One micron is 1/1000 mm (1/25,000 of an inch). Airborne particles are usually described in microns. Generally speaking, the human eye can see debris and dust that are approximately 25 microns in size.
To understand just how small this is, consider that a single hair from your head averages about 70 microns in diameter or 30 times larger than the largest fine particle. The size of a given particle helps to determine the degree of potential threat to human health.
Particles ranging from 0.3 to 0.9 micron present the greatest health concern because they are small enough to get past the tiny hairs that line our breathing passages and are too large to be easily exhaled. These irritating mid-range particles include house and textile dust, pollen, pet dander, dust mites and their feces, many bacteria, auto exhaust, mold spores, and particles from laser printers and copiers.
0.1 to 0.3 micron = Dust mites and allergens
0.3 to 1 micron = Tobacco smoke, metallic fumes and bacteria such as staphylococcus
1 to 5 micr>5 to 10 micr>10 micr>
Because mid-range particles are more likely to become lodged in lung tissue, they are suspect in a wide range of health problems related to indoor air pollution, from headaches and dizziness to cardiovascular disease and cancer. In particular, pollen, pet dander, mold spores and dust mite particles are known to trigger asthma episodes and allergy attacks.
While smaller particles (0.1 to 0.3 micron) can be inhaled and exhaled more easily than mid-range particles, even these minute particles may irritate breathing passages and lungs. Smaller particle filtration is particularly beneficial to people living with allergies, asthma, other respiratory conditions, or cardiovascular disease.