The Practicals of Particles

Sep 10, 2010

Have you ever looked at a sunbeam shining through a window? You will find airborne particles otherwise unseen by the naked eye. These particles are likely to be greater than 50 microns in diameter. A micron is a unit used to measure distance equal to a millionth of a meter. Imagine dividing a tiny, little millimeter into 1,000 smaller units… that’s a micron.

People can see these floating particles and can observe the build up of dust on surfaces. But are these particles really a concern? Most of these airborne particles settle out of the air and are never inhaled. Those visible particles that are breathed in are typically removed by the nose or upper respiratory tract. As for concentrations of these visible particles in a typical indoor environment, you may have a few hundred per cubic meter of air.

Now let’s make it interesting and discuss the particles that you cannot resolve with your naked eye.   Once you get down to approximately  5 microns in diameter, these particles can get past your nose and upper respiratory tract, depositing into the deeper reaches of the lung. At 5 microns, expect to have particles measured in the 10’s of thousands per cubic meter. Particles at 1 micron can reach your lungs’ alveoli where the air you breathe interfaces with your bloodstream. At 1 micron, expect to see 100’s of thousands of particles per cubic meter. Going even further,  you will typically find millions of particles per cubic meter at 0.5 microns.

What is the composition of these particles? They typically find the following constituents: organic carbon (OC), nitrates (NO3), sulfates (SO4), elemental carbon (EC), iron (Fe), silicon (Si), potassium (K), and zinc (Zn). Here are links to two articles that studied the chemical makeup of particulates:

There are indoor air quality contaminants that can also be considered particles.  Here is a list of some common ones:

  • asbestos fibers
  • pollen grains
  • mold spores
  • airborne bacteria
  • environmental tobacco smoke
  • cat allergens