Cleanroom Particle Count Limits Under ISO 14644-1: Classes, Sizes and Testing Requirements

Cleanroom particle count limits under ISO 14644-1 define the maximum cumulative concentration of airborne particles permitted at specified sizes for each ISO Class.

A particle counter may display thousands-or even millions-of particles per cubic metre.

But does that mean the cleanroom passes?

Not necessarily.

The number displayed on the instrument is only one part of cleanroom classification. A defensible result also depends on:

• the required ISO Class;
• the particle-size threshold being assessed;
• whether the counter is showing cumulative or differential counts;
• the condition of the room during testing;
• the number and position of sampling locations;
• the volume of air sampled at each location;
• the particle counter’s flow rate and calibration status; and
• any applicable pharmaceutical, medical-device, customer or regulatory requirements.

ISO 14644-1 provides the internationally recognised framework for classifying the air cleanliness of cleanrooms and clean zones according to airborne particle concentration. The standard considers cumulative particle populations at threshold sizes between 0.1 µm and 5 µm.

However, a cleanroom cannot be formally classified merely by comparing one instrument reading with an online ISO limit table.

A particle-limit table tells you the acceptance value. The complete testing method determines whether the result is valid.

This practical guide explains cleanroom particle count limits, how ISO Classes 1 to 9 should be interpreted, why the 0.5 µm channel is commonly referenced, how sample volume affects results, and the mistakes Perfect Pollucon LLP LLP commonly looks for while planning or reviewing particle-count testing.

What Does ISO 14644-1 Classify?

ISO 14644-1 classifies air cleanliness according to the number concentration of airborne particles in:

• cleanrooms;
• clean zones; and
• certain separative devices covered by the ISO 14644 framework.

The classification system includes ISO Classes 1 to 9:

• ISO Class 1 represents the lowest permitted particle concentration.
• ISO Class 9 permits the highest particle concentration within the classification system.
• Intermediate decimal classes may also be specified where justified.

The classification applies to cumulative particle populations at selected threshold sizes from 0.1 µm to 5 µm.

ISO 14644-1 does not, by itself, classify:

• viable microorganisms;
• particle toxicity;
• particle chemical composition;
• surface contamination;
• room differential pressure;
• airflow velocity;
• HEPA-filter leakage;
• temperature;
• relative humidity;
• room recovery time; or
• overall product sterility assurance.

These may form part of cleanroom qualification or environmental monitoring, but they are separate from airborne-particle classification.

Classification Is Different From Routine Monitoring

ISO 14644-1 addresses classification of air cleanliness by particle concentration.

ISO 14644-2 addresses the minimum requirements for a monitoring plan intended to provide continuing evidence of cleanroom or clean-zone performance.

The difference is important.

Classification asks:

• Does the cleanroom meet the specified ISO Class?
• In which occupancy condition?
• At which particle sizes?
• Using how many sampling locations?
• Using what sample volume?

Routine monitoring asks:

• Is cleanroom performance remaining stable?
• Are particle levels increasing over time?
• Have internal alert or action limits been crossed?
• Are critical locations showing deterioration?
• Is investigation or corrective action required?

A single handheld particle-count reading is not automatically an ISO classification.

Similarly, passing a periodic classification test does not prove that a room remained under control throughout the period between tests.

How Should an ISO Classification Be Stated?

A statement such as:

“The cleanroom is ISO Class 7”

is incomplete.

A meaningful classification statement should identify:

1. the ISO Class;
2. the occupancy state; and
3. the particle size or sizes used for classification.

A clearer statement would be:

ISO Class 7, operational, at ≥0.5 µm and ≥5 µm.

Without these details, the reader cannot determine:

• whether the room was empty or in use;
• whether the result was based on the 0.3 µm or 0.5 µm channel;
• whether the test represents actual production conditions; or
• whether two reports are technically comparable.

ISO 14644 Particle Limits

The following table provides practical reference values for the maximum cumulative concentration of particles permitted for the nine integer ISO Classes.

