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Encasing Pore Size

Pore Size & Encasing Efficiency

In 1999 a team of researchers at the University of Virginia published a study entitled, "Evaluation of materials used for allergy bedding: Effect of pore size in blocking cat and dust mite allergen." The stated purpose of this study was "to develop a method for testing encasement materials made of breathable fabrics." A new testing protocol was indeed developed which is now used to determine two things: 1) the ability of woven fabrics to block cat and dust mite allergen and 2) the breath-ability of a fabric as measured in liters of air per minute that can be drawn through it. You might be wondering why cat allergen was used in the study, after all, who has a cat in their pillow? Cat allergen is a potentially very small allergen, so fabrics that can block all or most of it are the most effective allergy mattress covers.

"Although mite fecal particles are 10 to 40 µm [microns] in diameter, a significant proportion of cat allergen is associated with particles less than 10 µm in diameter. Because of the size of particles carrying Fel d 1 and the quantity of this allergen in dust samples, cat allergen was used to provide a more rigorous test of the fabric's ability to block passage of allergens."

Another result from this study that many have focused on had to do with something called mean pore size. Pore size is a reference to the openings where woven fibers intersect in a piece of material. Here's what was published:

"Dust mite allergens (Der f 1 and Der p 1) were blocked below detectable limits by fabrics of less than 10 µm in pore size. Fabrics with an average pore size of 6 µm or less blocked cat allergen (Fel d 1)."

"For woven fabrics, the key factor in blocking allergens is pore size. Our results show that a woven fabric with an average pore size of 6 µm or less will block common indoor allergens below detectable limits."

"Our current judgement is that fabrics of 2 µm to less than 10 µm in pore size will effectively block passage of all dust mite allergen and would be suitable for use on pillow cases and mattresses."



Mean pore size is best determined by using a capillary flow test. PMI (Porous Materials, Inc.) has performed capillary flow tests for many textile manufacturers and is a leading authority in this field of study. You can learn more about how those tests are performed by visiting PMI's web site. The U.Va. study did not develop a test method or protocol for determining mean pore size. Often the reporting of pore size is done either by the manufacturer of the fabric (like we do) or by the retailer selling the finished goods. In addition to the results reported by the manufacturers of fabrics used in our encasings, we have employed PMI to test samples we sent them. The results of that testing are posted here.

It's important to remember that there is no requirement to use a company like PMI when reporting mean pore size. Anyone with a microscope and a lens with a built-in micrometer (used to measure things in terms of microns) can give an opinion concerning the mean pore size of a woven fabric by simple observation. And this leaves open a number of questions. For example, how many samples are needed to come up with a "mean"? When using a simple observation technique, the answer to that question is left up to the one doing the reporting. Additionally, the pore size of fabric will not remain completely uniform simply due to the fact that it is woven material. That means some pores are larger than others. So that leaves open the possibility of taking samples until the desired result is reached - either a high number or a low number. Consequently, it becomes important to find out how a reported mean pore size was determined and to request a copy of the independent test results.

In the past, National Allergy did not rely exclusively on the reporting of pore size to determine product efficacy but rather chose to rely only on tests that IBT Reference Lab performed to establish how good a barrier an encasing fabric would be. The primary test employed the exact same protocol developed in the U.Va. study discussed above whereby an attempt was made to vacuum cat and dust mite allergen through the fabric. Random samples of fabric were sent to IBT for these tests, and we relied on the results to establish the barrier efficiency of the fabrics we used. IBT no longer tests fabrics for companies like ours, so we rely on the PMI pore analysis to determine fabric efficacy. As you can see, we really try to do our homework before making encasings for you!

There is a notable push in the marketplace by companies and individuals who want doctors to insist on the much more subjective measurement of pore size as the main determinant of performance. We're glad to publish that information for all of our covers to which it applies (waterproof encasing like BedCare Classic™ or encasings made with a non-woven fabric like BedCare Economy™ don't have "pores"), and you will find it in the product description for all of our woven fabric styles. We report what the fabric manufacturers tell us, but please be aware that these are still very subjective numbers because they are averages and anyone else taking the readings could come up with a different average.

* Data taken from Vaughn JW, McLaughlin TE, Perzanowski MS, Platts-Mills TAE. Evaluation of materials used for bedding encasement: effect of pore size in blocking cat and dust mite allergen. J Allergy Clin Immunol 1999; 103:227-31.

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