Air is seemingly empty to us because the human eye cannot see many of the particles floating around. Particles can be human skin scales, bacteria, viruses, soot, rust, pet dander, pollen grains, mold spores, car-tire particles and more. Particles can be smaller than .01 micron in size. A micron is one-millionth of a meter, and the human eye cannot see anything under 40 microns.
To put this in perspective, the diameter of a single human hair is about 100 microns.
Household dust can range anywhere from smaller than .01 micron to 10 microns. Mold spores can be 2 microns to 10 microns. Bacteria are normally 0.1 micron to 10 microns and viruses are even smaller. Animal dander can be 5 to 20 microns in size.
So, what does this have to do with healthy indoor environment?
Particles travel on airflows created by HVAC systems and ventilation and, the smaller the particle, the longer it remains airborne. Some are even permanently airborne. The longer it remains airborne, the more time available it can be inhaled deeply into the lungs. Bacteria and viruses carry diseases. Some cause asthma and allergy symptoms. These particles that we can’t see need to be filtered out as much as possible.
Products such as HEPA filters remove 99.97% of particles. Air purifiers can be beneficial but can also be detrimental. An air purifier that ionizes air or creates hydroxyl ions creates an irritating gas to provide that fresh air smell.
The best way to removes these particles is to remove the source altogether. For example, if mold spores are coming off an old rug, get rid of the rug.