Worksheet: Cleanroom Recovery Time Test (100:1 Method)

This worksheet runs and records a cleanroom recovery time test using the 100:1 method: challenge the room with particles, stop the challenge, and measure the time for the concentration to fall to one-hundredth of the challenge value. It applies to non-unidirectional (turbulently ventilated) rooms; a true Grade A unidirectional zone demonstrates continuous sweep instead (see airflow velocity testing). Replace every <<FILL: ...>> placeholder and set your own acceptance target with a documented justification. A worked example and a filled specimen follow. This content is educational and general; confirm the method against ISO 14644-3 before you rely on it.

Step 1: define the test basis

Field	Entry
Room / area ID	`<<FILL>>`
Grade / ISO class (state tested)	`<<FILL: e.g. Grade C, ISO 8 in operation>>`
Considered particle size	`<<FILL: typically >=0.5 um>>`
In-operation (or at-rest, per justification) class concentration limit	`<<FILL: particles/m3>>`
Target challenge concentration (approximately 100x the limit)	`<<FILL: particles/m3>>`
Challenge generation method	`<<FILL: e.g. PAO/DEHS or polystyrene latex aerosol generator>>`
Counter ID and calibration due date	`<<FILL>>`
Acceptance target for recovery time and its justification	`<<FILL: e.g. <=20 minutes, justified by changeover hold-time and at-rest definition in <<document ID>>>>`

Step 2: generate and confirm the challenge

Introduce the aerosol challenge at the defined location(s) with the HVAC running normally.
Confirm with the counter that the concentration reached approximately the target challenge value before stopping the challenge.
Record the actual peak challenge concentration achieved.

Field	Entry
Actual peak challenge concentration reached, particles/m3	`<<FILL>>`
1/100 of actual peak (the pass threshold to reach)	`<<FILL: calculated>>`
Challenge stop time	`<<FILL>>`

Step 3: log the decay

Log concentration at regular intervals from the moment the challenge stops. Add rows as needed.

Elapsed time (min)	Particle concentration (particles/m3)	Below 1/100 threshold? (Y/N)
0	`<<FILL: = peak challenge>>`	N
`<<FILL>>`	`<<FILL>>`	`<<FILL>>`
`<<FILL>>`	`<<FILL>>`	`<<FILL>>`
`<<FILL>>`	`<<FILL>>`	`<<FILL>>`

Step 4: determine the recovery time

Field	Entry
Elapsed time at which concentration first and consistently stayed at or below the 1/100 threshold	`<<FILL: minutes>>`
Measured recovery time (record this value)	`<<FILL: minutes>>`
Acceptance target (from Step 1)	`<<FILL>>`
Pass / Fail	`<<FILL>>`

Step 5: downstream use of this result

State where this measured recovery time is used operationally, so the number has a documented purpose rather than sitting unused in a file.

Downstream use	Entry
At-rest definition / post-personnel-exit settle period	`<<FILL: e.g. settle period set at 1.4x measured recovery time>>`
Changeover hold time between product campaigns	`<<FILL>>`
Post-excursion re-entry criterion	`<<FILL>>`

Step 6: sign-off

Role	Name	Signature	Date
Tester	`<<FILL>>`
Reviewer (Facilities/HVAC SME)	`<<FILL>>`
Approver (QA)	`<<FILL>>`

References

ISO 14644-3:2019, Cleanrooms and associated controlled environments, Part 3: Test methods (recovery test, described in original wording here; refer to the standard for its exact procedure). EU GMP Annex 1 (2022), for the expectation that recovery time be demonstrated for non-unidirectional cleanrooms and used to support operational claims.

Confirm the current version of each standard before issue.

Worked example

Challenge a Grade C room to 35,200,000 particles/m3 at the considered size, about 10 times the ISO 8 in-operation limit of 3,520,000/m3. The 1/100 pass threshold is 352,000/m3.

Elapsed time (min)	Particle concentration (particles/m3)	Below threshold?
0	35,200,000	N
5	9,800,000	N
10	1,450,000	N
13	520,000	N
14	340,000	Y
16	210,000	Y

The concentration first drops below 352,000/m3 at 14 minutes and stays below it afterward. Measured recovery time: 14 minutes. Against an acceptance target of 20 minutes or less (justified by the site’s changeover procedure), the room passes, and 14 minutes is recorded as the qualified recovery time, used downstream to set a 20-minute post-exit settle period before the room is declared at rest.

Filled specimen

Field	Entry
Room / area ID	Formulation suite FS-04
Grade / ISO class	Grade C, ISO 8 in operation
Target challenge	35,200,000 particles/m3 (approximately 10x the ISO 8 limit of 3,520,000/m3)
Actual peak challenge reached	36,100,000 particles/m3
1/100 threshold	361,000 particles/m3
Measured recovery time	14 minutes (first and consistently below threshold)
Acceptance target	<= 20 minutes, justified by the 20-minute changeover hold time in SOP-FAC-022
Result	Pass
Downstream use	Post-exit settle period set at 20 minutes before declaring at-rest; changeover hold time confirmed adequate
Sign-off	A. Mensah (tester), R. Gomez (QA approver), 18 September 2026

Common inspection findings this worksheet prevents

Recovery time never measured for a non-unidirectional room, leaving the at-rest definition and changeover hold time with no supporting data.
An acceptance target copied from another firm’s published number (commonly 15 or 20 minutes) with no justification tied to this room’s own operational claims.
The challenge concentration not confirmed against the target before stopping, so the 1/100 threshold calculation is built on an assumed, not measured, starting point.
A measured recovery time that is never fed into any actual procedure (settle period, changeover, re-entry criterion), so the test exists only on paper.

How to adapt this worksheet

Set your room’s grade, considered particle size, and class limit from your own classification basis.
Confirm your challenge generation equipment and counter are calibrated and appropriate for the concentrations involved.
Set your own acceptance target and write down the operational basis for it in Step 1, rather than copying a commonly cited number.
Complete Step 5 for every test; a recovery time with no stated downstream use is a gap an inspector will find.