Cleanroom HVAC and contamination control field guide

A cleanroom HVAC system, and the particles you cannot see

A cleanroom HVAC system holds the air in a room clean enough that a particle you cannot see, one of millions floating in a single breath of ordinary room air, does not ruin the work happening inside. Make a semiconductor and a particle a fraction of the width of the circuit lands on the wafer and kills the chip. Fill a vial of injectable drug and a single microbe-carrying particle contaminates the dose. The system that prevents that is the HVAC, run far past anything a comfort system ever does.

It gets there four ways at once. It pushes large volumes of HEPA- or ULPA-filtered air through the space at a high air-change rate. It moves that air in a controlled pattern so particles get swept out instead of stirred around. It holds a pressure cascade between rooms so air always flows from the cleaner space toward the dirtier one and never the reverse. And it locks temperature and humidity inside a tight band the process demands.

None of that is judged by eye. A cleanroom is clean because it was designed to an ISO class and then certified by particle count and filter-integrity testing, and it stays clean only as long as it keeps passing those tests. The filtration side of this is covered in the air filtration and MERV field guide, and the room-to-room and building pressure side in the building pressurization field guide. This guide is about how the whole system holds a class.

What is an ISO cleanroom class?

An ISO cleanroom class is a limit on how many airborne particles of a given size are allowed in a cubic meter of the room's air, set by ISO 14644-1. The scale runs from ISO 1 to ISO 9, and lower is cleaner. ISO 1 is the most demanding class on the scale, and ISO 9 is still a controlled space, just a relaxed one. The class is the spec the whole system is built and certified to, and it drives the filtration, the air-change rate, and the airflow regime.

The reference size most people quote is 0.5 micron. An ISO 5 room allows no more than 3,520 particles 0.5 micron or larger per cubic meter. ISO 7 allows 352,000. ISO 8 allows 3,520,000. Each step up the scale is a factor of ten, and the standard sets limits at several particle sizes, not just 0.5 micron, so a room is classified across the sizes that matter for its work.

The older language still on a lot of drawings is US Federal Standard 209E, retired in 2001 but not forgotten. It counted particles per cubic foot, so its Class 100 means 100 particles 0.5 micron or larger per cubic foot, which lands near ISO 5. Class 10,000 is near ISO 7 and Class 100,000 near ISO 8. When a spec says Class 100, confirm which standard and which particle size it means before you design to it, because the conversion is close but not exact and the edition controls.

What the system actually controls

The target is particles, the count and the size of them, because particles are what destroy the product. A particle in this work is anything suspended in the air: a skin flake, a fiber, a speck of dust, a droplet of aerosol, a bit of the operator's own shedding. People are the largest source in most rooms, which is why gowning matters as much as filtration. The system exists to drive the particle count under the class limit and hold it there while work is happening.

In some industries the target widens. In pharmaceutical and medical work the concern is not the inert particle count alone but viable contamination, the live microbes a particle can carry, so the room is controlled for both non-viable particles and microbial load. In semiconductor work the concern reaches past particles to molecular contamination, trace gases at parts-per-billion levels that no particle filter catches.

Either way the principle holds. You cannot judge the contamination by looking. The particles are invisible, the microbes are invisible, the molecular contaminants are invisible, and the only way to know the room is in class is to measure it.

HEPA and ULPA, the final filter at the ceiling

The filtration that makes a cleanroom clean is the HEPA or ULPA filter at the ceiling, the last thing the air passes through before it enters the room. A HEPA filter captures 99.97 percent of particles at 0.3 micron, the most penetrating particle size, the size hardest for any filter to catch. Above and below that size it does even better. ULPA goes further, to 99.999 percent or better at around 0.12 micron, and it shows up where the class is tightest.

These sit above the MERV scale that rates general air filters, and the air filtration and MERV field guide covers where HEPA and ULPA fall relative to MERV and why a standard rooftop unit cannot move design airflow through one. In a cleanroom the HEPA is almost never the only filter. Prefilters and intermediate filters take the coarse and medium load upstream so the expensive final HEPA loads slowly and lasts, the same staged approach a built-up air handler uses.

The number that matters in this guide is not the rated efficiency on the box. It is whether the installed filter, in its frame, with its gasket, passes an in-place leak test. A HEPA that tests 99.99 percent in the factory means nothing if it has a pinhole in the media or a gap at the seal once it is mounted. That is the filter-integrity test, and it is covered below.

