Cleanroom Particle Counting: The 5 Micron Issue

By Tim Sandle, Ph.D, M.A., BSc (Hons), CBiol, MSBiol., MIScT
The IEST Journal has an interesting article on cleanroom metrology by Lothar Gail. Within the article i an exploration of the 5 micron particle size issue. AS those involved with cleanrooms will know there are two particle count sizes looked for within cleanrooms: 0.5 and 5.0 micron.
The FDA emphasise in the Guide to Aseptic Filling that the 0.5 micron size is te important one for determining if the environment is below or above the accepted evel of particles and in doing so draws upon the ISO 14644 cleanroom standard. However, more controversially with Europe, the EU GMP Guide states that both particle sizes are important.
In arguing against the need to measure 5.0 microns, Gail states:

EC GMP requiring the detection of 5-µm particles with a sample volume of at least 1 m³ for ISO Class 5 classification and monitoring, overlooks some essential facts:

• 5-µm particle counts in an ISO Class 5 environment should be avoided in principle due to background noise level and poor resolution. The poor reliability of 5-µm particle counts cannot be fully compensated by increasing the measuring time.
• 5-µm particle determination proves to be about 10 times more expensive and timeconsuming than 0.5-µm particle counts.
• Currently there is no scientific evidence that 5-µm particle detection offers any improvement for cleanroom hygiene control. EC GMP regulation impedes international harmonization of cleanroom qualification and monitoring procedures.

Even with the latest development of particle counters that offer substantially higher sampling flow rates, the situation does not improve: Areas of ISO Class 5 normally are as small as possible. A particle counter with high sample flow rates cannot be placed in that area since the high sample flow is withdrawn from a small volume. In small areas such as pass-throughs, when the sample flow air is returned into the environment, the pressure differential may be affected; when the sample flow air is returned into the measured area, the air change rate may be affected.