The distribution of the biological content is also important. A thousand bacteria in a single 10 µm particle may represent a different health risk than a thousand 1 µm particles each with one bacterium. Obviously, if the bacteria are not viable or have lost the ability to be infective or pathogenic, the health hazard has changed dramatically. Current research is suggesting that nonviable content in the aerosols—including lysed cellular material—may contribute to the health risk or, to the contrary, provide benefit to the immune response in some cases. The currently used unit of measure, ACPLA, and current state-of-the-art detection capabilities, cannot take these factors into account.
A further limitation of ACPLA is that it does not contain any indication of particle size and particle size distribution, characteristics of the aerosol clearly important in evaluating health hazard to humans. For aerosolized particles with biological content, the aerodynamic diameter is a critical parameter in determining the final site of deposition in the respiratory tract and must be considered.
In light of these limitations, the committee developed a new, robust framework for evaluating the health hazard posed by biological aerosols. The next chapter describes the recommended framework and its mathematical and conceptual reasoning.