AirSentinel<sup>®</sup> is a new low cost, compact ultraviolet-based light induced fluorescence (UV-LIF) bio-aerosol threat detection trigger. Earlier UV-LIF triggers, for example, FLAPS, BARTS, BAWS, Bioni, and BioLert, have used UV <i>laser</i> sources to induce fluorescence of biological aerosols. Two recent developments from the DARPA MTO SUVOS program, BAST and TAC-BIO, use UV LEDs for the same purpose, thereby broadening the term UV-LIF to mean laser or LED induced autofluorescence. All of these earlier triggers interrogate aerosols on a particle-by-particle basis on- the-fly. The major trade-off for these instruments is cost, size, and complexity versus counting efficiency (probability of detection) with the lower size end of the respirable range being most difficult to detect. AirSentinel<sup>®</sup> employs a different approach to UV-LIF detection: aerosol concentration by collection on a surface, surface interrogation, and surface rejuvenation prior to repeated concentration and interrogation cycles. Aerosol particle concentration via impaction on a surface addresses the issue of small particle counting efficiency since the fluorescence from the sum of the particles is the sum of the fluorescence signals from the collected particles, typically hundreds or thousands in number. Surface interrogation for a LIF signal is accomplished by illumination with a 280 nm and/or a 365 nm LED. As expected, test results show better relative detection performance using 280 nm excitation LEDs for bio-toxin simulants and somewhat better performance at 365 nm for standard Bacillus globigii spore targets. AirSentinel<sup>®</sup> beta technology is currently in long term testing in a number of public and other government buildings.
The Receiver Operating Characteristic curve (ROC) has long been used in medical applications to compare screening and diagnostic methods. As the threshold used by any screening or diagnostic method is changed, the operating characteristics of the method, such as the number of true positive and false negative determinations changes as well. The ROC curve is one way to characterize the changes in order to compare different methods. This definition, however, is difficult to apply to chemical and biological sensors detecting the release of a toxic agent given that there is more than one ROC curve. There is a continuum of ROC curves corresponding to a continuum of release levels. A new definition of ROC curves has been adopted for chemical and biological sensors which will reduce the continuum of curves to a single curve. This paper presents a methodology to estimate ROC curves using this new definition.
A rationale for evaluating bioaerosol sensor technology for building protection applications is presented. Issues associated with bio-threat sensor systems for buildings include sensor performance metrics, standards and cost. The low-cost AirSentinel bioaerosol sensor is highlighted as an example of an approach that addresses the issue of affordability.