Toward biological aerosol reference standards

One issue that has persisted since the beginning of what might be referred to as a modern era of biological aerosol research (since the 1990's), is absence of reference biological aerosols that would permit quantitative comparison among different experimental studies. We believe sufficient technical progress has been made bridging the diverse fields of biology, chemistry, physics and engineering to consider implementing well-characterized biological aerosols as reference standards. Establishment of methods and procedures which result in reliable and repeatable generation of well-characterized biological aerosols would enhance a wide range of different topics under investigation, and permit wider utility for data acquired from individual efforts. In this article we discuss some of the challenges and limitations for two general approaches for biological aerosol generation: solvent evaporation from liquid suspension droplets, and dry powder dispersal. We provide detailed descriptions of an example for each of these two approaches in which sufficient control is demonstrated over particle size distribution, total particle composition, biological constituent quantification and biological state (viability or enzymatic activity) to serve as a comparison among different experimental investigations. These two specific cases are intended as examples, not necessarily as prescriptions.