The high air-change rate

A cleanroom moves far more air than a comfort space. An office might see 4 to 8 air changes per hour. A cleanroom runs tens to hundreds, and the rate climbs as the class tightens. The air-change rate is how the room dilutes and sweeps out the particles people and processes generate, so a cleaner class needs more changes to hold the lower count.

Rough field figures, and they are figures to start from rather than design to: an ISO 8 room might run on the order of 10 to 25 air changes per hour, an ISO 7 room on the order of 30 to 60, and an ISO 6 room higher still. Once the class reaches ISO 5 and cleaner the room usually switches to unidirectional airflow, where the rate is so high it is expressed as an air velocity instead of changes per hour, equivalent to several hundred changes an hour.

The actual rate is not a fixed lookup. It depends on the class, the airflow pattern, the particle load the process and the people generate, and the recovery the design has to hit. ISO 14644 and the engineer's calculation set the number for the specific room, so treat any chart as a starting point and confirm against the design.

Unidirectional or non-unidirectional airflow?

The airflow regime is the pattern the filtered air moves in, and it splits into two types. Unidirectional flow, often called laminar, moves the air in one direction at a uniform velocity, ceiling to floor in most cleanrooms, like a piston pushing particles straight down and out the low returns before they can settle on the work. It is the cleanest pattern and it is what ISO 5 and cleaner rooms use, typically at a face velocity around 0.36 to 0.54 m/s, roughly 70 to 100 feet per minute.

Non-unidirectional flow, often called turbulent or mixed flow, supplies filtered air from ceiling diffusers and lets it mix through the room, diluting the particle concentration and carrying particles to the returns. It is what ISO 6 through 9 rooms use, and it controls contamination by dilution and air-change rate rather than by displacement.

The split is not arbitrary. Below a certain count you cannot dilute your way to the class, because the people and the process keep adding particles, so you switch to sweeping them out in a uniform stream before they reach the product. Which regime a room needs follows from its class and its process, and ISO 14644 and the engineer set it. The work that demands the cleanest air, an open drug fill or a wafer step, gets unidirectional flow right over the critical point even when the room around it is non-unidirectional.

Fan filter units in the ceiling grid

Most modern cleanrooms build the supply into the ceiling as fan filter units. An FFU is a self-contained module: a fan and a HEPA or ULPA filter in one box that drops into the ceiling grid and pushes filtered air straight down into the room. Tile the ceiling with them and you get even, distributed clean air without a central air handler ducted to every diffuser, and you can add or move units as the room changes.

How much of the ceiling is covered in FFUs scales with the class. A cleaner class needs more coverage to hit its air-change rate and airflow pattern. As a rough guide, an ISO 5 unidirectional room runs high coverage, often 60 to 100 percent of the ceiling, while an ISO 7 room might need only 15 to 25 percent. Treat those as planning numbers. Ceiling coverage is a cost-estimating shortcut, not an ISO performance parameter, and the real driver is the air-change rate and velocity the room has to certify to, which the engineer calculates.

The tradeoff with FFUs is that every unit is a fan, and the fans run continuously. That is convenient for layout and expensive to run, which the energy section comes back to.

What is a pressure cascade?

A pressure cascade is a series of rooms held at stepped pressures so each cleaner room is positive to the dirtier room next to it, and air always flows from clean to dirty across every opening. Open a door and the air spills out of the cleaner space toward the dirtier one, carrying particles away from the product instead of toward it. The cascade is how a cleanroom keeps dirty air out without relying on the door being shut.

The step between adjacent rooms is small. A common design figure is 10 to 15 Pa, roughly 0.04 to 0.06 inches of water column, between rooms of different class, with ISO 14644-4 referencing a differential in the range of 5 to 15 Pa. Each room in the suite sits a step above the one outside it, so the pressure drops in stages from the cleanest core out to the corridor.

The building pressurization field guide covers the mechanics of holding a space at a pressure, the supply-versus-exhaust balance, the relief, and how a building automation system keeps a pressure on setpoint. The cleanroom-specific point is the direction and the order: the cascade must run clean to dirty at every step, and a reversed or collapsed step is a contamination event whether or not the particle counter has caught it yet. The exact differentials come from the design and the validated protocol, so hold them to the project, not to a number carried in from the last job.

Positive for the product, negative for the hazard

Which way the cascade points depends on what you are protecting. Most cleanrooms run positive, cleaner-to-dirtier, because the thing being protected is the product and the enemy is the outside particle. Push air out of the clean space and the dirty air cannot get in. That is the default for semiconductor work, for sterile drug fill, for medical-device assembly.

