Knowledge Center

In a cleanroom operations are the major source of micro-organisms. These organisms are carried by particles. The average size of microbe carrying particles is larger than 15 µm. So measuring particles larger than 15 µm is worthwhile, especially because the chance a particle carries a micro-organism increases with particle size. In Grade A and Grade B environment the ratio of mcp’s and particles is higher than in Grade C or Grade D. Below the relation between depositing particles and depositing micro-organisms “m” is shown.

The real relation depends on the cleanroom installation and operations. Once this is determined particle deposition monitoring can predict the microbial deposition rate.

The removal of a particle by air flow depends on type, shape, size and location of a particle and the local air flow and available surfaces to deposit on. In a diluting air flow the average percentage of of particles that deposit is given in de graph below. This is the result of a theoretical analyses and measurements in a ISO 7 cleanroom.

Particle counters give valuable information on airborne particles and the performance of the cleanroom installation. Unfortunately, they don’t measure macro-particles, even when they are calibrated up to 25 µm because larger particles easily get trapped in the collection system and cannot be measured. An APMON can measure the larger particles much better. The data can be used to determine the contamination risk of products and processes and show the impact of the way the cleanroom is used. The particle the APMON measured cannot be removed by the cleanroom installation. So if the particle deposition is too high the way the cleanroom is used should be improved.

If particle counters are positioned in close proximity of the ventilation system, operational impact measurements are less accurate. Presumably the operational quality is not good enough. Particle deposition data will show which operational activities are detrimental and should be improved.

The better the cleanroom class the better the particle deposition rate level should be. This is determined by the better operational procedures. However, if these or not in place the particle deposition rate level can be much higher.

In ISO 6 cleanrooms (in operation for particles ≥ 0,5 µm) average particle deposition rates of 5 to 50 particles ≥ 20 µm per dm² per hour are measured.

In ISO 7 cleanrooms (in operation for particles ≥ 0,5 µm) average particle deposition rates of 10 to 100 particles ≥ 20 µm per dm² per hour are measured.

By improving the cleanroom cleaning programme, frequency and cleanroom discipline, the particle deposition rate can be improved. Presumably will stay the same when the air supply flow rate is reduced, as long the overpressure is maintained. That way energy usage can be reduced.

In the past, particle deposition was measured by placing and collecting witness plates and inspecting these under a microscope. This is an expensive and laborious way of monitoring. Hence, it has disappeared.

New particle deposition monitoring equipment that makes use of various new vision techniques make particle deposition rate monitoring more accessible. In various industries, the likelihood of quality loss by macro particles is increasing due to larger critical surface areas.

The new methods show the influence of the part of the cleanroom (air cleanliness with respect to macro-particles) that is missed in the cleanroom standards.

In hospitals the risk of unwanted contamination depends on the type of surgery. Important are the size of the opened area (wound), the time of exposure the number and contact area of surgeons fingers, hands and instrument and the sensitivity of the patient. In high risk surgery antibiotics are used preventive when the risk of infection is high. In an operating theatre there are two different areas: a protected area with cool unidirectional downflow of HEPA filtered air and a background which is diluted with HEPA filtered air. The particle concentration in the protected area is about 100 time lower that the back ground. Further there are surrounding rooms for preparation of instruments and personal. Since the patient will spread many micro-organism it is difficult to monitor the quality of the operating rooms by microbiological measurement methods. The relation between micro-organisms and macroparticles particle deposition monitoring can be used when microbial testing cannot be executed. Realtime particle deposition monitoring also shows the moment(s) the particle deposition is high. The APMON can give valuable information on the operational quality of an operating room.