When people consider buying a home air filter they often judge value by reading the efficiency rating. This kind of information is often misleading.
Efficiency ratings
When you read the term efficiency rating you would think Ah ha! This will tell me if this filter will do the job. Not so. The rated efficiency pertains to the particle size. For example, a HEPA filter may have a 99.97% efficiency rating. This refers to its performance with particles of 0.3 microns or larger.
In any given device using a HEPA filter, the true efficiency could be as low as 80% due to leakage, damage, contamination, humidity and other factors. If you are looking at a HEPA-type or a HEPA-like filter, the actual efficiency may be as low as 55% or less at 0.3 microns.
Another factor to consider when looking at efficiency ratings is that they can only apply to the air which actually goes through the filter. Depending upon the size of the room and location of a pollutant source, much of the bad air may never make it to the filter. Again, if you are in the room, it is entirely possible that you will breathe the air first. You then become the air filter.
Gas filtration
Home air filters are usually considered based on their ability to remove particle contaminants. This does not relate to the removal of gaseous products and undesirable odors. This type of removal is left to the absorption abilities of filters such as activated carbon filters. It is well known that activated carbon in its granular form is effective in removing many gaseous contaminants. But most filters on the market use an inferior product consisting of carbon fiber pads which are impregnated with activated carbon dust. The performance is greatly reduced.
ULPA filters
An Ultra Low Penetration Air filter is supposed to have an even higher efficiency than the HEPA filter and is used in modern clean rooms like hospitals, electronics manufacturing and so on. The disadvantage of using these units in homes or offices is the reduction of air flow which results in a lower turnover of air. As a result, most room purifiers with HEPA filters would probably be more effective in reducing particulate.
Usage over time
Many home air filters show excellent results but only for a few days or even hours of usage. Frequent clogging of filters is a real problem. Some advertisers try to turn this into a positive by claiming greater efficiencies due to the inability of smaller and smaller particulate to pass through. In reality, the denser the filter, the harder it becomes to force air through it and therefore we are faced with less volume of clean air entering the room.
