Particle deposition rate gives a direct relation between air cleanliness and surface contamination. Meaning, particle deposition rate can be used as a cleanroom requirement at a critical location and as a monitoring parameter especially in cleanrooms with personnel.
ISO 14644-17:2021 provides guidance on the application of particle deposition rate in the three steps of contamination control: setting requirements, establishing control and demonstrating control during operation.
The particle deposition rate is influenced by both the cleanroom ventilation and the way the cleanroom is used (operation). The application of ISO 14644-17 will help to find the optimal combination of cleanroom ventilation and cleanroom operation and to reduce energy consumption.
Particle deposition rate
Given this direct relationship, the likelihood of product contamination is likely determined by the deposition rate of critical particles, the vulnerable product area and the time of exposure. One or more of these three factors should be decreased to reduce potential product contamination.
In cleanrooms with personnel, often the critical particles size is larger than 5 μm and the standards on air cleanliness ISO 14644-1 & -2:2015 offer limited information on the likelihood of contamination by these particles. In turbulent (non-unidirectional) cleanrooms the removal efficiency of airborne particles by ventilation reduces rapidly for larger particles. Increasing air change rate can help, but needs a lot of energy. A doubling of the air change rate leads to quadrupling of the required energy. Proper operational control can lead to lower airborne concentrations of macro-particles (> 5 µm), near visible particles (> 25 µm) and visible particles (> 50 µm) and reduces the likelihood of deposition of these particles. Important are the human behaviour and cleaning programme.
One or more of these three factors should be decreased to reduce potential product contamination
The particle deposition rate is determined by the airborne particle concentration at a particular point of time and location and the particle deposition velocity. The particle deposition velocity can be expressed as a cumulative velocity assuming that most particles come for personnel. The airborne concentration is determined by the introduction of particles during operations and removal by airflow and deposition.
Particle deposition increases the concentration of particles on surfaces. Surface particles can be transferred to a product during direct or indirect contact, but also after resuspension by deposition. Resuspension depends on surface concentration and external forces like footsteps and turbulent airflow. The change of surface cleanliness by deposition is used to measure particle deposition rate during a particular time of exposure. A witness plate or test plate with known initial surface concentration is used to monitor the particle deposition rate at a location of interest. In case the exposure times are short the particle deposition can be monitored real time.
The number of particles larger than or equal to a particle size of interest per sample time gives information on the impact of local activities and this can be used to improve operational control.
The particle size distribution shows the impact of ventilation for particles that can be removed by airflow (up to 40 µm), the impact of particles dispersed by personnel (up to 100 µm) and the impact of cleaning (≥ 100 µm).
To determine a particle deposition rate level the assumption is made that the number of deposited particles is reciprocal proportional to the particle size and expressed in the equivalent number of particles ≥ 10 µm per sqm per hour. This approach is the same as applied for surface cleanliness, where the equivalent concentration of particles ≥ 1 µm per sqm is defined (ISO 14644-9:2022).
Particle deposition rate limit
The set up of a contamination control solution starts with the determination of the requirements for air cleanliness, particle deposition rate and surface cleanliness. Therefore it is important to find determine what can threaten functionality and the severity of the consequences of contamination.
Mostly the critical particle size is used. Particles can also carry microorganisms. The larger the particle the more likely it will carry microorganisms.
The consequence severity of certain particles can be high or low. The risk of contamination is the combination of the likelihood and the severity of these consequences. In a risk assessment one or more critical particle sizes are determined and their maximum acceptable number on the functional surface of a product or process. This number times the critical particle size divided by the vulnerable area in sqm gives the final surface cleanliness of the product in the equivalent number of particles ≥ 1 µm per sqm.
Failures can be analysed to determine the killer particle size and number
To determine the critical particle sizes and their acceptable number, intensive investigations are needed. These investigations can be both theoretical and experimental. During experimental investigation, various sizes and numbers of particles are seeded onto test products and the consequences are evaluated. Failures can be analysed to determine the killer particle size and number. Often when these investigations have not been done, people set their requirements too high to be on the safe side. However, in practice that associated cleanliness levels can not be met, but production does not show any problems. That means a lot of wasted energy from using too much supply air.
The difference between the final and initial surface cleanliness in a cleanroom determines the acceptable contamination. The larger this difference the easier the cleanliness can be controlled. The time the vulnerable surface is exposed then determines the particle deposition rate limit or particle deposition rate level limit.
The rate limit determines the required air cleanliness at the location of interest. Guidance is given in ISO 14644-17:2021. However, the impact of the air change rate is not taken into account. For particles that cannot be removed by airflow the operation control programme should reduce the presence of nearly visible and visible particles.
Establishing particle deposition rate control
To establish control, the cleanliness requirements and intended use are the inputs for the design of a clean facility. The intended use should be described in an initial operations control programme. At the moment, a new version of ISO 14644-5 is in preparation. This will provide guidance on the elements of an operations control programme.
