 The American Society of Heating, Refrigeration and Air Conditioning Engineers defines lung damaging particles as 0.2 to 5.0 micrometers in size. Larger particles are filtered by the body’s natural protection system in the nose, in the throat and trachea before they can penetrate deeper into the lungs. Smaller particles because of their physical properties can penetrate deep into the lungs and reach bronchioles, bronchi and the critical alveoli.
Most maladies, including infections, occur in the lungs. This critical component should be the focus of particulate protection.
The chart below shows the number of particles by count in a typical atmospheric air sample. Please note that 99% of all airborne particles are under 1-micron in size. Filters should be selected for the most offending particle size, which would be 0.35-micron as that size is the mass-mean diameter of all suspended particulate.
The typical office environment particle number by count can be up to double the level of atmospheric air. Internal pollution contributors such as carpeting, natural deterioration of the building’s materials office equipment, and of course people, are the prime sources for the increased level of indoor pollution.  Many authorities now recognize a MERV 13 air filter as the discriminate choice for controlling indoor air quality. With an initial efficiency of 60% on particles 0.3 micrometers in size a MERV 13 fine fiber filter continues to gain efficiency during operation approaching 90% efficiency after a short period of operation.
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