Particle Size Comparison
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| Type | Microns (diameter) | Time to Settle |
| Skin Flakes | 20 - 50 | 5 seconds |
| Dust | >10 | |
| Pollens | 15-25 | |
| Mite Allergens | 10-20 | 5 minutes |
| Common Spores | 2-10 | |
| Bacteria | 1-5 | |
| Cat Dander | 0.5-1.5 | 10 hrs |
| Tobacco Smoke | 0.1-1 | |
| Metal/Organic Fumes | <1 | |
| Cell Debris | 0.01-1 | |
| Viruses | <0.1 | 10 days |
You will note that the larger the particle the quicker it will fall out of the air on its own. This can be both good and bad.
Think about this for a moment. If they settle out too quickly they are not likely to make it to a filter before landing on a surface. Once landed, they will remain until something stirs them up again; probably a person walking by. If the person is walking away from the air filter at the time, which way do you think it will move?
The smallest particles tend to remain in the air indefinitely but may not be pulled into the air filter due to other influences such as the movements of the occupants. Eventually they will enter the air filter. Let's hope they are not too small for the filter you purchased.
Look closely at the chart. At less than 0.3 microns, a few of these pollutants are small enough to pass right through a HEPA filter. The most hazardous, at least in terms of size are small enough (less that 1 micron) to penetrate deep into your respiratory tract. Some can even end up in your blood stream.
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