The exception is when the product itself is the hazard. A room handling a potent compound, a live pathogen, a cytotoxic drug, or any material you must keep from escaping runs negative, so air flows inward and the hazard stays contained. A biosafety lab and a containment suite for potent pharmaceutical compounds invert the usual logic: the room is held negative to the spaces around it even though it is a controlled, filtered environment, because containing the hazard outranks protecting the product from outside particles.

The hard case is when you have to do both, protect a sterile product and contain a hazardous one at the same time. That is solved with airlocks that step the pressure so containment and cleanliness hold together, often a negative-pressure sink or a positive bubble between the spaces. The direction is a design decision driven by what you are protecting, and it belongs in the contamination control strategy, not in a rule of thumb. Get the direction wrong and the system works perfectly at protecting the wrong thing.

Temperature and humidity, held tight

A cleanroom controls temperature and humidity far more tightly than a comfort space, and the setpoints come from the process, not from human comfort. Temperature gets held in a narrow band, often within a degree or two of setpoint, because process equipment drifts with temperature, photolithography especially, and because gowned operators in full coverings overheat fast in a warm room.

Humidity is the one that bites. Too low and static electricity builds, and static is a particle magnet and an outright hazard to electronics, pulling charged particles onto a wafer and risking electrostatic discharge that destroys a device. Too high and you risk condensation on cold surfaces, corrosion, and microbial growth, which is a contamination problem in pharma and a defect problem in semiconductor. Many rooms hold relative humidity in a band somewhere around 30 to 60 percent, with the exact window set by the process.

The values are not generic. A wafer fab, a tablet line, and an aseptic fill room each have their own temperature and humidity windows, set by the equipment and the material and written into the validated conditions. Treat the setpoint and the tolerance as process specifications. The HVAC job is to hold them inside the band the process and the protocol require, not to a comfort default.

Why most of the air is recirculated

A cleanroom recirculates most of its air. The volumes needed to hold a class are so large that conditioning that much outdoor air from scratch would be ruinous, so the bulk of the air is captured at the low returns, passed back up through the HEPA filters, and pushed into the room again. The air is already clean and already conditioned, so recirculating it through the final filters is what makes the high air-change rate affordable at all.

Outdoor air still has to come in, for two reasons. People in the room need ventilation air, and the room needs a surplus of supply over exhaust to hold its positive pressure and feed the cascade. So the system runs a large recirculated loop with a smaller stream of conditioned makeup outdoor air added in, sized to cover the ventilation and the pressurization, the same supply-versus-exhaust accounting the building pressurization field guide works through.

The catch is that all of that air is moving all of the time, and moving it costs fan energy on a scale a comfort building never sees.

Airlocks and gowning, keeping the people-borne dirt out

People are the dirtiest thing in a cleanroom, shedding skin, hair, and fibers constantly, so the entry sequence is built to strip that load before a person reaches the clean space. Entry runs through airlocks and gowning rooms, each a pressure step in the cascade and each a stage in a protocol that has to be followed every time.

An airlock is a small room between two pressure zones with interlocked or procedurally controlled doors, so the two doors are not open at once and the pressure step is never short-circuited. It buffers the cleaner space from the dirtier one during entry. The gowning room is where the operator puts on the cleanroom garments, the coverall, hood, boots, gloves, and mask appropriate to the class, in a defined order that keeps the clean side of each garment clean. Material entry gets its own path, often a pass-through or a separate airlock, so incoming supplies do not ride in on the people route.

The protocol is the control, and it fails the way protocols fail: a propped door, a skipped step, a glove touched to a dirty surface, two airlock doors open together because someone was in a hurry. A perfectly designed cascade leaks through a propped airlock door. The gowning and entry discipline is as much a part of holding the class as any filter.

Room layout and the cascade order

The layout puts the cleanest room at the core and steps the cleanliness down outward, so the cascade and the people-and-material flow line up with the pressure. The critical process sits in the cleanest space, surrounded by less-clean support rooms, surrounded by gowning and airlocks, surrounded by the general corridor. Pressure drops as you move out, and traffic moves through the steps in order.

That adjacency is a design decision with consequences. A dirty support function next to the clean core, a material path that crosses a people path, an airlock that opens the cleanest room straight to a corridor, each is a contamination route built into the floor plan that no amount of filtration fixes later. The flow of people and the flow of material both have to run from clean to dirty in a way that matches the pressure cascade.

