There are many situations where it would be desirable to determine biological aerosol levels in surrounding air.  Many human, animal and plant pathogens such as viruses and bacteria travel from one host to another either by attachment to an otherwise benign aerosol particle or through a purposeful aerosol lifecycle stage such as a spore.  Such bioaerosols are particularly disadvantageous in pharmaceutical manufacturing facilities and clean rooms of any type. 
There are a limited number of ways to monitor bioaerosols.  The company Proengin of Saint-Cyr-l'École, France markets a product that passes a stream of sampled air through a hydrogen flame.  Literature from the company indicates that the spectrometric signature created by oxidation of the bioaerosol can be used to detect many different types of pathogenic organisms.  However, it is likely that many carbonaceous materials will present very similar signatures and this approach may be too susceptible to falsely categorize organic aerosols as biological in nature to have much utility except in certain applications. 
A second method that has been the subject of considerable research and development, particularly by the U.S. Department of Defense, is based on UV fluorescence.  In this approach, advantage is taken of the broad characteristic of organism biochemicals to fluoresce when subjected to ultraviolet light.  Fluorescing chemicals are ubiquitous in living organisms, and may be compounds such as aromatic amino acids, NADH, and flavins.  In these devices, a stream of air is made to pass through a beam of ultraviolet light (with the light beam usually positioned perpendicularly to the particle beam).  Fluorescence emitted from any aerosol particles is then collected optically and converted into a corresponding electronic signal by a photodetector, either particle-by-particle or in an ensemble integrated mode.  In both cases an electronic signal is generated that is proportional to the local bioaerosol concentration. 
This has advantages over the flame photometry approach in that it is likely to have less false positives from atmospheric organic debris, there are no consumables, and highly flammable hydrogen is not needed.  In addition, differences in fluorescence intensity may help discriminate against some natural or manmade organic aerosol particles that are not of interest.  A particle-by-particle approach is preferred because it allows a more detailed examination of aerosol composition, but the intensity of fluorescent light emitted by a single bioaerosol particle is very small, generally only 1/1000 or less the intensity of light scattered by the particle.  Hence, highly efficient methods of collection and electro-optic conversion are required. 
Preferred detection devices are the photomultiplier tube (PMT) and avalanche photodiode.  The PMT has very high gain but may be somewhat bulky and fragile, while solid state devices such as the avalanche photodiode can be quite small but have less gain and may not be capable of operation at higher temperatures.  Which approach is more suitable depends on the application. 
At first glance, it would seem desirable to design the excitation optics and air stream injector so that each particle's fluorescence signature is maximized.  This has been the approach most often used in aerosol particle sizing devices.  Particles are forced through an aperture and made to pass through the focused beam from a laser diode.  This produces a large photon pulse which is converted to an equivalent electronic pulse by the detector.  These pulses are then summed, and the sampled concentration calculated based on the known air flow.  One disadvantage of this analog approach is that there is no way to know how many of the counts are truly associated with particle transits as opposed to background noise pulses (such as generated internally by the detector) without halting the sampling process and measuring the background count rate.  This would be extra work for an operator and force a period of time when no sampling was being performed. 
There may also be a tradeoff between maximizing the fluorescence signature of particles that are detected, and the uniform detection of a significant proportion of particles in the air stream.  For example, the image spot size from a solid state laser may be made very small and the local light intensity very high so that particles passing through the focal point will be intensely excited.  It is exceedingly difficult, however, to focus an air stream containing 1 to 10 micron particles to a correspondingly small cross-section.  In typical embodiments either only a fraction of the particles in the air stream pass through the laser beam's focal point and many particles sampled by the measuring device pass through undetected, or the particles are forced to pass through such a small aperture that there is a risk of the aperture clogging over time.  In addition, the radial intensity distribution around the propagation axis of a laser beam is typically Gaussian and particles passing at different distances from the focal point would see varying levels of UV excitation, making it very difficult to quantitatively estimate the overall aerosol sample's particle size distribution. 
Essentially all bioaerosol detectors reported in the literature or sold on the market use mirror optics to collect and focus signal fluorescence onto a photodetector.  This is due in part to a lack of low-cost refractive optics at ultraviolet wavelengths and to a preference on the part of those knowledgeable in the art to use metal reflective optics since, to the first order, they are insensitive to the working wavelength.  Also, the curved mirrors used are focused on the intersection of the light beam and air stream, thereby maximizing the reflection of the faint fluorescent signals created.  This may, however create a large expense in system manufacture and maintenance, as the reflective optic components can be quite costly and are very difficult to clean without damaging the reflective surface.