ISO Cleanroom Classification Table

Units: maximum cumulative particles per cubic metre of air.

Important Use Warning

This table is provided as an educational and calculation reference. It is not a substitute for the official standard.

A blank entry does not mean that the allowable concentration is zero. It means that the class-size combination is not ordinarily used in the integer classification table because of measurement, sampling or statistical limitations.

The purchased standard should be consulted before:

• calculating an intermediate ISO Class;
• using a particle threshold not shown in the table;
• interpreting extremely low concentrations;
• evaluating ISO Class 5 at 5 µm;
• deciding the minimum locations for a specific floor area; or
• issuing a formal statement of conformity.

What Does “Cumulative Particle Count” Mean?

ISO classification limits are cumulative.

A cumulative reading at ≥0.5 µm includes every particle detected at or above 0.5 µm, including particles in larger channels such as:

• 1.0 µm;
• 2.5 µm;
• 5.0 µm; and
• 10 µm, where that channel is available.

It does not mean particles only between 0.5 µm and 1.0 µm.

Cumulative count

All particles equal to or larger than the selected threshold.

Differential count

Particles falling between two adjoining size channels.

For example, a differential 0.5 µm value may represent particles between the 0.5 µm and 1.0 µm channels.

Common report-review mistake: A differential reading at 0.5 µm should not be compared directly with the cumulative ISO limit for particles ≥0.5 µm.

The instrument mode must therefore be recorded in the field sheet and test report.

Important ISO Class 5 Particle Limits

ISO Class 5 is frequently referenced in pharmaceutical, medical-device, electronics and other contamination-sensitive operations.

The most commonly searched value is:

ISO Class 5 limit at ≥0.5 µm: 3,520 particles per cubic metre.

However, a reading below 3,520 particles/m³ does not by itself prove that the room is ISO Class 5.

The conclusion depends on whether:

• the agreed room condition was maintained;
• the minimum number of locations was sampled;
• enough air was collected;
• the instrument was suitable and within calibration;
• cumulative counting mode was used; and
• each required location complied.

Important ISO Class 7 and ISO Class 8 Limits

ISO Class 7

ISO Class 8

A reading of 20,00,000 particles/m³ at ≥0.5 µm may comply with ISO Class 8 but would exceed the ISO Class 7 limit.

A particle concentration cannot therefore be described as simply “high” or “low.” It must be compared with:

• the required class;
• the correct particle size;
• the correct room condition; and
• the correct unit.

Why Is the 0.5 µm Limit Commonly Referenced?

The 0.5 µm threshold is widely used because:

• it is available on most cleanroom particle counters;
• it provides useful separation between ISO Classes;
• many historical cleanroom specifications referred to it;
• pharmaceutical requirements commonly consider it; and
• it is relevant to many contamination-control applications.

The ISO Class 5 limit at ≥0.5 µm is approximately 100 particles per cubic foot. This is why ISO Class 5 is often associated with the older expression “Class 100.”

However, this does not mean that:

• only the 0.5 µm channel matters;
• every facility should monitor only at 0.5 µm;
• the 0.5 µm channel is automatically the most critical for every process; or
• classification at 0.5 µm replaces process-specific risk assessment.

The appropriate sizes should be selected according to:

• the applicable standard;
• product sensitivity;
• process risk;
• customer requirements;
• sector-specific requirements; and
• the verified capability of the particle counter.

Particles per Cubic Metre Versus Particles per Cubic Foot

ISO limits are expressed in particles per cubic metre.

Some instruments and older specifications use particles per cubic foot.

The conversion is:

Particles/m³ = particles/ft³ × 35.3147

Particles/ft³ = particles/m³ ÷ 35.3147

Example: ISO Class 5 at 0.5 µm

ISO Class 5 limit:

3,520 particles/m³

Conversion:

3,520 ÷ 35.3147 = approximately 99.7 particles/ft³

This is normally described as approximately:

100 particles/ft³

Example of a Unit Error

Suppose an instrument shows:

2,000 particles/ft³ at ≥0.5 µm

It would be incorrect to compare 2,000 particles/ft³ directly with the ISO Class 5 limit of 3,520 particles/m³.