Get the layout right and the cascade, the gowning sequence, and the traffic all reinforce each other. Get it wrong and the HVAC spends its life fighting a floor plan that wants to move dirt the wrong way.

Recovery, how fast the room cleans itself up

Recovery is how fast a room returns to its class after something disturbs it, a door opening, a burst of activity, a process event that throws particles into the air. A cleanroom does not hold a perfect count every instant. It gets disturbed, and the measure of a healthy system is how quickly it pulls the count back under the limit.

Recovery is a defined test. ISO 14644-3 describes a recovery test that challenges the room with particles, then measures the time to return to the target concentration, often expressed as a 100:1 recovery time, the time to drop the count by two orders of magnitude. A room with adequate air changes and a good airflow pattern recovers fast. A room that recovers slowly is telling you the air-change rate or the airflow pattern is short, even if it can hold the class when nothing is happening.

The number to hold is set by the design and the protocol for the room's class and use. Recovery is one of the tests that separates a room genuinely in control from one that only looks clean when it is empty and undisturbed.

How is a cleanroom certified?

A cleanroom is certified by testing, not by inspection of how it looks. ISO 14644-1 sets the classification by particle count, ISO 14644-3 gives the test methods, and a room is in class only when it passes the measured tests at the agreed occupancy state. There are several tests, and each proves something different.

Particle count is the test that sets the class. A calibrated particle counter samples the air at a number of locations fixed by the room size, and the counts at each particle size have to fall under the class limit. Airflow is measured next, velocity and volume for unidirectional rooms and air changes per hour for non-unidirectional rooms, to confirm the room moves the air the design called for. Filter integrity is the in-place HEPA leak scan, covered in the next section, which proves the installed filters and their seals have no leaks. Pressure differential is measured across every step of the cascade to confirm the clean-to-dirty direction and the differentials. Recovery is timed where the protocol requires it.

The occupancy state matters and gets written down. ISO 14644 defines three: as-built, with the room finished but no equipment, at-rest, with equipment running but no people, and operational, with equipment running and people working. A room can pass at-rest and fail operational, because the people and the process are the particle source, so the state the room is certified in has to match how it will be used. The validation protocol, the engineer, and the standard set which tests, how many points, and which state. Certify by the count and the scan, not by the look of the place.

The HEPA integrity test, finding the leak

The filter-integrity test is the in-place leak test of the installed HEPA or ULPA filters, and it is the test that catches the leak a particle count can miss. A challenge aerosol, historically DOP and now commonly PAO or DEHS, is introduced into the air upstream of the filter at a known concentration. A technician then scans the entire downstream face of the filter, the media, the frame, the gasket, and the seal to the ceiling, with a photometer or a particle counter, moving the probe slowly across every inch.

What the scan finds is penetration. A pinhole in the media, a hairline crack in the frame, a gasket that did not seat, a bolt left loose, any of these lets a thread of unfiltered air through, and the photometer reads a spike right over it. The acceptance is a penetration limit, commonly 0.01 percent for a HEPA, and a reading over it is a leak to repair or a filter to replace. A leak you can patch within limits gets patched per the standard. A leak you cannot means a new filter.

This is the test that proves the filtration is working as installed, not as rated in a catalog. A factory-rated 99.99 percent HEPA with a leak at the gasket is, at that spot, no filter at all. ISO 14644-3 and the IEST recommended practices give the method and the limits, and they are what the certifier scans to.

Continuous monitoring and the EMS

Certification is a snapshot. Monitoring is the ongoing watch that the room stays in class between certifications. Critical rooms run continuous monitoring of particle count, room pressure, and temperature and humidity, fed to an environmental monitoring system that trends every parameter and alarms when one drifts out of band. A pressure step that collapses, a particle count that climbs, a humidity excursion, the EMS catches it while it is happening, not at the next six-month certification.

The trend is as useful as the alarm. A filter slowly loading, a damper drifting, a fan losing performance, these show up as a slow walk in the data long before they trip a limit, and the trend is what lets you act before the room falls out of class. A room with no monitoring is a room you only know about when the periodic test fails, which means it may have been out of class for weeks.

The monitoring data and the periodic readings are records that have to be kept and produced on demand, especially in a regulated facility. Capturing pressure checks, particle counts, filter changes, and certification results in the field as the work happens, with a tool like tradeos, builds the record from the actual readings instead of reconstructing it before an audit.

