PMx refers to small particles suspended in the air, also called particle pollution. Dust is a form of particle pollution, as are smoke particles or liquid droplets. PM is categorized by particle size, not type. PM2.5 refers to particles which are 2.5 microns in diameter. PM10 particles are 10 microns. Some PM pollution is visible but others can too small to see without instruments. Fine particles can be very dangerous because the small particles can become embedded in the lungs, causing a variety of health problems.

Indoor PM pollution is of interest in the science of COVID and other disease propagation because of fine liquid dropments produced by infected persons. Such particles are defined by OSHA as Bioaerosols. Even though viruses are very small (COVID is less than 0.14 microns), the liquid droplets that carry them are well within the range of sizes detected by PMx sensors. PMx sensors can, within limits, resolve a statistical distribution of particle sizes, but they cannot determine the type of particle, for example liquid vs solid.

Currently, the EPA has primary and secondary standards for PM2.5 (annual average standards with levels of 12.0 µg/m3 and 15.0 µg/m3, respectively; 24-hour standards with 98th percentile forms and levels of 35 µg/m3) and PM10 (24-hour standards with one-expected exceedance forms and levels of 150 µg/m3). There are no regulatory requirements today for exposure to 1 micron particles (PM1.0) , though this is likely to change because the science is clear regarding the risk of these smaller particles. WellStat devices measure PM1.0, in addition to PM2.5 and PM10.

WellStat devices use a reflected laser type sensor. Laser PM sensors are the most accurate available today. A Laser Sensor detects particle concentration in air by using an optical sensing method consisting of a laser light emitting diode (LED) and a photo sensor which are optically-arranged in the device. The photo sensor detects the reflected laser LED light by dust particles in air. The sensor can detect the particles and distinguish their sizes from the pulse pattern produced by light reflected by the particle. Multiple detectors are used to increase accuracy. The size of the dust particle detected is an approximation of the mass distribution of particle sizes. For example, if the PM10 reading is twice the PM2.5 reading, then the total mass of particles 10microns in diameter is approximately twice the mass of 2.5micron particles. A PM10 reading does not indicate that the detected particles are 10 microns in diameter. iESMach uses calibrated NIST-traceable test dust with a certified mass-distribution of particle sizes to validate the performance of sensors used in WellStat