ISO 14644 Cleanroom Classes Explained for Hospital Operation Theatres

ISO 14644-1 classifies a room by one thing only: how many airborne particles of a given size are present in a cubic metre of its air. It says nothing about air change rate, nothing about filter grade, nothing about bacteria, and — crucially — nothing about whether the room is empty or full of surgeons. Everything a hospital argues about after a failed validation traces back to one of those four omissions.

This is the reference page for that. The classification table, the translation to the Class 100 and Class 10,000 names Indian tenders still use, which class actually belongs where in a hospital, how the class is engineered, and how it is proved.

The ISO 14644-1 classification table

The standard defines classes ISO 1 (cleanest) to ISO 9 (roughly ordinary filtered indoor air) using a formula that scales the limit by class number and particle size. The table below gives the maximum permitted concentration in particles per cubic metre of air, equal to or larger than the stated size.

ISO Class	≥0.1 µm	≥0.2 µm	≥0.3 µm	≥0.5 µm	≥1 µm	≥5 µm
ISO 1	10	–	–	–	–	–
ISO 2	100	24	10	4	–	–
ISO 3	1,000	237	102	35	8	–
ISO 4	10,000	2,370	1,020	352	83	–
ISO 5	100,000	23,700	10,200	3,520	832	–
ISO 6	1,000,000	237,000	102,000	35,200	8,320	293
ISO 7	–	–	–	352,000	83,200	2,930
ISO 8	–	–	–	3,520,000	832,000	29,300
ISO 9	–	–	–	35,200,000	8,320,000	293,000

The dashes are not omissions. The current revision of ISO 14644-1 deliberately leaves cells blank where the concentration is either too low or too high to be counted with acceptable statistical confidence by a standard optical particle counter. The most consequential blank is ISO 5 at ≥5 µm — it used to exist, it was removed, and Indian specifications that still demand a “≥5 µm result for the ISO 5 zone” are asking for a number the standard no longer defines. Check which revision your specification was written against; classification tables and sampling rules have both changed between revisions.

Two things follow from the table that people find counter-intuitive.

Each class step is a factor of ten. ISO 7 permits ten times the particle load of ISO 6 and one tenth that of ISO 8. There are no intermediate grades in the standard, though ISO 14644-1 does allow a class to be specified to one decimal place where a project genuinely needs it.

A class is meaningless without a particle size. “ISO Class 7” on its own is shorthand for the ≥0.5 µm limit, because that is the size almost everyone measures. If a specification names a different size, the limit changes and so does the instrument setup.

Translating the old Class 100 / 10,000 names

Indian tender documents, hospital briefs and vendor datasheets are still full of “Class 100 laminar flow” and “Class 10,000 OT.” Those names come from US Federal Standard 209E, which was cancelled in 2001 and superseded by ISO 14644. The names survive because they are short and because a generation of specifications was written using them.

FS 209E counted particles ≥0.5 µm per cubic foot. ISO 14644-1 counts per cubic metre. That is the whole translation, and it is why the numbers look nothing alike.

FS 209E class	Particles ≥0.5 µm per ft³	ISO 14644-1 equivalent	Particles ≥0.5 µm per m³	Where it shows up in hospitals
Class 1	1	ISO 3	35	Semiconductor work, not hospitals
Class 10	10	ISO 4	352	Specialist pharmaceutical, not hospitals
Class 100	100	ISO 5	3,520	Ultraclean surgical zone under a laminar canopy; sterile compounding hood
Class 1,000	1,000	ISO 6	35,200	Occasionally specified for high-risk theatre zones
Class 10,000	10,000	ISO 7	352,000	The conventional general operating theatre
Class 100,000	100,000	ISO 8	3,520,000	OT periphery, sterile storage, CSSD packing, ante-rooms
(no equivalent)	–	ISO 9	35,200,000	Ordinary filtered indoor air

Two practical warnings when you meet these names in a live document.

The equivalence is close but not exact. ISO 5 and Class 100 are treated as interchangeable in practice, and for a hospital the difference is not clinically meaningful. But they are limits from two different standards with two different measurement and sampling regimes, so a certificate issued under one is not automatically a certificate under the other.

Never leave both names in the same clause. A tender that says “ISO Class 5 (Class 100) at ≥5 µm, tested as per FS 209E” is asking for something incoherent. Pick ISO 14644-1, state the class, state the particle size, state the occupancy state, and state the test method. If the client’s brief uses the old names, translate them in the specification and note the translation.