Pharma and semiconductor, two different worlds

Cleanrooms split into two broad worlds that share the physics and almost nothing else. Pharmaceutical and life-science cleanrooms run under Good Manufacturing Practice, and in Europe under EU GMP Annex 1, which grades rooms A through D and ties them to ISO classes while adding what the semiconductor world does not measure: viable contamination. A pharma room monitors microbes with settle plates, contact plates, and active air samplers alongside the non-viable particle count, because a sterile product is destroyed by a single live organism. The 2022 revision of Annex 1 pushed contamination control strategy, protection of first air, and airflow visualization to the front.

Semiconductor and microelectronics cleanrooms care about the inert particle, and they care about it at sizes pharma never worries about, down toward the size of the circuit features themselves. They also chase a contaminant pharma mostly sets aside: airborne molecular contamination, trace gases at parts-per-billion or parts-per-trillion levels that corrode surfaces or poison a process and that no particle filter touches. SEMI standards categorize those molecular contaminants, and the fab adds gas-phase chemical filtration to handle them.

The practical line for the HVAC: a pharma room is designed around microbial control and the regulated validation that proves it, while a fab is designed around ultrafine particles and molecular contamination at concentrations that make the particle count look generous. The class might be the same. The contamination control strategy behind it is not.

Commissioning and validation

A cleanroom is not done when the air is balanced. It is done when it is commissioned and, in a regulated facility, validated, with documented proof that it performs as specified under the conditions it will run in. Balancing sets the airflows. Commissioning confirms the whole system, the filters, the cascade, the temperature and humidity, the controls, and the alarms, does what the design intended. Validation, in pharma, goes further and proves it on paper to a standard an auditor will accept.

In pharmaceutical work that proof follows the IQ, OQ, PQ sequence. Installation qualification confirms the system was built and installed per the design and the specifications. Operational qualification confirms it performs across its operating range, the airflows, pressures, recovery, and alarms all tested and documented. Performance qualification confirms it holds the class and the conditions over time under real operating loads, often with the microbial monitoring running. The protocol is written and approved before the testing starts, and the room is not released until the protocol is satisfied.

This is the part that gets shortcut under schedule pressure, and it is the part that fails an audit or a product lot later. Who signs off, which tests, which acceptance criteria, and which occupancy state are all set by the validation protocol, the quality unit, and the applicable regulation, not by the installer's judgment. Do not treat a balanced room as a validated one. The validation protocol controls, and a room without a completed, approved protocol is not a qualified cleanroom no matter how clean the particle counter reads.

The energy a cleanroom burns

A cleanroom is an energy hog, and the fan power is why. Moving tens to hundreds of air changes an hour through dense HEPA and ULPA filters, continuously, takes far more fan energy than any comfort system, and that load runs every hour the room is classified, which is usually all of them. The filters add static, the high airflow demands it, and the fans pay for it around the clock.

There is room to cut the bill without giving up the class. High-efficiency FFU motors, ECM and EC fans, cut the fan power for the same airflow. And many rooms can run a setback when the space is unoccupied and idle, dropping the air-change rate to a lower turndown that still holds a reduced cleanliness, then ramping back before work resumes. The turndown has to be validated so the room recovers to class in time, and whether a given room can set back at all depends on its process and its protocol. The energy is real, the savings are real, and both come from the same fans.

Maintenance and the recertification interval

A cleanroom holds its class only as long as it is maintained, and the lifecycle has a rhythm. Prefilters load fast and get changed often, on their pressure drop, to protect the final HEPA filters behind them, the same staged change-out the air filtration and MERV field guide lays out. The final HEPA and ULPA filters last for years but eventually load or fail a scan and get replaced, and a replacement filter has to be leak-tested in place before the room goes back to service. FFU fans wear and lose performance, and a fan that has drifted down quietly pulls the air-change rate with it.

The gowning consumables, the cleaning protocol, and the room cleaning itself are part of the maintenance, not separate from it, because a cleanroom cleaned wrong, with the wrong wipe or the wrong direction, just redistributes the particles. And the certification is not a one-time event. Cleanrooms are recertified on an interval, commonly every six months for the tighter pharmaceutical classes and at least annually for many others, with the interval set by the class, the regulation, and the protocol.

Skip the recertification and you are running on the assumption the room is still in class, which is exactly the assumption the testing exists to replace. Confirm the interval against the governing standard and the validation protocol for the specific room.

