Though HEPA filters may seem to be a new technology, the concept was actually developed during the Second World War by the US Atomic Energy Commission. It was first made to capture and remove radioactive dust from the air that could have presented a hazard to workers. The HEPA filter was designed to protect the human respiratory system. In fact, the US Centers for Disease Control (CDC) guidelines for preventing transmission of tuberculosis recommend that HEPA filters be used. They are designed to filter out 99.97% or more of particles that are 0.3 microns in diameter or larger.
A micron is a metric unit of measure that’s equal to one millionth of a meter. It is 1000 times smaller than a millimeter. The unaided human eye cannot see particles that are under 10 microns in diameter. Particles of 0.3 microns are the most difficult particles to trap within a filter because of their size. When inhaled, a 0.3 micron particle easily enters the body’s respiratory system, so that the body’s immune system is vulnerable to attack from them. That is the main reason why doctors highly recommend HEPA filters for controlling particles that aggravate asthma and allergies.
HEPA-based air purifiers are fine to leave on constantly. In fact, it is recommended for continuous cleaning of the air. Additionally, if a HEPA purifier is turned off for a long period then switched back on, some of the particles could find their way back into the air. Leaving a HEPA air purifier running continuously is best. When you do shut the HEPA air purifier off for a period (for example, while you will be on vacation and your home will be unattended) it is a good idea to go ahead and change the filters before turning it back on.
Keep in mind when choosing an air purifier that a higher wattage unit does not necessarily mean there is a stronger fan motor. Check the Clean Air Deliver Rate (CADR) number to learn how much air the fan moves. Higher wattage simply corresponds to the amount of electricity used to run the unit. Also, energy efficient air purifiers generally cost more, but they may also be quieter, which is an added bonus. Portable HEPA air purifiers with high quality energy efficient fan motors should from 80 to 200 watts on machines with CADR ratings of 350 to 600.
Not all of the particles that a HEPA purifier cleans from the air are as small as the 0.3 micron diameter minimum that HEPA filters filter out. Many other contaminants are much larger. For example, a human hair is 70 to 100 microns in diameter, and the droplets in a human sneeze are generally 10 to 100 microns in diameter. Pet dander can be anywhere from 0.5 to 100 microns in diameter, while pollen ranges from 5 to 100 microns in size. Plant spores are generally in the 6 to 100 micron range, while mold ranges from 2 to 20 microns in diameter. Smoke particles can be very tiny: 0.01 microns to 1 micron, while dust mite debris is larger: 0.5 to 50 microns. Most household dust ranges from 0.05 microns to 100 microns, skin flakes range from 0.4 to 10 microns, and bacteria are anywhere from 0.35 to 10 microns in diameter.