Correct conversion:

2,000 × 35.3147 = 70,629 particles/m³

That concentration substantially exceeds 3,520 particles/m³.

PPS field observation: Unit mismatch is one of the easiest errors to miss because both values may appear reasonable when the unit is omitted.

Every result table should clearly state whether the value represents:

• raw particles counted;
• particles per litre;
• particles per cubic foot; or
• particles per cubic metre.

Treatment of 5 µm Particles

Particles at and above 5 µm require special care.

Larger particles settle more readily and may be lost:

• at the sampling probe;
• inside long sampling tubes;
• at bends in the tube;
• while travelling to the counter; or
• within components of the sampling system.

The ordinary ISO Class 5 table does not provide a conventional classification limit at ≥5 µm.

A value such as approximately 29 particles/m³ should therefore not automatically be presented as the general ISO Class 5 limit under the current classification framework.

Where 5 µm particles are important, the testing team should review:

• the applicable ISO macroparticle provisions;
• the customer specification;
• pharmaceutical or sector-specific requirements;
• probe orientation;
• sampling-tube length and bends;
• particle-transport losses; and
• the instrument’s verified performance.

Practical warning: Do not copy an old ISO Class 5 value for 5 µm particles into a report without checking the standard edition and the source of the requirement.

The applicable purchased standard and regulatory or customer specification must govern the conclusion.

As-Built, At-Rest and Operational Conditions

Particle concentrations may differ substantially depending on the condition in which the room is tested.

As-Built

In the as-built condition:

• the cleanroom installation is complete;
• building services are connected and operating;
• process equipment may not yet be installed;
• production materials are absent; and
• personnel are absent.

This condition primarily evaluates the room and HVAC installation before operational contamination sources are introduced.

At-Rest

In the at-rest condition:

• the cleanroom is complete;
• equipment is installed;
• equipment is operating in an agreed condition; and
• personnel are not performing normal production activities.

The exact at-rest condition should be defined in the approved test protocol.

Operational

In the operational condition:

• the cleanroom is functioning in the specified operating mode;
• the defined number of personnel is present;
• equipment is operating; and
• normal or simulated activities are being conducted.

Operational counts are frequently higher because of:

• personnel movement;
• gown shedding;
• material handling;
• door opening;
• machinery;
• packaging activities; and
• process-generated particles.

Why the Condition Matters

Suppose a room passes ISO Class 7 when tested at rest.

That result does not prove that the room meets ISO Class 7 during production.

Similarly, a low count obtained immediately after cleaning in an empty room should not be presented as representative of normal operations.

PPS field observation: Reports sometimes state only that a room was “tested and found satisfactory” without defining the occupancy state. Such a conclusion is difficult to defend during an audit.

The occupancy state should appear in:

• the test protocol;
• field data sheet;
• report heading;
• result table; and
• final classification statement.

Minimum Number of Sampling Locations

The minimum number of sampling locations is based mainly on the area of the cleanroom or clean zone.

The 2015 edition uses a reference table. The testing agency should not rely on the older square-root shortcut without checking the current standard.

Selected practical examples include:

Where the actual floor area falls between two table values, the next higher applicable area is used.

The full official sampling-location table must be consulted for formal classification.

Example: 18 m² Room

An 18 m² room exceeds the 10 m² entry.

The next applicable reference area is 24 m².

Minimum locations:

6

Example: 25 m² Room

A 25 m² room exceeds 24 m².

The next applicable reference area is 28 m².

Minimum locations:

7

Example: 100 m² Room

A 100 m² room falls within the 104 m² entry.

Minimum locations:

16

Minimum Does Not Always Mean Sufficient

The area-based number establishes the minimum number of classification locations.