This programme will cover the design of a cleanroom, the source strength of all particle sources, mainly the number of operators, the selected cleanroom garments and entree procedures. ISO 14644-4:2022 provides guidance on cleanroom design, construction and start up. In this standard it is recommended to use the ventilation equation to determine or check the supply air volume to achieve the required air cleanliness. For a dedicated design, knowledge of source strength of personnel and equipment will become more important. The standards on the assessment of cleanroom suitability of equipment and cleanroom consumables (ISO 14644-14:2017 and ISO 14644-18:FDIS2023) stimulate the generation of source strength data. In existing cleanrooms, comparison of the air cleanliness in the at rest state and the operational state and knowledge of the air supply, ventilation efficiency and cleanroom use can provide useful source data.
When all aspects have been verified with the use of ISO 14644-3:2019 and the facility fulfils agreed cleanliness requirement and all relevant personnel is trained, the established control can be commissioned
With respect to near visible particles (> 25 µm) and visible particles (> 50 µm) the way the cleanroom is used determines the air cleanliness for these particles and by that, the particle deposition rate. Next to human behaviour the cleaning of all surfaces is important. Surfaces accumulate particles until these are removed by cleaning. These particles must also be considered as an important contamination source. This is also the case for goods that are transferred into a cleanroom.
When all aspects have been verified with the use of ISO 14644-3:2019 and the facility fulfils agreed cleanliness requirement and all relevant personnel is trained, the established control can be commissioned. If the established control fulfils the initial cleanroom requirements, the cleanroom can be handed over and regular operations can start.
Demonstrating particle deposition rate control
Cleanroom classification denotes the cability of the environment. For classification a light scattering airborne particle counter is used. The procedure is described in ISO 14644-1:2015.
The operation will impact both the air cleanliness for airborne floating and depositing particles. The airborne particle counter can also be used to monitor the air cleanliness at one or more critical locations. It provides information on the effectiveness of the cleanroom ventilation system with respect to generated particles by the operation at one or more critical locations (see ISO 14644-2:2015).
An airborne particle counter takes an air sample and transports this to its measurement chamber. Macroparticles (> 5 µm) can get trapped and not be counted. The larger these particles the more difficult it is to count them and also their concentration is relatively low. Therefore, for larger particles the monitoring of the particle deposition rate is more useful. It provides information over a wide range of particle sizes. Many of these particles are difficult to remove by air flow. A proper operations control program must limit the number of macroparticle, near visible and visible particles in the room.
The particle deposition rate can be monitored by placing witness plates with a known surface cleanliness at locations of interest and collecting them after some hours of exposure during operation and measuring the surface cleanliness after exposure. There are also instruments that can monitor the particle deposition rate in real time.
Particle deposition rate monitoring can also be used next to settle plates for microbiological control. The collected particles on a settle plate are the same as collected on a witness plates.
The ratio between deposited particles and microbe carrying particles is determined by the established control. Once this relation is determined, it can be used for making earlier decisions. In grade B environment about a third of the deposited particles will be a microbe carrying particle.
Monitoring the surface cleanliness of various environmental surfaces can be used to determine the effectiveness of the cleaning programme and the change of surface cleanliness by particle deposition. Important surfaces are the floors, workbenches and equipment. Surfaces that are not necessary in the cleanroom should be removed. Then they do not become a source and do not need to be cleaned and monitored.
Area of application
The applications of ISO 14644-17:2022 are all critical activities where personnel is in the same controlled clean environment as the vulnerable product or process. Such a controlled environment is ventilated with clean filtered air in a non-unidirectional manner. Application fields span a number of high tech industries.
In these applications the critical particle size will always be larger than 5 micrometres. In case a product (or process) is sensitive for smaller particles, a segregation between operator and vulnerable product by a unidirectional air flow or physical separation must be made. In a separative device often a combination of unidirectional airflow and physical separation is made to create good air cleanliness levels (ISO 14644-7 in revision).
As an example, a vulnerable area of 10 square cm, that accepts 1 particle ≥ 25 µm in 1 hour, leads to a particle deposition rate level limit of 2,500 particles ≥ 10 µm per sqm per hour.
In cleanrooms with personnel particle deposition rates better than 500 particles ≥ 10 µm per sqm per hour can be achieved, but in many industries the particle deposition rate levels up to 50,000 have been observed. In a cleanroom with 20 air changes per hour it is possible to reach a particle deposition rate level of 500 by improving the operations control programme.
The application of particle deposition rate will not only help to improve cleanroom operation, but will also help to reduce the air supply to achieve the required cleanliness levels.
Conclusion
The ISO TC209 developed a series of standards on cleanrooms and associated controlled environments. With respect to particles the focus has been on airborne particles up to 5 micrometers, but in many applications larger particles, even visible particles, need to be controlled.
The new ISO 14644-17:2021 standard on particle deposition rate applications is useful for products that are sensitive to macro particles, microbe carrying particles, near visible and/or visible particles in cleanrooms with personnel. Especially when critical areas are large and/or exposure times are long.
Though perfect for high-tech applications, microbial deposition rate is proportional to the particle deposition rate and therefore can be applied in the control microbiological contamination in life sciences and hospitals.
