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Dust Mites

By Michael Hilton

House dust mites were first identified as a major source of allergen in house dust in 1964. The major source of allergen from dust mites is fecal matter and the remains of dead mites. After inhalation, as much as 15% of the population show sensitivity to dust mite allergen. House dust mites are found in almost every environment capable of supporting plant and animal life. There are hundreds of species of mites that exist throughout North America, but two principle species are found in the United States, dermatophagoides farinae (der f 1 and dermatophagoides pteronyssimus (der p 1).

House dust mites receive the majority of their nutritional needs (food and moisture) from dead human skin scales which are shed daily. Because of environmental and feeding requirements, dust mites prefer humid conditions (above 55% humidity) with an ambient room temperature of 72°-79°. This explains their high concentration in pillows and mattresses. When humidity levels are maintained below 50%, most adult mites will die within 7-10 days.

In homes, the highest concentration of dust mites can be found in mattresses and upholstered furniture. A typical profile of dust mite allergen location might be 10,000 nanograms per gram (ng/g) – mattress, 8,000 ng/g - upholstered chair, 2,000 ng/g – carpet, 1,000 ng/g – draperies. As many as 2,000,000 live mites may inhabit the average mattress. With the average pillow, 30% of its weight is comprised of dead human skins scales and dust mite allergen. Can you imagine how much heavier your mattress has become?

Much of the house dust mite research over the past 30 years has concentrated on mite avoidance and, to a lesser extent, eradication of live mites. Very little research has examined allergen removal and few cause and effect studies have been undertaken to identify risk exposures from specific identified allergen sources such as mattresses, pillows, upholstered furniture and carpet. Yet, allergists routinely make recommendations of allergen avoidance by removing these sources even though a direct link of allergen exposure from these sources has never been established.

Effectiveness of Allergen Removal

Several studies have taken place recently examining the effectiveness allergen removal through routine carpet cleaning and quantifying airborne dust mite allergen content above carpeted surfaces. Each of these studies has demonstrated the effectiveness of carpet cleaning in removing dust mite allergen and carpets effectiveness in trapping allergen and preventing its release into the breathing zone.

In a two-year, south Florida school study, examining the effectiveness of routine maintenance in lowering dust mite allergen levels, dust mite allergen levels were reduced by an average of 92% with each routine carpet cleaning. An additional 12 month south Florida school study reinforced these findings. In the two-year carpet cleaning study, allergen levels were tracked throughout the study period with a regular carpet care program in effect. Allergen levels, which were considered quite high at the beginning of the study continued to decline throughout the study period and gradually increased only after regular carpet care had been discontinued.

While mite allergen can be found in most environments on most surfaces, a never been established. It is known that the average size of live mites is 100 to 600 microns and the average size of dust mite allergen averages 6 to 10 microns. What is not known is how particles of this size can become airborne or enter the breathing zone to produce an allergic reaction.

Several evaluations have taken place recently to evaluate the amount of dust mite allergen airborne above a carpeted surface. In these evaluations, airborne samples were taken at 4 inches, 24 inches and 42 inches above a carpeted floor at various locations within an occupied classroom. The investigation revealed no allergen released from the carpet due to the heavy weight of the allergen and the unique properties of carpet in trapping these particles.

A related study was performed to evaluate the airborne release of dust mite allergen and to evaluate equipment used to detect allergen. In an extreme test, a heavily loaded classroom carpet was evaluated for allergen release during normal playtime classroom activities, while vacuuming with a standard vacuum bag, and without a vacuum bag to introduce dust particles directly into the air (This activity was accomplished by removing the vacuum bag and disconnecting the vacuum dust containment system).

Results revealed no airborne allergen was detected at any height during typical classroom activities and during regular vacuuming procedures, but allergen was detected during vacuuming without the dust containment system in place. Vacuuming without any filtration system in place, while considered an unacceptable maintenance practice, allowed the researchers to gauge the effectiveness of the sampling equipment. It also should be noted that the majority of the airborne dust mite allergen captured during the vacuuming (without a dust containment system attached) was captured at the four-inch (from floor) measurement with very little allergen captured at 24-inches and 42-inches (a child's sitting and standing height). This was surprising considering the considerable "push" you might expect in using the Kirby G-4 vacuum cleaner that was utilized for the test.

Based on this "extreme" test, you should not expect dust mite allergen to become airborne from within the carpet pile during normal activities. Carpet surface sampling following the extreme test (using scotch tape to collect surface allergen) revealed excessive amounts of allergen lying on the carpet surface after vacuuming without a dust containment system in place. Subsequent sampling with the vacuum containment system in place revealed all of the allergen lying on the surface was removed by a follow-up vacuuming. This indicates the effectiveness of vacuuming in removing both embedded dust mite allergen and surface allergen.

Did you know

Dust mites were identified as a primary allergen in 1964. At that time, carpet was rarely used in homes. Allergists routinely recommended that carpet be installed to reduce airborne dust mite allergen. Current data suggests this may be true.

About the Author
Michael Hilton was the original creator of Carpet Buyers Handbook. Having owned and operated a carpet wholesale company, Hilton has a vast knowledge about all-things carpet related as well as other types of flooring.

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