What to document

A cleanroom lives and dies by its records, and in a regulated facility the record is the product. If a drug lot is questioned, the question is whether the room was in class when the lot was made, and the only answer is the documentation, the particle counts, the pressure logs, the filter certificates, the validation protocol, and the monitoring trends for that period. A room with no record cannot prove it was ever clean.

Capture the class and the occupancy state it was certified in, the certification test results across particle count, airflow, filter integrity, pressure, and recovery, the filter certificates and the in-place scan results, the validation protocol and its sign-offs, the continuous monitoring trends, and the maintenance and recertification history. Record who tested, to what protocol, and against what acceptance criteria, so a reviewer or an auditor can reproduce the result instead of taking it on faith.

Common mistakes

• Designing the wrong cleanliness class or airflow regime for the process, so the room cannot hold the count the work needs.
• Running a reversed or collapsed pressure cascade, so air flows dirty to clean across a step.
• Replacing or installing HEPA filters and never running an in-place DOP or PAO leak scan.
• Propped airlock doors, skipped gowning steps, or two cascade doors open at once.
• Certifying a room at-rest and using it operational, where the people and process push the count past the limit.
• No recovery test and no recertification, so the room runs on the assumption it is still in class.
• Ignoring tight humidity control, letting static build the dry way or condensation and microbial growth the wet way.
• Treating a balanced room as a validated one, with no completed, approved protocol behind it.

Field checklist

Standards and references

ISO 14644 is the standard family that governs cleanrooms, and the parts divide the work. Part 1 sets the classification by airborne particle count, ISO 1 through 9. Part 2 covers the monitoring that demonstrates continued compliance. Part 3 gives the test methods, the particle count, airflow, filter integrity, pressure, and recovery tests. Part 4 covers design and construction, including the pressure cascade differentials. Part 5 covers operations. Cite the part that governs the point, and confirm the edition, because the standard has been revised and the class limits and methods moved with it.

The IEST recommended practices sit alongside ISO, giving detailed methods for HEPA and ULPA testing and cleanroom operation that the industry leans on, and the older US Federal Standard 209E still appears as legacy class language on drawings. Regulated industries layer their own requirements on top. Pharmaceutical sterile manufacturing follows GMP and, in Europe, EU GMP Annex 1, with its A through D grades and its viable-contamination requirements, and FDA guidance in the US. Semiconductor work follows SEMI standards, including the categorization of airborne molecular contamination.

None of these replaces the project specification, the equipment manufacturer's requirements, or the validation protocol. The class drives the filtration, the air-change rate, and the airflow regime. The cascade has to run clean to dirty. And the room is proven by particle count and filter integrity, not by appearance. Hold the specific class limits, air-change rates, pressure differentials, and test methods to ISO 14644, the engineer, and the approved validation protocol, and confirm the edition and the jurisdiction before citing any of them on a submittal.

Units, terms, and conversions

Cleanroom work carries its own vocabulary, and the same idea shows up in a few unit systems across an ISO standard, a GMP document, and a manufacturer's data sheet.

Particle concentration is counted per cubic meter in ISO 14644 and per cubic foot in the legacy 209E language, at particle sizes given in microns, the micrometer, written um. Airflow is a face velocity in meters per second or feet per minute for unidirectional rooms and air changes per hour for non-unidirectional rooms. Pressure differential is in pascals in most cleanroom work and in inches of water column on US drawings, where 1 inch of water column is about 249 Pa, so the 10 to 15 Pa cascade step is roughly 0.04 to 0.06 inches. Filter penetration is a percentage, and efficiency is its complement, so a 99.99 percent filter has 0.01 percent penetration.

Cleanroom

A room controlled to a defined airborne particle limit, classified and certified to an ISO 14644 class

ISO 14644 class

The cleanliness class set by particle count per cubic meter, ISO 1 to 9, lower is cleaner

HEPA / ULPA

High-efficiency filters; HEPA 99.97% at 0.3 micron, ULPA 99.999% or better near 0.12 micron

Air change rate

The room volume exchanged per hour, the measure of how fast the air is diluted and swept

Unidirectional / non-unidirectional

Laminar piston flow versus turbulent mixed flow, set by the class

FFU

Fan filter unit, a combined fan and HEPA or ULPA module set into the ceiling grid

Pressure cascade

Stepped room pressures holding each cleaner room positive to the dirtier one, clean-to-dirty flow

Filter integrity test

In-place DOP or PAO aerosol scan of an installed filter for leaks in media, frame, and seal

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