Viable aerosol measurement: Drawing from parallel experiences

01/01/2003

by Robert P. Donovan

My December 2002 column discussed the measurement of viable particles in microelectronic ultrapure water (UPW). The measurement of viable aerosol particles in the cleanroom environment housing production processes is also growing in importance.

In pharmaceutical manufacturing, the presence of viable aerosol particles during the manufacturing of aseptic pharmaceuticals—pharmaceuticals which must achieve and maintain sterility without relying on post-manufacturing heat treatments—can endanger an entire product line. As with the measurement of small, viable particles in UPW, the standard methods for measuring the concentration of sub-micron, viable aerosol particles still depend on time-consuming culturing methods that report results only after lengthy incubation periods.

Thus, biologically-contaminated environmental conditions do not generally become known for a day or more after the end of a sampling period—which can be a lengthy process in itself. During this time period, much product will be produced and discarded. Real-time or near-real-time measurements of the concentration of viable aerosol particles are every bit as urgent as the measurements of the viable particle concentrations in microelectronics UPW.

Common methods for collecting samples of viable aerosol particles for culturing are impaction, filtration and liquid impingement.1 Actually, large aerosol particles (those >5 microns in diameter) can be collected and counted under a microscope without culturing. The rotorod sampler, for example, uses adhesive-coated rods spun at high speed in the air space under the test to collect representative aerosol samples of relatively large bioaerosol particles, such as pollen, by impaction.

These deposits can be subsequently counted under a microscope. Unfortunately, this technique does not necessarily distinguish viable from nonviable bioaerosol particles and is not appropriate for monitoring cleanroom aerosol particles, which are much smaller and less concentrated than those efficiently sampled by the rotorod sampler.

Sampling devices such as the Andersen sampler and the all-glass impinger also rely on inertial impaction to separate aerosol particles from an air stream and are designed to either deposit submicron-sized bioaerosols directly onto agar-coated plates—the Andersen sampler—or into a liquid matrix with subsequent transfer to a culture media—the all-glass impinger.

Membrane filters are a high-efficiency third method for collecting bioaerosol samples, but the desiccating conditions of a dry filter render many species nonculturable. Sample collection time and the survivability of captured bioaerosols are important considerations for all culturing methods.

Recent advancements

In principle, it's also possible to identify and enumerate viable aerosol particles directly from the environment without removing a sample and culturing it—like an optical particle counter (OPC) counts particles in an air stream without collecting them. One example is TSI Inc.'s (St. Paul, Minn.) ultraviolet aerodynamic particle sizer (UV-APS). This commercially available instrument measures the aerodynamic diameter, light-scattering intensity and intrinsic fluorescence of an aerosol particle more or less in real time.

It spans the size range 0.5 to 15 microns, a size range over which particle relaxation time varies by nearly three orders of magnitude. Particle aerodynamic diameter is measured by particle time of flight (TOF) between two precisely positioned laser beams located immediately downstream of a nozzle that sharply accelerates the airflow.

TOF depends upon particle relaxation time and empirical curves are used to convert TOF into particle aerodynamic diameter. The two TOF laser beams also scatter light that can be used to measure the particle's light scattering diameter. Finally, the TOF signals initiate a pulse from a UV laser that stimulates the intrinsic fluorescent response of bioaerosols (but not inorganic aerosols), enabling the instrument to classify a light scattering particle as viable or nonviable.

While this instrument is commercially available, it is expensive and not yet widely used in cleanrooms; however, it does illustrate one method that has already demonstrated the capability to distinguish viable aerosols from nonviable aerosols in near real time.

Growing concerns about biological attacks initiated by terrorists have stimulated a broad, intensive research effort to develop speciating, rapid-responding detectors of bioaerosol agents.2 The Gulf War (1990/91) jump-started interest in this capability.3 These efforts are likely to spin-off additional instrumentation capable of near-real-time monitoring of viable aerosol particles in cleanrooms, although costs remain a barrier.

Many of the techniques used in aerosol sampling and enumeration can be adapted to liquid matrices and vice versa. Techniques described for rapid measurement of viable particles in UPW can often be configured to measure viable aerosol particles. III

Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories and a monthly columnist for CleanRooms magazine. He can be reached at [email protected].

References

• 1. Jensen, P. A. and M. P. Schafer, "Sampling and Characterization of Bioaerosols," NIOSH Manual of Analytical Methods, 1/15/98.
• 2. "An Introduction to Biological Agent Detection Equipment for Emergency First Responders," NIJ Guide 101-00, December, 2001.
• 3. Field Analytical Chemistry and Technology 3 (4-5), John Wiley & Sons Inc., 1999.