the room’s air is replaced with filtered air.
- Pressure cascade: cleaner rooms are kept at higher pressure than less clean neighbors, so air leaks out, not in.
- Temperature and humidity: controlled for the process, for static, and for comfort in full gowns.
To show how this turns into hard design numbers, here is how that one NASA facility standard sets them. These are NASA’s requirements for its own facilities, not universal rules; every project sets its own in the owner’s requirements.
| Item (MSFC-STD-246E example) | Requirement |
|---|---|
| Air changes, conventional flow Class 8 | At least 15 per hour |
| Air changes, conventional flow Class 7 | At least 20 per hour |
| Unidirectional flow filter face velocity at initial certification | 0.46–0.56 m/s (90–110 ft/min) |
| Unidirectional flow filter coverage | 80% or more of the wall or ceiling |
| Minimum pressure differential, cleanrooms | 12.5 Pa (0.05 in. water) |
| Filter stages | Pre-filter, medium-efficiency filter, final HEPA |
Gowning: the people side of cleanliness
People are a major particle source, so gowning is serious. ISO 14644-5:2025 requires an operations program that includes a gowning program and training. The details depend on the facility, but the pattern is consistent. MSFC-STD-246E, for example, says garments are put on and taken off only in the gowning area, and lists a top-down order:
- Cap, snood or hood, with facial hair cover or face mask as needed
- Frock or coverall
- Shoe covers or booties
- Gloves, with all zippers, snaps and ties fastened before entry
In drug manufacturing, the FDA guidance recommends gowns that are sterilized, nonshedding and cover the skin and hair. On a construction site that is turning into a cleanroom, expect a “clean build” protocol: restricted entry, tacky mats, wipe-downs of tools and materials, and gowning that gets stricter as the space gets closer to certification.
Why the class drives design and construction
The target class is one of the first decisions on a fab project, and it ripples through everything:
- Building size and shape: unidirectional flow needs room for air handling above and return paths below, which is why fabs often have tall ceiling plenums and subfab levels.
- Mechanical systems: more filtered air means bigger fans, more filters, more energy and more cooling. ISO 14644-4 includes energy-efficient design for this reason.
- Materials and finishes: walls, ceilings, floors and fixtures are chosen to shed few particles and be easy to clean.
- Construction sequence: dirty work has to finish before clean work starts, and protocols tighten in stages.
- Start-up and certification: ISO 14644-4 covers start-up and verification; the room is tested against its class before handover.
Roles involved, step by step through a project
Here is how the work flows and who does it. I recruit for most of these roles:
- Requirements: owner process and facility engineers set the class, occupancy state and environmental limits.
- Design: A&E firm architects, mechanical (HVAC) engineers, electrical engineers and process utility engineers turn that into drawings.
- Coordination: BIM coordinators fit air handling, ductwork, process piping and power into tight spaces.
- Construction: project engineers, superintendents, cleanroom specialty contractors and QA/QC staff build it under clean-build protocols.
- Safety: EHS staff manage risks during construction and tool install. See my guide to safety jobs at battery and chip plants.
- Commissioning and certification: commissioning agents and test technicians verify airflow, pressures and particle counts.
- Operations: facilities teams and technicians keep the room in class every day.
Many of these roles are on project sites away from home, so check travel and per diem terms. Site roles also usually need an OSHA 10 or 30 card.
What’s different at Korean-affiliated fab and battery projects
- Owner standards on top of ISO: owners often bring their own facility standards from projects abroad. Expect to work to both the ISO class and the owner’s internal spec.
- Terms in two languages: cleanroom terms are often used in English even in Korean conversations, but bilingual staff help a lot when explaining test results or nonconformances.
- Fast schedules: clean-build deadlines can mean long weeks during start-up. Ask about overtime and shift patterns up front.
Questions to ask in the interview
- What ISO class are the main process areas, and in which occupancy state?
- Is the airflow unidirectional, conventional, or mixed by zone?
- What clean-build protocol does the site use, and when does it start?
- Who certifies the rooms, and what is my role in testing?
- Are there owner cleanroom standards beyond ISO 14644 that I should read?
FAQ
Is ISO 5 cleaner than ISO 7?
Yes. Lower numbers are cleaner. At 0.5 µm, ISO 5 allows 3,520 particles per cubic meter and ISO 7 allows 352,000, per the FDA’s table.
What does “Class 100” mean?
It is the old U.S. name based on particles 0.5 µm and larger per cubic foot. The FDA says an ISO 5 concentration equals Class 100.
Is ISO 14644-1 still current?
Yes. ISO says the 2015 edition was reviewed and confirmed in 2021 and remains current. Part 4 was updated in 2022 and Part 5 in 2025.
Do I need cleanroom experience to get hired on a fab project?
Not always, but it helps. If you lack it, learn the basics in this post and show related experience, such as controlled environments, HVAC, commissioning or strict QA programs.
Where do I get the full particle limit table?
From ISO 14644-1 itself, sold through ISO and national standards bodies. Your employer may already have a copy.
Not legal advice. I summarize ISO standards and do not reproduce them; project requirements decide. Sources: ISO pages for ISO 14644-1:2015, 14644-4:2022 and 14644-5:2025; FDA Guidance for Industry, Sterile Drug Products Produced by Aseptic Processing (Table 1, Section V.A); NASA MSFC-STD-246E, Standard Design and Operational Criteria for Controlled Environmental Areas. Accessed October 2026.
Engineering exam prep and references on Amazon
Learn the airflow and pressure basics first; they explain most cleanroom rules.
- Cleanroom design books
- Cleanroom technology fundamentals
- HVAC design handbooks
- Contamination control guides
- Commissioning process guides
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