SM-4000 Real-Time Personal Silica Monitor The SM-4000 air sampler is mounted in the OSHA defined breathing zone allowing for a more accurate representation of workers exposure. Having a Real-Time Optical Sensor placed in the breathing zone, reduces inner wall dust deposition, experienced by other Real-Time optical devices on the market. In addition, having a gravimetric filter cassette directly behind the optical sensor allows for maximum particle deposition on the filter and thus a more accurate representation of worker exposure. The SM-4000 offers Real-Time Concentration Readings and the DustComm Software allows for graphical analysis and comprehensive time history reporting.