Additional locations may be appropriate near:

• exposed products;
• filling points;
• open processing operations;
• material-transfer points;
• doors and airlocks;
• operator workstations;
• return-air grilles;
• heat-generating equipment;
• airflow disturbances;
• difficult-to-clean areas; or
• locations with a history of high readings.

The room should be divided into approximately equal sections, but the exact sampling points should also consider airflow and contamination risk.

The testing agency should not select only the locations most likely to produce favourable results.

Minimum Sample Volume

Enough air must be collected at each location to provide a meaningful result for the largest particle size being considered.

The minimum calculated sample volume is:

Vs = 20 × 1,000 ÷ Cn,m

Where:

• Vs = minimum sample volume in litres;
• 20 = the number of particles that would be expected if the concentration were at the class limit; and
• Cn,m = the applicable class limit in particles/m³ for the largest selected particle size.

The test must also satisfy the minimum sample-volume and minimum sampling-duration provisions contained in the standard.

In practical application, at least:

• 2 litres should be collected at each location; and
• the sample should run for at least one minute.

Where the calculated volume requires a longer period, the longer sampling time governs.

The same selected sample volume should generally be used at all classification locations.

Worked Example 1: ISO Class 5 at ≥0.5 µm

Consider an 18 m² cleanroom requiring classification as:

ISO Class 5, operational, at ≥0.5 µm.

Step 1: Determine the Minimum Locations

For 18 m², use the next applicable reference area of 24 m².

Minimum locations:

6

Step 2: Identify the Limit

ISO Class 5 at ≥0.5 µm:

3,520 particles/m³

Step 3: Calculate the Sample Volume

Vs = 20 × 1,000 ÷ 3,520

Vs = 5.68 litres

At least 5.68 litres should therefore be collected at each location.

Step 4: Calculate Sampling Time

Suppose the counter operates at:

2.83 litres per minute

Sampling time:

5.68 ÷ 2.83 = 2.01 minutes

The sample must therefore run for slightly more than two minutes.

A practical programmed duration may be three minutes.

Volume collected in three minutes:

2.83 × 3 = 8.49 litres

Step 5: Convert the Raw Count

Suppose 28 particles at ≥0.5 µm are counted during the 8.49-litre sample.

Concentration:

28 × 1,000 ÷ 8.49

= 3,298 particles/m³

Comparison:

3,298 < 3,520 particles/m³

That location complies.

What if 30 Particles Were Counted?

30 × 1,000 ÷ 8.49

= 3,534 particles/m³

Comparison:

3,534 > 3,520 particles/m³

That location exceeds the limit.

This example demonstrates why:

• sample volume matters;
• raw particle counts must be normalised correctly; and
• small differences can affect classification when the concentration is close to the limit.

Worked Example 2: ISO Class 8 at ≥0.5 µm

Consider a 100 m² room requiring ISO Class 8 classification.

Minimum Locations

For 100 m², use the 104 m² reference entry.

Minimum locations:

16

Applicable Limit

ISO Class 8 at ≥0.5 µm:

35,20,000 particles/m³

Calculated Sample Volume

Vs = 20 × 1,000 ÷ 35,20,000

Vs = approximately 0.00568 litres

This calculated volume is extremely small.

It does not override the minimum volume and sampling-duration provisions.

A particle counter operating at 2.83 litres/minute collects:

2.83 litres in one minute

The one-minute minimum would therefore govern in this example.

Worked Example 3: Converting Particles per Cubic Foot

Suppose the counter reports:

8,000 particles/ft³ at ≥0.5 µm

Convert to particles/m³:

8,000 × 35.3147

= 2,82,518 particles/m³

Compare with the ISO Class 7 limit:

• Measured: 2,82,518 particles/m³
• ISO Class 7 limit: 3,52,000 particles/m³

The result is below the numerical ISO Class 7 limit.

However, the room can be declared compliant only when the other classification requirements have also been satisfied.

Can Passing Locations Be Averaged With a Failed Location?

No.

Every applicable sampling location should be evaluated against the class limit.