Which class belongs where in a hospital

There is no universal mandate assigning classes to hospital spaces, and different national guidance documents land in different places. What follows is common Indian design practice, consistent with the way HTM 03-01, ASHRAE 170 and NABH expectations are usually reconciled. Treat it as a starting point for a design discussion, not a rule.

Space	Typical classification	Occupancy state usually specified	Notes
Ultraclean surgical zone — joint replacement, implant orthopaedics, neurosurgery, transplant, cardiac	ISO 5 within the canopy footprint	At rest, with a separate operational or microbiological acceptance criterion	Achieved by a unidirectional canopy, not by the room as a whole
General major operating theatre	ISO 7	At rest	The single most common Indian OT specification
OT periphery, outside the canopy in an ultraclean theatre	ISO 7, occasionally ISO 8	At rest	Specifying ISO 5 for the whole room is an expensive misunderstanding
Scrub bay, sterile store, sterile corridor	ISO 7 to ISO 8	At rest	Positively pressured; class often specified loosely or not at all
Pre-operative holding, PACU	Usually unclassified	–	Specified by air change rate, filtration and pressure instead
Isolation / negative pressure room	Usually unclassified	–	Defined by pressure regime, exhaust and filtration, not particle class
Intensive care unit	Usually unclassified; where a class is asked for, ISO 8	At rest	ACH, filtration grade and pressure are the meaningful parameters
Neonatal intensive care	Usually unclassified	–	Temperature, humidity, acoustics and air quality dominate; particle class rarely specified
CSSD — sterile packing and sterile storage	ISO 8 typical for packing and sterile store	At rest	Pressure cascade from sterile store down to washing area matters more than the class number
CSSD — washing / decontamination	Unclassified, negative	–	Should never be positively pressured toward clean areas
Pharmacy sterile compounding	ISO 5 inside the hood or isolator, ISO 7 buffer room, ISO 8 ante-room	Both states, per the applicable pharmacy standard	This is the one hospital area where the pharmaceutical model applies directly
Burns unit	Sometimes ISO 7 for high-dependency burns	At rest	Specified case by case

The pattern worth internalising: outside the operating theatre and the pharmacy clean suite, particle classification is rarely the right specification tool. For an ICU or a NICU, air change rate, filtration grade, pressure relationship, temperature and humidity describe the requirement better and cost less to verify. Specifying “ISO Class 8 ICU” in a tender adds a validation cost without adding clinical value, and it does not substitute for the parameters that actually matter. Those are set out in NABH requirements for operation theatres for theatre spaces.

How a cleanliness class is actually achieved

A class is an outcome. Five engineering decisions produce it, and none of them appear in ISO 14644-1.

Air change rate. Dilution is the primary mechanism in a conventionally ventilated theatre. Indian OT practice generally starts at 20 air changes per hour for a general theatre and 25 to 30 for high-risk specialties. More air changes reduce steady-state particle concentration and — more importantly — shorten recovery after a contamination event. Above roughly 25 to 30 ACH in a turbulently ventilated room, returns diminish sharply while fan energy keeps climbing.

Filtration grade, and what EN 1822 actually says. Filter classes under EN 1822 are defined at the Most Penetrating Particle Size (MPPS), typically in the 0.1 to 0.3 µm region, which is where filter media are least effective. That is a deliberately harder test than the familiar American “99.97% at 0.3 µm” figure, and the two are not the same claim.

EN 1822 class	Overall efficiency at MPPS	Local (leak) efficiency at MPPS	Typical hospital use
E11	≥95%	not defined	Final filter in non-critical AHUs
E12	≥99.5%	not defined	Occasionally as a fine filter stage
H13	≥99.95%	≥99.75%	Terminal filter for general theatres and many clean areas
H14	≥99.995%	≥99.975%	Terminal filter for ultraclean canopies and high-risk theatres
U15	≥99.9995%	≥99.9975%	Pharmaceutical and semiconductor; rarely justified in hospitals

The local efficiency column is the one that matters on site. It is the basis of the installed-filter integrity scan, in which an aerosol is introduced upstream and a probe is traversed across the downstream face and the frame seal to find leaks. A filter certified in the factory and leaking at its gasket delivers nothing. Insist on an in-situ scan after installation, and after every replacement.

