Filter & Filter Media Discussion
HEPA (high efficiency particulate air) and ULPA (ultra low penetration air) designations describe a general class of filter. Usually, it is found that an ULPA filter is better than a HEPA filter, all else being equivalent. However, it is not of great concern whether or not a filter is of HEPA or ULPA classification. What is critical is the filtration efficiency of the filter media in question. There are even designations like VLSI, but the bottom line is: it doesn’t matter what you call it, it only matters how the filter performs.
What must be understood is that any filter media has a “worst case” efficiency rating at some particulate size. This means that to cite an efficiency other than worst case is merely marketing. Efficiency will be better at larger and smaller particulate sizes. Also, this worst case rating is based upon a certain rate of airflow. To exceed the airflow rate is to lessen efficiency.
Keep in mind that there are various methods to determine this rating. Some methods are based upon measuring total filter “leakage”, while other methods rate based upon finding and measuring “pinhole” leaks, or tiny flaws, at some point on the filter media. It could be entirely possible to find a pinhole leak that would in effect have a rating of, for example 97.5% at 0.3 micron, while the same filter as a whole could be rated at 99.97% at 0.1 micron.
What this means is that there is a lot of room for manipulating the data to obtain the desired result. What is truly needed by the industry is a “standard” test that all filter makers must use to rate filters. Until this is available, and until such tests are conducted by an outside party, such as a not-for-profit lab, there will continue to be confusion.
Another point is that different particulate sizes are governed by different “rules”. Typically, particles in the 0.2 to 0.3 micron range are the hardest to capture, for a number of reasons. Generally speaking, there are two distinct regions in which to categorize particles: those greater than 0.5 micron and those less than 0.5 micron in size
Particles greater than 0.5 micron tend to be trapped by Direct Interception or Inertial Impaction. Those less than .5 micron tend to be trapped by Diffusional Interception. These smaller particles are governed by what is known as Brownian Motion Theory. Simply stated, this means that a particle will go in whatever direction it wants following whatever path it wants and is not unduly affected by such trivialities as airflow rate and direction.
Statistical analysis will show that these smaller particles do “stop”, or are “trapped” when they run into something. At the very least, an obstruction may impart a change of direction, however the odds of a particle traveling through some media tend to be inversely proportional to the media density and/or thickness (the more things that exist to run into or be trapped by, the lower the odds of completely penetrating the media).
It should be noted that the filter material is also hydrophobic. That is to say that it is a barrier to water, airborne moisture, etc. This has two beneficial side effects. First, particles are not able to escape filtration by clinging to water droplets. Second, as the moisture is trapped, this effectively increases the filter efficiency.
In fact, it could be stated in general that as a filter “clogs up” it becomes able to trap particles more efficiently, simply because holes are plugged. This is true only to a point. An analogy would be a screen to let small rocks through but stop big rocks. Eventually the number of big rocks will prevent any small rocks from getting through, but it is possible that the weight of the big rocks will destroy the screen.
What must be understood is that there is much, which is claimed by many filter suppliers. Claims should be based upon real studies and real facts, and worst case data should be used. It means nothing to claim efficient removal at 0.01 or 0.001 micron, because of the laws of physics governing these particles and because “worst case” ratings do not take place at this particle size, and because it would be extremely difficult to prove these ratings — or that there are even any dangerous contaminants of this size generated by the surgical procedures themselves. Additionally, you may want to try and find equipment even capable of measuring particles this small.
The best bet is to monitor medical journals and keep abreast of the studies taking place, as well as to continue to use what works, is tried and true, and cost effective.