A high result at one point should not be hidden by averaging it with low readings from other parts of the room.

The average is 2,30,000 particles/m³, which is below the limit.

But Location 3 exceeds the applicable limit.

The room should not be declared compliant based on the overall average.

Possible causes of a localised high count include:

• recent door opening;
• personnel movement;
• poor airflow distribution;
• contamination near equipment;
• an unsuitable probe position;
• damaged filtration;
• activity near a return-air zone; or
• an abnormal event during sampling.

Which Channels Can the PPS Particle Counter Measure?

The PCE-PCO 1 available to PPS provides six particle-size channels:

• 0.3 µm;
• 0.5 µm;
• 1.0 µm;
• 2.5 µm;
• 5.0 µm; and
• 10 µm.

The manufacturer specifies a flow rate of 0.1 cubic foot per minute, equivalent to 2.83 litres per minute, and provides cumulative, differential and concentration modes.

The manufacturer also states:

• counting efficiency of 50% at 0.3 µm; and
• counting efficiency of 100% above 0.45 µm.

These specifications are stated with reference to ISO 21501.

Classification Thresholds the Instrument Can Measure Directly

The available channels correspond to commonly referenced ISO thresholds at:

• ≥0.3 µm;
• ≥0.5 µm;
• ≥1.0 µm; and
• ≥5.0 µm.

Thresholds It Cannot Measure Directly

The counter does not provide channels at:

• 0.1 µm; or
• 0.2 µm.

PPS should therefore not claim classification based on those particle thresholds using this instrument.

What About the 2.5 µm Channel?

The 2.5 µm channel may be useful for:

• investigation;
• trend monitoring;
• indoor-air studies;
• process observations; or
• customer-specific requirements.

However, 2.5 µm is not one of the standard particle thresholds shown in the integer ISO table above.

Read more about Indoor air quality Monitoring Services by Perfect Pollucon

What About the 10 µm Channel?

The 10 µm channel is outside the ordinary 0.1 µm to 5 µm ISO 14644-1 classification range.

It may provide useful diagnostic information but should not be represented as an ordinary ISO classification threshold.

Does Having the Correct Channels Make an Instrument Suitable?

Not automatically.

A particle counter having the required channels does not, by itself, prove suitability for formal classification.

The testing agency should also review:

• current calibration status;
• calibration traceability;
• particle-size accuracy;
• counting efficiency;
• flow-rate accuracy;
• size resolution;
• coincidence error;
• zero-count performance;
• probe design;
• sampling-tube losses; and
• instrument condition.

ISO 21501-4 describes calibration and verification methods for light-scattering airborne particle counters used in clean spaces. ISO currently lists ISO 21501-4:2018 and Amendment 1:2023, while a future revision is also shown as being under development.

PPS technical principle: The manufacturer’s specification describes the instrument design. The calibration certificate provides evidence relating to the performance of the particular instrument. Both must be reviewed.

Why a Zero-Count Check Matters

Before classification sampling begins, the particle counter should undergo the required operational checks, including a zero-count check using the correct zero filter and the manufacturer’s procedure.

The zero-count check can help identify:

• contamination inside the counter;
• contamination in the inlet;
• particles remaining after a previous measurement;
• leakage in the sampling path;
• electronic false counts; or
• inadequate purging.

Operating the counter in an apparently clean room is not a substitute for using the correct zero filter.

Where the instrument continues to report particles while connected to the zero filter, the cause should be investigated before classification testing continues.

Practical warning: Formal classification should not begin without the accessories, calibration records and verification checks required by the instrument manufacturer and the applicable method.

ISO Limits Versus Internal Alert and Action Limits

The ISO classification limit is the maximum permitted concentration for the specified class.

It should not automatically be adopted as the routine alert level.

A room operating continuously just below the ISO maximum may technically pass while having very little operating margin.

Internal Alert Limit

An alert limit provides early warning that performance may be moving away from the qualified or normal condition.