Note also that the filter grade does not set the room class. An H14 filter installed in a room with a leaky ceiling, insufficient air changes and eight people moving around will not produce ISO 5. Filter grade and room class are independent specifications and both belong in the tender.

Unidirectional versus turbulent airflow. A conventional theatre is turbulently ventilated: clean air is supplied, mixes with room air, dilutes contamination and is extracted. It can reliably hold ISO 7. It cannot hold ISO 5 over an open wound, because mixing is the mechanism.

Unidirectional (laminar) airflow works differently. A large filter array above the operating table delivers air as a coherent downward stream at a controlled velocity — commonly in the 0.25 to 0.45 m/s region, with the exact figure set by the canopy design and the standard being followed — sweeping particles shed by the surgical team away from the wound and the instrument trolley rather than mixing them into the room. This is what buys ISO 5, and it buys it only within the canopy footprint. Step outside the canopy and you are back in the ISO 7 room. Understanding that boundary prevents both over-specification and disappointment.

Pressure cascade. Clean spaces must be positive to less clean ones so that leakage flows outward. The theatre sits at the top of the cascade, then the sterile corridor and clean zone, then the protective zone, with dirty utility and sluice negative and separately extracted. Indian specifications commonly use ≥2.5 Pa as a floor between adjacent spaces and often work a 5 to 15 Pa cascade across the whole complex. Pressure only works if the envelope is tight enough to hold it, which leads directly to the last point.

Room sealing. Every unsealed penetration is a bypass around your entire filtration investment. Panel joints, door seals, view panel perimeters, light fitting frames, gas terminal boss plates, pendant collars, sprinkler escutcheons, grille frames, cable entries and the ceiling-to-wall junction all leak if not detailed and executed properly. A theatre that cannot hold its pressure differential with the doors closed is a theatre that will not hold its class, and no amount of extra fan capacity fixes it economically. This is the strongest engineering argument for a modular envelope over site-built construction, and it is covered in more depth on our modular operation theatre page.

At-rest versus operational: the point most specifications miss

ISO 14644-1 defines three occupancy states, and a classification result is meaningless unless one of them is stated.

State	Definition	What it tells you
As-built	Construction complete, services connected and functioning, no production equipment, no people	That the shell, ducting and filters were installed correctly. Useful as a construction milestone, useless clinically
At rest	Equipment installed and running as agreed, no people present	The performance ceiling of the installation. This is what almost every OT certificate in India actually reports
Operational	Equipment running and the specified number of people carrying out the specified activity	What the patient is actually exposed to

The gap between at-rest and operational is not marginal. People are the dominant particle source in an operating theatre — a gowned person sheds continuously, and shedding scales with movement. Door openings collapse the pressure cascade momentarily and admit corridor air. Electrosurgical plume, gauze, drapes, packaging and warming devices all add particle load. A theatre certified ISO 7 at rest, with six staff and a busy list, is a materially dirtier room while the surgery is happening.

The regulated pharmaceutical world handles this openly. EU GMP grades are defined in both states, and the drop is explicit:

GMP grade	At rest	In operation
Grade A	ISO 5	ISO 5
Grade B	ISO 5	ISO 7
Grade C	ISO 7	ISO 8
Grade D	ISO 8	not defined

Look at Grade B: the same physical room is ISO 5 empty and ISO 7 in use. That is the honest scale of the effect, and it is why a hospital that specifies only an at-rest class has specified only half a requirement.

What to do about it in practice:

• State the occupancy state for every acceptance criterion. “ISO Class 7 at rest, verified per ISO 14644-1” is a specification. “ISO Class 7” is not.
• Where operational performance matters clinically, add a second criterion in the operational state — either a particle limit or, more usefully in a theatre, a microbiological limit measured during real activity.
• Add a recovery test. This is the most informative single test for a theatre and the most commonly omitted. It challenges the room and measures how long the ventilation takes to return it to class — a 100:1 reduction is the usual construction, and many Indian specifications set the target in the 15 to 20 minute region. Recovery time predicts operational behaviour far better than an at-rest particle count does, because it measures the room’s ability to clear the contamination that surgery generates.
• Use microbiology as the operational check. Particle counters cannot distinguish a skin scale carrying bacteria from an inert dust particle. Active air sampling and settle plates during real cases are the measurement that connects to surgical site infection, and conventional theatres are commonly held to around 35 CFU/m³ in use with ultraclean zones an order of magnitude tighter. Confirm the figures against the guidance your infection control policy adopts.

