What does the smaller ISO rating number mean?

Direct answer: Under the same particle size and evaluation conditions, the smaller the ISO grade number, the lower the allowed suspended particle concentration, and the stricter the particle control requirements.

The sentence itself is not difficult to understand. But in actual projects, just remembering 'the smaller the number, the cleaner' is not enough. Because the grade number never appears alone, it is followed by particle size, area and operating status. It is easy to draw wrong conclusions when comparing numbers without these conditions.

Grade numbers describe particle limits

The classification logic of the ISO 14644 series is: select the particle size, count the cumulative number of particles in the air per unit volume, and then compare it with the limit value given by the standard. The smaller the number, the stricter the corresponding limit.

For example, ISO 5 corresponds to a lower allowed particle concentration than ISO 7, and ISO 8 is more relaxed than ISO 7. This order is fixed in the standard.

But please note that the premise of this comparison is 'the same particle size'. If one document states a concentration of no less than 0.5 μm and another states a concentration of no less than 5 μm, it is meaningless to compare the two numbers together. The particle size and caliber are not uniform, and the size relationship between grade numbers loses reference.

Small numbers do not mean sterility

This is another understanding that is easily amplified.

The cleanliness level evaluates the concentration of suspended particles in the air, not the number of microorganisms or the sterility state. ISO 3 or ISO 4 is a small number, but it describes the level of particle control and does not mean that the space is free of bacteria or viruses.

Control of biological contamination requires separate disinfection, sterilization, pressure differential and personnel management measures. A high cleanliness level only provides a more controllable background environment for these measures, but cannot replace them.

Therefore, if 'sterility' is required in the project file, you cannot just write an ISO level and end it. The required microbiological testing, disinfection procedures and equipment verification still need to be arranged separately.

Comparing grades requires unified evaluation conditions

Level numbers can be compared, but only if the conditions are aligned.

Area scope : Is it the entire room, or inside a certain workstation or equipment? Low concentrations measured in a local area cannot be used to represent the entire room.

Running states : Results from empty, static, and dynamic states can vary significantly. When using static data to compare dynamic requirements, the numbers may look beautiful but are not actually valid.

Particle size conditions : The cumulative concentrations under different particle sizes are different, and the corresponding limits are also different. There are applicable restrictions on some combinations of grades and particle sizes in the standards and cannot be calculated by oneself.

Sampling method : Measurement point location, sampling volume, and instrument capabilities all affect results. If the two reports have different methods, even if they both write ISO 5, they cannot be directly compared together.

Only after unifying these conditions can the comparison of grade numbers be meaningful.

Return to device configuration from target level

After understanding the meaning of the numbers, you will have a clear direction when choosing equipment.

Lower numbers usually mean tighter filtration, higher air supply volume, or more complete airflow organization. However, the specific configuration cannot only be applied based on the level number, but also depends on the protection object, exposure path and on-site conditions.

For example, in a local area inside a closed device or a large room where people move around frequently, the device configuration may be completely different even if the target level is the same. Local solutions may use EFU with high-efficiency filters; room solutions may use FFU arrays or centralized air supply.

Aili Industrial has manufacturing capabilities in motors, fans, control systems, sheet metal, air filters and complete machine assembly. When doing clean level projects, you can put filters, fans, structures and control conditions in the same project for verification, so that the equipment configuration and on-site evaluation boundaries are consistent.

Related products can be viewed FFU clean air supply products、HEPA and ULPA filters , equipment-side applications can be referred to EFU equipment side fan filter unit。

Project judgment example

The static test result of a certain equipment cavity is close to ISO 3. Based on this, the project team believes that the entire production line has reached the same level. However, in the room where the production line is located, people move around frequently and materials continue to enter and exit, so the dynamic particle concentration is significantly higher. Taking the local static results as the whole line level will cause problems in actual production.

Another project compared two reports, one written as ISO 5 and one written as ISO 6, and directly believed that the former was better. However, a closer look revealed that the particle size, caliber and operating status of the two reports were different, and the data were not comparable.

Both cases illustrate that the numerical size itself is clear, and the problem is the comparison condition.

Key points for selection and acceptance inspection