Crossing it may trigger:

• trend review;
• repeat monitoring;
• observation of operator practices;
• inspection of the area;
• review of recent maintenance; or
• increased monitoring frequency.

Internal Action Limit

An action limit normally triggers a documented response, potentially including:

• formal investigation;
• product or process impact assessment;
• cleaning;
• HVAC inspection;
• filter assessment;
• corrective action;
• maintenance; or
• requalification.

Internal alert and action limits should be developed using:

• qualification data;
• historical trends;
• normal operating performance;
• process risk;
• product sensitivity;
• instrument uncertainty;
• customer requirements; and
• sector-specific expectations.

There is no universal percentage of the ISO limit that is suitable for every facility.

Why a Limit Table Alone Is Not Enough

A table answers only one question:

What is the maximum cumulative concentration for the selected class and particle threshold?

It does not demonstrate that the test was conducted correctly.

A technically useful report should include:

• room name and identification;
• room dimensions and floor area;
• required ISO Class;
• occupancy state;
• particle sizes selected;
• applicable standard and edition;
• cumulative or differential mode;
• minimum required locations;
• actual locations tested;
• location plan;
• sampling height;
• critical locations added;
• sample volume;
• sampling duration;
• instrument flow rate;
• instrument make, model and serial number;
• calibration details;
• zero-check result;
• operational observations;
• location-wise readings;
• units;
• applicable limits;
• individual pass or fail status;
• deviations or abnormal events; and
• a clear classification conclusion.

A report containing only several readings and the word “Pass” may not provide enough information for independent technical review.

Common Cleanroom Particle-Count Mistakes

1. Comparing differential readings with cumulative limits

ISO classification limits are cumulative.

Always confirm the counting mode.

2. Mixing particles/m³ and particles/ft³

The values differ by a factor of approximately 35.3.

Units should appear next to every reading.

3. Comparing the wrong channel

A ≥0.3 µm count should not be compared with a ≥0.5 µm limit.

4. Converting PM2.5 or PM10 mass readings into particle counts

PM2.5 and PM10 readings in µg/m³ represent estimated mass concentration.

ISO cleanroom classification is based on particle-number concentration.

A reliable universal conversion is not possible without information about:

• particle density;
• shape;
• size distribution;
• composition; and
• optical characteristics.

5. Testing too few locations

One convenient reading in a large room does not establish the classification of the whole room.

6. Sampling too little air

A very short spot reading may produce a number without satisfying the sample-volume requirement.

7. Failing to define the room condition

An at-rest result should not be presented as operational performance.

8. Averaging a failed location with passing locations

Each classification location should satisfy the applicable limit.

9. Sampling only below clean-air supply points

A point directly below filtered supply air may not represent the actual working area.

10. Ignoring large-particle losses

Long tubes, bends and inappropriate probes may prevent larger particles from reaching the counter.

11. Treating ISO Class 5 at 5 µm as a simple table value

The applicable standard, regulatory requirement or customer specification must be reviewed.

12. Assuming an instrument’s “ISO mode” completes the classification

The instrument cannot independently confirm that:

• enough locations were tested;
• enough air was sampled;
• the room condition was correct;
• the correct standard was applied; or
• the calibration was valid.

13. Repeating a high reading until a passing result appears

A repeat may be justified after identifying an abnormal condition.

However, the original result should not simply disappear.

The report should document:

• what happened;
• why the measurement was repeated;
• what corrective action was taken; and
• how the final conclusion was reached.

14. Using an expired calibration certificate

An instrument may appear to function normally while being outside the required calibration period.

PPS Field Observations

Based on practical monitoring and technical-report review, several issues repeatedly create confusion.

Room area is not confirmed before mobilisation

Without the room dimensions, the testing team cannot properly determine:

• minimum locations;
• sample duration;
• likely test time; or
• resource requirements.

“Operational” means different things to different people

One person may interpret operational as “HVAC running.”

Another may interpret it as “normal production with operators and equipment.”

The condition must be agreed in writing.

