Passive Air Sampling

Passive air sampling is done by means of settle plates: agar plates are left open and exposed in clean rooms for defined periods. They are used widely in Europe where they have been strongly advocated over many years in the work of Whyte (1986), but less so in the U.S., except in facilities manufacturing for export to Europe.

The principles of the settle plate were empirically demonstrated by Whyte (1986). Most airborne microorganisms are associated with physical particles of 12-^m diameter or larger (Whyte, 1986), which are heavy enough to settle out of air by gravity.

Sykes (1970), in earlier research, challenged this concept. He calculated a mean settling time of seven minutes for particles of 5-^m diameter (the particle size normally associated in European regulatory literature with airborne microorganisms) through a one-foot column of still air. Probably only the heaviest particles will be collected on settle plates laid out horizontally (as is the almost universal practice) on flat surfaces in turbulent or unidirectional air flow clean rooms.

It is also important to consider the significance of 1 cfu on a settle plate. What does it represent? One single viable microorganism? Or several tens or hundreds of microorganisms carried on a single skin particle? This restricts the value of settle plates for consistent comparison with quantitative limits, though their qualitative value is not debatable.

Table 2.2 Some Characteristics of Available Active Air Samplers

Sampler

Sampling rate (litres/minute)

Weight (kg)

Recovery of microorganisms

Slit-to-agar sampler (Casella)

Slit-to-agar sampler (Mattson Garvin)

Centrifugal sampler (RCS)

Centrifugal sampler (RCS-Plus)

Filtration sampler (Sartorius MD-8)

Perforated atrium sampler (PBI)

Perforated atrium sampler (Merck)

Perforated atrium sampler (VAI)

175, 350, 525 or 700

40 (effective) 50 (effective) 130

90 100 175

16 (with pump)

Agar in 150-mm Petri dishes

Agar in 150-mm Petri dishes

Agar in flexible plastic strips unique to centrifugal samplers

Agar in flexible plastic strips unique to centrifugal samplers

Gelatin membrane transferred to standard 90-mm Petri dish containing agar

Agar in 55-mm Rodac Petri dishes

Agar in standard 90-mm Petri dishes

Agar in standard 90-mm Petri dishes

There has been some debate about how long settle plates may be left open in clean rooms before the effects of desiccation impair the ability of the agar to support the growth of microorganisms. The PDA recommends 30 minutes in its 1981 Monograph No. 2 (PDA, 1981), the Parenteral Society in the U.K. recommended four hours in 1990 (Parenteral Society, 1990), while Whyte and Niven (1987) argued that the viability of microorganisms on agar plates was not significantly affected by desiccation for exposure periods of up to 24 hours. It is most likely a function of the depth of agar in the Petri dish and the condition of the agar when introduced into the clean room.

All practising microbiologists will have some experience of seeing agars drying out, and agars with desiccated "skins" on their surfaces. They should have technical or procedural mechanisms in their laboratories to prevent such plates from being used, or at least to ensure that data from such plates are discarded.

Regardless of these objections settle plates are popular with many microbiologists, as they are inexpensive, do not disrupt, in most instances, protective air flow patterns, and require no great technical expertise to generate data. They can sometimes be laid out by production rather than QA personnel, though some regulators have insisted that these practices should be observable for independent QA audit.

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