Particle counters are devices that function primarily using the principles of light scattering, although other technologies may also be employed. Light scattering by particles use instrumentation comprising a high-intensity light source (e.g., a laser), a controlled media flow (e.g., air, gas, or liquid), and highly sensitive light-gathering detectors (a photo detector). While the technologies vary by device and intended function, basically these devices rely on light obscuration by particles. This is the principle that states the presence of particles blocks some of the light from the photodetector, typically through either absorbance or light scattering. The photodetector records the obscuration of light and converts this to an electrical signal, this signal is then correlated to a specific sized particle using a pulse height analyzer, reporting these findings as particle counts. These devices have become increasing popular on some mold related assessments and remediation projects. But how reliable is this technology when attempting to understand the airborne mold spore populations and in determine if those populations are normal or abnormal (i.e., if a mold problem exists)? 

My professional opinion is that a mold assessor should seek to provide as much technically accurate and specific information as possible before drawing a conclusion on whether a mold problem exists or not. Shortcuts, like the one I described above, may be quick and convenient to some, but may not provide an honest, scientific description of the environment being assessed.