Sampling for Airborne Biological Contaminants:

A RATIONAL APPROACH

Nothing we do raises as many questions as microbial air sampling.

While the science of microbial air sampling is fairly straightforward, somewhere along the line our approach to it has become much like that of a backward soothsayer who divines the question after first being given the answer. For example, we often first buy the sampler and then try to fit it into our operation

Sampling strategies differ when applied to quality control, validation, or research. Each requires a differing degree of sampling sophistication and a different approach to data analysis. All microbial aerosol samplers must be judged in terms of their capability to collect microbes under different operating conditions while minimizing the environmental stress on the organisms collected. There is, therefore, no single sampling method that is suitable to all occasions.

Another point to remember: rarely is microbial air sampling conducted as a quality control point of a product. More often than not, air sampling is done to determine the cleanliness, or absence thereof, of the environment in which a product is manufactured. Or in this instance, to answer the familiar question: how clean is clean?

Sampling Theory

Microbial air samplers are characterized by:

* Mode of capture,

* Flow rate and flow characteristics, and

* Collection efficiency as a function of particle size and shape.

As a rule, aerosol collection devices that exhibit the lowest shear forces collect samples where microorganisms have the highest viability. Conversely, these samplers usually have the lowest physical efficiencies in terms of numbers of airborne particles collected. The microbiological collection efficiency, therefore, depends largely upon the sampling method used.

The primary objective of any sampling program is to produce a set of samples that are representative of the source under investigation, and, that these samples are suitable for subsequent analysis. Because the air we sample in any environment is not homogeneous even in a clean room that exhibits good laminar airflow, there can be no duplicate samples. We therefore need to consider sampling conditions, sampling time, and sample size as limitations in our data collection scheme.