Laser Particle Counters are instruments that come in a variety of sizes and shapes for different uses, from checking biological contamination level in your breathing air to monitoring cleanrooms during microchip manufacturing. Laser particle counters have ranges of effectiveness from as high as 25 microns to as low as 0.1 microns.  Laser particle counters are customized by the manufacturer to check specific regulations.

Even the least expensive laser particle counters can usually detect particles as small as 0.5 microns, which is adequate to detect dust, mold spores, pollen, some bacteria and other biological contaminates and is well suited for Indoor Air Quality professionals who test air for health related contaminants. Laser particle counters used for IAQ are generally handheld and can sometimes test related environmental factors such as temperature and humidity.

Besides handheld, laser particle counters come in two other varieties, portable and remote.  Portable are bigger than handheld, are less mobile but can do more things, like print test results, run other environmental sensors and run unattended.  Remote laser particle counters come in a variety of sizes and are permanently installed in a facility as part of a monitoring system controlled from a central computer room.

All laser particle counters have one thing in common, laser counting technology.  Here's how it works. A sample of air is sucked into the instrument.  The air passes through a chamber where there is a laser diode on one side and a photo detection cell on the other.  The laser shines through the air sample toward the photo detection electronic cell.  Particles within the air sample scatter or block the light from the laser.  Detection electronics are able to determine the size of a particle based on the pattern of light as it is reflected by the particle.  The photo electronic cell passes a signal carrying information about the size of each particle to counting electronics.  Size on count information is then recorded and displayed. 

http://www.particlecounters.org/laser/#moreinfo