Measurement and validation

Instrument. A discrete-particle optical counter with a valid, traceable calibration certificate, set to the particle sizes named in the specification, with a stated flow rate and isokinetic probe where required.

Number of sampling locations. This is where a lot of Indian test reports are quietly wrong. The older revision of ISO 14644-1 used the square root of the floor area in square metres to set the number of sampling points, and that rule is still copied into tender documents and test templates today. The current revision replaced it with a lookup table derived from a statistical confidence model, which generally requires more points for a given area. Check which method your test agency used. A report that says “number of locations = √A” is following a withdrawn rule.

Where to sample. Locations distributed to represent the whole room, at working height, with at least one point in each critical zone. In a theatre that means points at the operating table position, at the instrument trolley position and at the periphery — not three points clustered where the counter was convenient to place.

Sample volume. Each location must be sampled long enough to give the result statistical meaning: the standard requires a minimum volume per location and enough volume that a room exactly at the class limit would yield a countable number of particles. In practice this makes cleaner classes take much longer to test, which is one reason under-sampled ISO 5 certificates exist.

Supporting tests. Classification alone does not describe a theatre. A complete validation package should also include airflow volume and air change rate, downflow velocity and uniformity under any unidirectional canopy, installed filter integrity scan, room pressure differentials across the full cascade, recovery test, temperature and humidity performance, illumination, and containment or smoke visualisation where airflow patterns are in question.

Frequency. ISO 14644-2 sets maximum intervals for re-classification — commonly six months for ISO 5 and cleaner and twelve months for ISO 6 to ISO 9 — and expects a monitoring plan between those points. Re-validate additionally after any filter change, any AHU or ducting modification, any building work adjacent to the theatre, and any unexplained cluster of infections. Confirm the current intervals in the standard rather than relying on habit.

Who performs it. An agency independent of the contractor who built the theatre, with accredited calibration on every instrument, issuing a signed report that states the occupancy state, the method, the sampling plan, the instrument serial numbers and the acceptance criteria alongside the results.

Common specification mistakes in Indian tenders

Mistake	Why it goes wrong	Better clause
Class stated with no occupancy state	Every party assumes the state that suits them; disputes at handover	”ISO Class 7 at rest, verified in accordance with ISO 14644-1”
Class stated with no test method or sampling plan	Report produced under a withdrawn or invented method	Name ISO 14644-1 for classification and ISO 14644-3 for the supporting tests, and require the sampling plan in the report
ISO 5 specified for the entire theatre	Enormous cost for no clinical gain; usually unachievable in a room with doors and people	”ISO Class 5 within the unidirectional canopy footprint; ISO Class 7 elsewhere in the theatre”
ISO 5 specified for routine general surgery	Ultraclean ventilation is justified for implant and high-risk surgery, not for every list	Match the class to the case mix, and design the canopy only where it earns its cost
Filter grade treated as equivalent to room class	”H14 HEPA therefore ISO 5” is a non-sequitur	Specify the filter grade, the installed integrity test and the room class as three separate requirements
Mixing EN 1822 and US filter definitions in one clause	”H14, 99.97% at 0.3 micron” mixes two incompatible test bases	Specify EN 1822 classes with efficiency stated at MPPS, or ISO 29463
Old FS 209E names left in the document	Cancelled standard; no valid test method to cite	Translate to ISO classes and note the translation
ISO class demanded for ICU, NICU or wards	Validation cost with no clinical benefit; distracts from the parameters that matter	Specify ACH, filtration grade, pressure relationship, temperature and humidity
No recovery test	The most predictive test of operational behaviour is missing	Require a 100:1 recovery test with a stated target time
No revalidation interval or owner named	Certificate expires quietly; discovered at assessment	State the interval, put it in the AMC scope with fixed dates
Contractor validates its own work	No independence; findings tend not to appear	Require third-party verification appointed by the hospital

Related reading

• NABH Requirements for Operation Theatres: A Practical Checklist — the full environmental parameter set, zoning, finishes and the documentation an assessor asks for.
• HTM 02-01 vs NFPA 99: Which Medical Gas Standard Applies in India? — the same specification discipline applied to medical gas.
• Modular Operation Theatre — envelope, filtration, laminar flow and the validation package delivered at handover.

Specifying a theatre, or holding a validation report you are not sure you can defend? Request a validation and specification review and we will tell you what your current documents actually prove.