Results are shared without units

A value such as “2,500” is meaningless unless it is identified as:

• raw particles;
• particles/litre;
• particles/ft³; or
• particles/m³.

The report does not include a location plan

Without a plan, the reviewer cannot determine whether the test covered:

• the complete room;
• critical work areas;
• entrances;
• return-air zones; or
• only convenient locations.

Calibration is mentioned but not traceable

A report may state that a calibrated instrument was used without identifying:

• instrument serial number;
• certificate number;
• calibration date;
• due date; or
• calibration laboratory.

Results pass, but the trend is deteriorating

A room may remain below the ISO limit while particle levels have increased significantly compared with earlier tests.

Classification answers whether the room meets the class during the test.

Trend analysis indicates whether environmental control may be deteriorating.

PPS field learning: Most disputes about particle-count reports begin with missing information about units, room condition, locations, sample duration or instrument status-not complicated mathematics.

PPS Checklist Before Cleanroom Classification

Scope review

• Confirm the purpose of testing.
• Obtain the cleanroom layout.
• Confirm room dimensions.
• Identify the required ISO Class.
• Identify the required particle sizes.
• Confirm the occupancy state.
• Review the applicable ISO edition.
• Review customer, GMP or sector-specific requirements.
• Clarify whether the assignment is classification, routine monitoring or investigation.

Sampling plan

• Determine the minimum number of locations.
• Divide the room into representative sections.
• Add critical locations where justified.
• Identify the sampling height or work plane.
• Calculate minimum sample volume.
• Calculate sample time from the counter flow rate.
• Define how abnormal events and repeat tests will be handled.

Instrument readiness

• Verify make, model and serial number.
• Confirm the required particle channels.
• Review the calibration certificate.
• Confirm the calibration due date.
• Verify the specified flow rate.
• Check battery and data storage.
• Carry the correct zero filter.
• Inspect the probe, tubing and accessories.
• Confirm cumulative mode.
• Confirm reporting units.

Site-condition verification

• Verify the room identification.
• Confirm HVAC operation.
• Confirm equipment status.
• Confirm personnel condition.
• Record cleaning and recovery status.
• Record doors and airlock conditions.
• Note unusual activities.
• Confirm that the agreed occupancy state exists.

Before sampling

• Complete the zero-count check.
• Confirm sample time and volume.
• Confirm the selected channels.
• Position the probe correctly.
• Avoid unnecessary tubing.
• Record the exact location.
• Avoid obstructing airflow.

During sampling

• Minimise unnecessary movement near the probe.
• Record door openings and abnormal activities.
• Maintain a consistent sample volume.
• Record each result.
• Do not discard adverse readings without justification.
• Repeat measurements only according to the approved method.

Report review

• Confirm the units.
• Confirm cumulative readings.
• Check each location against the correct limit.
• Ensure a failed point was not concealed through averaging.
• Review 5 µm results carefully.
• Attach the location plan.
• include calibration details.
• Include the zero-check result.
• State the occupancy condition.
• Record limitations and deviations.
• Provide a clear conclusion.

Frequently Asked Questions

What is the ISO Class 5 limit at 0.5 micron?

The maximum cumulative concentration is:

3,520 particles/m³ at ≥0.5 µm.

What is the ISO Class 7 limit at 0.5 micron?

The maximum cumulative concentration is:

3,52,000 particles/m³ at ≥0.5 µm.

What is the ISO Class 8 limit at 0.5 micron?

The maximum cumulative concentration is:

35,20,000 particles/m³ at ≥0.5 µm.

Can PM2.5 readings be converted into ISO particle counts?

Not reliably.

PM2.5 generally represents particle mass in µg/m³, while ISO classification is based on the number of particles per cubic metre.

Does one failed location mean the room fails?

Where an applicable sampling location exceeds the class limit, the room does not meet the specified classification based on that test.

The cause should be investigated before technically justified retesting.

Can the 0.3 µm channel be compared with the 0.5 µm limit?

