Definition: Air sampling in the context of microbiological assessment is the collection of air-borne microbial contaminants that may impact on product spoilage, product safety and human health.

Explanation: Collection of vegetative cells and spores may be achieved by passive or active methods.Passive methods usually involve settle plates whereas active methods include impaction and impingement devices.

The type of environment to be sampled varies from primary food production (GMO) to processed food factories, operating theatres, pharmaceutical clean rooms and compressed gases.

The relatively low concentration of microorganisms in air means that collecting them from this environment requires the sampling of large volumes of air. There are a number of techniques that allow the quantitative determination of microbial contamination or bioaerosols.

The collection methods available include; sedimentation (settle plates), impaction on solid surfaces (growth media), centrifugation, filtration, impingement in liquids and precipitation (electrostatic and thermal). The volume of air for sample collection depends on the device being used and on the anticipated concentration of the bioaerosol.

Where low concentrations of microbial contaminants are expected, e.g. clean rooms, food production and operating theatres, impaction methods are generally chosen. In highly contaminated environments then impaction techniques may 'oversample' even over short timescales and impingement or filter samples are more appropriate. With strict adherence to manufacturer's flow rates, sampling periods, culture media used, and device placement then most techniques will yield comparable results.

Source: http://www.rapidmicrobiology.com/PG/Air_Sampling.php