What is a d50 Value?

Physical Efficiency

D50 is the diameter of the particle at which a sampler or monitor is 50% efficient. Consequently, we tend to use d50 as a measure of the smallest particle that can be collected reliably. If a device reports a d50 of one micron, this means that it will reliably collect at least 50% of all particles of one micron in size or above. It will likely capture smaller particles as well, but for particles of at least one micron in size, the figure is 50%.

The GMP Annex 1 standard refers to having a sampler capable of efficiently detecting particles down to one micron in Grade A environments. We can interpret that as meaning a d50 of one; it’s 50% efficient at capturing particles of one micron in size.

However, the d50 value is only part of the story. It only refers to the physical efficiency. We need to consider the biological efficiency in terms of keeping the organisms alive and culturable too.

Biological Efficiency

This is the ability of an air sampling device to collect microbe particles efficiently, with minimal drying out or damage of the microbes by the velocity of the air coming though the collection slit or sieve. The constant air hitting the agar surface and consequent drying effect is also a factor that needs to be considered.

The biological efficiency is important because the standard has changed. The standard used to say that for a grade A clean room, there had to be less than one colony forming unit per cubic meter. Less than one implies zero; in a cubic metre of air, there’s either at least one colony forming unit, or there isn’t. However, some people were interpreting the standard as, “if I sample 12 cubic metres, and I get 11 colonies, then on average, that’s less than one.”

That wasn’t the intention of the standard and so the wording now says you have to have zero growth per cubic metre and that puts even more emphasis on having the best biological efficiency possible to make sure your instrument is capable of detecting down to the right level.

The CAMS ImpactAir range of samplers use a slit to agar technique which has a fixed radial slit over a rotating agar plate which means air is compacted onto fresh agar continually over a prolonged period of time in contrast to sieve samplers that impact air onto the same small areas of the agar surface and cause natural drying of the agar.

Independent studies by PHE, Porton Down show the ImpactAir is more efficient than the gold standard reference system and at least 1.8 times higher than 2 of the most common sieve samplers used today.

Summary

In summary, the d50 specification is about getting the live particles onto the agar (physics), and the biological efficiency is increasingly important because the standard has changed to zero growth.

The CAMS ImpactAir range has a d50 of 0.45 microns. This is the best d50 performance of any air monitoring system on the market today and this in conjunction with superior biological efficacy makes it the most appropriate air sampler for todays needs. CAMS ImpactAir devices are already in use today in Grade A environments all over the world.

To find out more about the ImpactAir range click here.