No.

A cumulative ≥0.3 µm reading normally includes more particles and must be compared with the applicable ≥0.3 µm limit.

Is one minute of sampling always sufficient?

No.

One minute may satisfy the minimum duration, but the calculated sample volume may require longer sampling.

Can a handheld counter be used for ISO classification?

Potentially, provided its channels, flow rate, counting performance, calibration, zero-count performance and other technical characteristics are suitable for the required classification.

Being handheld does not automatically make an instrument suitable or unsuitable.

Does a calibration certificate prove that the room is classified?

No.

Calibration is one part of the evidence.

The room condition, sampling locations, sample volume, particle channels and result evaluation must also satisfy the applicable requirements.

Is ISO classification the same as pharmaceutical Grade A, B, C or D?

No.

Pharmaceutical grades may refer to related particle concentrations, but they also involve sector-specific requirements, occupancy states, microbiological controls and monitoring expectations.

The applicable GMP document must be reviewed separately.

Conclusion

Cleanroom particle count limits appear simple when presented in a table.

But a defensible classification depends on the complete evidence chain:

Required class → particle size → room condition → sampling locations → sample volume → suitable calibrated instrument → location-wise evaluation → documented conclusion

The final review should not ask only:

“Was the reading below the limit?”

It should also ask:

• Was the correct limit selected?
• Was the correct particle channel used?
• Was the counter in cumulative mode?
• Were the units correct?
• Was enough air sampled?
• Were enough locations tested?
• Was the room in the specified condition?
• Did every required location comply?
• Was the instrument suitable and within calibration?
• Can an independent reviewer reproduce the conclusion?

Facilities requiring classification-related measurements or routine monitoring can review our airborne particle count testing service for cleanrooms and controlled areas.

Perfect Pollucon LLP LLP supports industrial, pharmaceutical, healthcare, laboratory, electronics and other controlled-environment facilities with:

• particle-count measurements;
• scope review;
• sampling-location planning;
• test-condition documentation;
• result interpretation; and
• technically transparent reporting.

The final testing scope remains subject to the applicable standard, agreed room condition, instrument capability and facility-specific requirements.

Important Standard-Use Warning

ISO standards are copyrighted technical publications.

This article provides general educational guidance and simplified technical reference information. It must not be used as the sole procedure for formal cleanroom classification.

Before conducting or claiming formal classification, the facility and testing agency should purchase, review and follow:

• the applicable edition of ISO 14644-1;
• ISO 14644-2 where monitoring is involved;
• ISO 21501-4 and its applicable amendment for particle-counter calibration and verification;
• applicable pharmaceutical or industry requirements;
• customer specifications;
• manufacturer instructions; and
• approved internal protocols and SOPs.

As of 25 July 2026, the ISO catalogue lists ISO 14644-1:2015 for classification and ISO 14644-2:2015 for monitoring. It also lists ISO 21501-4:2018 and Amendment 1:2023 for light-scattering airborne particle counters.

About Perfect Pollucon LLP LLP

Perfect Pollucon LLP LLP is an environmental and workplace-monitoring organisation supporting industrial and commercial facilities through field measurement, technical reporting and compliance-related services.

The organisation’s technical foundation is supported by more than 25 years of environmental monitoring and consulting experience, with practical exposure to industrial facilities, laboratories, controlled areas and compliance audits.

PPS follows a transparent reporting approach in which the instrument, testing condition, units, sampling locations, limitations and acceptance criteria are clearly documented.

Author

Harshal T. Gajare
Partner, Perfect Pollucon LLP LLP
Founder and CEO, EHSSaral

Harshal works on translating environmental, occupational-health and compliance requirements into practical systems, digital workflows and field-ready technical guidance for Indian industries.

Technical Reviewer

Organisational Technical Oversight

Tanaji S. Gajare
Founder, Perfect Pollucon LLP
More than 40 years of experience in environmental monitoring, pollution-control consulting and industrial compliance services

Technical review date: 25 July 2026

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