Is ULPA required for ISO 4 and ISO 5 clean rooms?

There is no one-size-fits-all answer to this question.

ISO 4 and ISO 5 describe the spatial particle concentration, while ULPA describes the filter penetration performance. The measurement objects of the two are different. In the same ISO 5 clean room, some projects can meet the standard using HEPA, while others must be ULPA approved. The difference is not in the room class itself, but in the workmanship, airflow organization and acceptance conditions.

Room grade and filter grade are not the same thing

Let’s separate these two concepts first.

ISO 4 and ISO 5 in ISO 14644 evaluate the number of particles per cubic meter of air under specified conditions. The allowable particle number of ISO 4 is one order of magnitude lower than that of ISO 5, and the control is tighter.

ULPA's U15, U16, and U17 evaluate the penetration rate of the filter under specified test conditions. The same goes for H13 and H14 of HEPA.

What the room needs to meet the standard is the system capability, not the single filter element index. Whether the air supply volume is sufficient, whether the airflow organization is reasonable, whether the seal is leaking, whether the pollution source is controlled or not - these factors jointly determine the effect of the room. Industry reference materials also mention that the actual cleanliness is recommended to be based on the actual measured value of the particle counter, and the estimate of the number of air changes is only a rough standard.

When is HEPA enough?

In the following situations, HEPA (such as H14) combined with reasonable system design can usually meet the requirements of ISO 5 or even ISO 4:

The system design is reasonable. The air supply volume is sufficient, the air flow coverage is complete, the return air path is smooth, and there are no obvious eddies and short circuits.

Pollution sources are controllable. The main pollution comes from the outside, and the dust production in the protected area is low, or there is local exhaust.

Sealed in place. The bypass air leakage at the end, box, and pipeline is well controlled, and unfiltered air will not bypass the filter material and enter the clean area.

The acceptance conditions are consistent with the design. The conditions for static or dynamic testing are clear and the measuring points are reasonably distributed.

The filtration efficiency of H14 for 0.3 micron particles reaches 99.995%. With sufficient air changes and good air flow organization, ISO 5 level is usually achievable.

When to Consider ULPA

ULPA may be more suitable in the following situations:

The process is sensitive to smaller particle sizes. In some semiconductor or precision manufacturing processes, the particle size of concern is on the order of 0.1 microns, and the penetration level of H14 may not be enough. ULPA has higher filtration efficiency at the 0.12 micron level.

Key workstations have certain high requirements. It’s not that the entire room needs to be upgraded, but that the microenvironment inside a certain exposed workstation or piece of equipment needs stricter particle control. At this time, ULPA can be configured locally, and the entire room still uses HEPA. There is no need to upgrade the entire room.

Project acceptance criteria are clearly required. Some customers' internal specifications or process documents directly specify the ULPA level. In this case, the document will be followed.

The system has margin support. The resistance of ULPA is usually higher than that of HEPA. Before upgrading, make sure that the fan, air duct and control system can withstand the change in resistance and will not cause a decrease in air supply volume.

A common misunderstanding

'ISO 4 is stricter than ISO 5, so you must use ULPA for ISO 4, and HEPA for ISO 5.'

There is no standard basis for this inference.

The difference between ISO 4 and ISO 5 is that the number of particles allowed is an order of magnitude different, but there is more than one way to achieve this difference. This can be achieved by increasing the number of air changes, optimizing air flow organization, strengthening sealing, or by upgrading the filter level. Which method to choose depends on system conditions, cost and maintenance requirements, and is not directly determined by the room level.

On the other hand, it may also be reasonable to use ULPA in ISO 5 rooms—if local workstations require it, or the system itself has the supporting conditions for ULPA.

Project judgment example

The particle monitoring of an ISO 5 clean room occasionally exceeded the standard, and the investigation revealed that the air supply volume was insufficient.

Will replacing HEPA with ULPA solve the problem at this time? uncertain. Insufficient air volume means that the efficiency of particle dilution and removal is insufficient. Replacing a higher-grade filter element will increase resistance, and the air volume may further decrease. The correct direction is to restore the air supply volume first, confirm the airflow organization, and then evaluate whether the filtration level needs to be adjusted.

Another example: A certain ISO 4 room uses H14, and all tests meet the standards. If the process has not changed and the acceptance conditions have not been tightened, there is no need to switch to ULPA for 'more security'. Level selection should be driven by needs, not habits or psychological expectations.

Selection and Acceptance Checklist

About Aili Industrial’s clean solution package

The selection of filtering levels for ISO 4 and ISO 5 projects is essentially a matter of matching 'process requirements' and 'system capabilities'.

Aili Industrial's manufacturing system covers filters, motors, fans, control systems and complete machine assembly, and can provide ULPA ultra-high efficiency filters and HEPA/ULPA products of different grades. Filter can be used with The FFU fan filter unit is matched to confirm the end configuration, and the equipment end application can be used with the EFU equipment-side fan filter unit . Check the installation conditions and interface dimensions with

When enquiring, provide the target area, particle control requirements, design air volume and acceptance method, which will help determine how to combine the filtration level and system configuration. Specific performance is subject to product technical data and project acceptance results.