Buyer guide

Cleanroom pressure differentials: cascades, values, monitoring

A pressure cascade holds each room above the less clean room beside it, so air leaks from clean to less clean and not the other way. Published minimums are small: EU GMP Annex 1 gives 10 Pa as a guidance value, FDA's 2004 guidance gives 10 to 15 Pa, and China's 2010 GMP gives not less than 10 Pa.

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By ZY Cleanroom · Updated 2026-11-01 · Sources at the end of the page

Published values

Values differ by sector, so state yours in the user requirements.

Source	Value	Applies to
EU GMP Annex 1, 2022, para 4.14	At least 10 Pa (guidance value)	Adjacent rooms of different grades
FDA aseptic guidance, 2004	At least 10 to 15 Pa (0.04 to 0.06 in. wg), doors closed	Adjacent rooms of different classification
FDA aseptic guidance, 2004	At least 12.5 Pa	Aseptic room against an unclassified room
FDA aseptic guidance, 2004	About 17.5 to 50 Pa	Isolators against surroundings
China GMP, 2010 revision, Art. 48	Not less than 10 Pa	Clean to non-clean areas, and between clean grades
UK HTM 03-01 Part A, 2021	Theatre 25 Pa, lay-up room 35 Pa, critical care room +5 Pa, isolation room −5 Pa	Hospital rooms
India NABH, 2018	At least 2.5 Pa	Operation theatre against adjoining areas

Cleanroom pressure differentials: cascades, values, monitoring

Building a cascade

With 10 Pa steps, a chain of four rooms puts the cleanest room 30 Pa above the unclassified corridor, as the table shows. That is arithmetic from the guidance value, not a requirement. Other steps may be justified by the process.

10 Pa is about 1 mm of water gauge. Force on a door is pressure times area, so 25 Pa on a 1.0 × 2.1 m door is 52.5 N.

Rooms handling pathogenic, highly toxic or radioactive material may need the cascade changed, including negative-pressure airlocks (Annex 1, para 4.14). HTM 03-01 gives −5 Pa to the general area for category 2 isolation rooms (Table 4).

Room	Example grade	Pressure above corridor
Corridor	Unclassified	0 Pa
Gowning airlock	Grade D	10 Pa
Preparation room	Grade C	20 Pa
Filling room	Grade B	30 Pa

Doors and airlocks

Pressure falls when a door opens. HTM 03-01 separates closed-door protection, where leakage flows from clean to less clean, from open-door protection, where airflow through the doorway replaces the lost pressure (para 8.18). A 1.4 m wide open doorway can pass 0.19 m³/s each way with no temperature difference (para 8.17).

Without pressure stabilisers or transfer grilles, doors are held partly open by air pressure (para 8.24).

Pressure stabilisers set the room differential with doors shut and close when a door opens, so air is forced through the doorway. They work in one direction only. Transfer grilles pass air both ways between rooms of equal class and pressure (paras 8.19 and 8.23). Lock windows shut or seal them where pressure is critical (para 8.20).

What to monitor

Fit pressure indicators between cleanrooms. Annex 1 says the warning signal should not be overridden without assessment.

Item	Practice	Source
Critical differentials	Monitor continuously and record. Warn at once on failure or a drop below the set limit.	Annex 1, para 4.16
Other differentials	Monitor and record at regular intervals.	Annex 1, para 4.16
Alarm delays	Assess and justify.	Annex 1, para 4.16
Frequency and alarms	Continuous through each shift, frequently recorded. Document alarms and investigate deviations.	FDA, 2004
Airflow direction	Visualise airflow at rest and in operation. Pressure alone does not show flow direction.	Annex 1, para 4.15

Who approves the room

The regulator approves the owner's facility, not the contractor. The contractor supplies pressure test results against your limits. To have a room list reviewed, send it to ZY Cleanroom, a cleanroom design-and-build contractor with its own panel factory in Tianjin, China.

Questions

Common questions

What pressure difference should a cleanroom have?

Commonly at least 10 Pa between adjacent grades: EU GMP Annex 1, China GMP 2010 and FDA (10 to 15 Pa). Hospital values differ: 2.5 Pa minimum in NABH, 25 Pa for a UK theatre.

Is higher pressure always better?

No. HTM 03-01 warns that rooms without relief paths hold doors partly open by air pressure.

Which rooms run at negative pressure?

Containment and infectious-disease rooms. HTM 03-01 gives −5 Pa for category 2 isolation rooms.

How often should pressure be recorded?

Annex 1 says critical differences should be monitored and recorded continuously and others at regular intervals. FDA says continuously through each shift.

How do you calculate a cleanroom pressure differential?

You do not calculate the pressure itself. Set the target, estimate the leakage through door gaps and other openings at that pressure, then make supply air minus extract air equal that leakage. Leakage grows with roughly the square root of pressure, so doubling the step takes about 1.4 times the air. Real gaps differ, so confirm by measurement.

What does a pressure relief damper do in a cleanroom?

It sets the room pressure with the doors shut by letting surplus air out at a set point. HTM 03-01 calls these pressure stabilisers and says they close when a door opens, which forces air through the doorway (para 8.19). Place them so that air moves from clean to less clean.

Related guides

Sources

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