Much has been published on exposure to dust mite allergen. It is known that as much as 15% of the U.S. population is sensitive to this primary allergen. However, most physicians stop short of blaming any allergen for a specific allergic episode because of the inability to isolate and identify the specific allergen causing the reaction. Because of this inability to identify a specific allergen for an allergic episode, most allergists practice an allergen avoidance theory of preventive medicine. The thought behind this theory is that if all allergens can be eliminated from an environment, which is impossible, the incidence of allergic episodes will diminish.
With regard to dust mites, many IAQ consultants who evaluate and categorize air quality complaints in facilities typically evaluate settled dust and make recommendations based on their findings. The evaluation of settled dust provides a good indication of the current levels of dust mite allergen in carpet dust, but it fails to establish an exposure risk or identify a route of exposure as a result of allergen contained in the dust. Current recommendations for dust mite allergen levels in carpet dust have been based on a database of prior sampling, without regard to problem and non-problem environments. In other words, dust samples have been collected, analyzed, and risk categories were created based on averages of the findings. No risk assessment study has ever been undertaken for these suggested levels. Based on these averages of collected samples, the following guidelines have been established for mite allergen content in settled dust.
IAQ consultants regularly use these levels in their analysis of indoor environments, but as mentioned, these averages of past sampling provide no real insight into whether actual health effects can be anticipated based on these levels. Good IAQ consultants are difficult to find, and even good consultants may give conflicting advice. Most IAQ consultants are brought into complaint situations, so much of the data that has been generated has been developed in environments where problems have existed. Until very recently, very little comparison data has been available to compare mite allergen levels in on-complaintenvironments. Few consultants have had the financial resources to collect comparison samples from non-complaint environments and few facilities are interested in knowing what mite allergen levels exist in non-problem environments. As a result, the classification of mite allergen into low, moderate, high, and severe categories has not taken into consideration the level at which mite allergen content begins to present an air quality concern. Consider the following mite allergen data collected from problem and non-problem environments.
These data would suggest that a classification system for mite allergen levels in carpet dust provides no real benefit in forecasting possible health effects using this categorization method.
The presence of allergen on/within a surface does not in itself identify a particular interior surface as a source of health complaints. To be considered a contributor to allergic disease, a route of exposure must be established and this has been a primary obstacle in verifying at which level dust mite allergen can become a problem. To be considered an irritant, mite allergen must be inhaled/ingested to produce an allergic reaction. It is known that mite allergen is extremely large in microscopic terms. In addition, since the allergen is primarily fecal matter, the allergen holds on to carpet fibers with much tenacity. The size of the allergen and the propensity to remain anchored to the attached surface have created problems for researchers who have tried to identify airborne dust mite allergen. Without positive proof that it can become airborne, a route of exposure has been difficult to establish a correlation.
Researchers have traditionally used anecdotal explanations for the inability to isolate airborne dust mite allergen. This stance has propagated what amounts to a paranoia defense, such asRJust because we can? find them doesn? mean their not there Researchers contend that because of it weight, the allergen only remains aloft for a limited time period. It is surprising that this paranoia defense has any place with regard to good science but IAQ consultants are typically paid to find problems and not paid on the basis on their ability to resolve complaints.
Recent investigations evaluating the propensity of dust mite allergen to become airborne do not support the paranoia defense. In a south central Florida Elementary School Study, a non-complaint classroom was evaluated for allergen content in carpet dust and an airborne investigation was initiated by the author of this site, to evaluate the efficacy of air sampling equipment to quantify airborne mite allergen. Dawson air pumps with .3 micron cowl asbestos filter media were used to collect samples. Samples were collected for 90 minutes at three sample sites at three heights during three sampling periods. Samples were collected at 4 inches from the floor, 24 inches from the floor, and 42 inches from the floor to simulate a child lying on the floor, seated at a classroom desk, and a child approximate standing height.
Carpet dust was evaluated for mite allergen and found to be in the severe risk for sensitization range (115,000 ng/gram of carpet dust), according to regularly accepted guidelines using prior sampling history. Children (27) were encouraged to remain active in the classroom for 90 minutes. Numerous activity games, exercises, running, jumping and other vigorous activities were performed while air sampling was performed.
After school, a separate sampling period was engaged using a vacuum cleaner in which the dust containment system was altered to allow all of the dust to be aerosolized. Vacuuming and sampling was repeated for 90 minutes at three sample sites at three heights.
Thirty minutes after the second (vacuuming) sampling activity concluded, the classroom was once again vacuumed with the dust containment system in place and sampling was performed during the 90-minute activity. Keep in mind, any allergen that had been entrapped within the carpet pile prior to the initial vacuuming was now lying on the carpet surface as a result of deposition from the air.
The results revealed no airborne allergen at any height was captured during the children activity, despite ?evere allergen levels found in the carpet pile (see graph) and vigorous activity. The second sampling with the dust containment system disengaged revealed that dust mite allergen, if airborne, could be captured. Another interesting finding was that most of the allergen was found at the 4 inch level followed by 24 inches and finally 42 inches. This suggests that even with a rigorous push from a vacuum cleaner with no dust containment system, the allergen may be too heavy to enter the respirable breathing zone on it own. Finally, no airborne allergen was detected at any height, while the vacuum cleaner was operated with its bag and containment system in-place, indicating that exposure to mite allergen during vacuuming may be overstated.
HVAC Air Filters As a follow-up, HVAC air filters from problem environments were evaluated for dust mite allergen content. The hypothesis was that if dust mite allergen were to be come airborne, they would surely be trapped by the air conditioning filter system. Unfortunately (or fortunately) no dust mite allergen was found in the air filters. Prior to this study the "paranoia" explanation would have prevailed. What does this say about the effectiveness of those HEPA air filtration dohickeys they sell on TV for 3 payments of $70?
Eliminating the concern from exposure to airborne mite allergen is a dramatic revelation. However, the concern for direct exposure is not eliminated. Very little research is available on the amount of allergen lying on the surface of interior surfaces and the risk of hand to mouth or hand to nose ingestion. Studies evaluating the reduction of these levels through daily vacuuming and regular carpet cleaning have proven successful in dramatically lowering the amount of allergen in carpet dust through extraction carpet cleaning. Surface levels can be reduced with a daily vacuuming program though no data is currently available for public use from the vacuuming studies. Preliminary results indicate vacuuming is extremely effective in removing allergen lying on the carpet surface. "One interesting observation made by Nishioka et al. (1999) was an estimate that only about 1% of bulk carpet dust was on the carpet surface and available for dermal contact. If this estimate was accurate and if it also reflects the fraction of dust available for potential resuspension, then one would not anticipate significant contributions of carpet to airborne particles. Although the point was not addressed in the Nishioka article, the amount of material available via dermal contact on a properly maintained carpet may actually be lower than on a smooth surface". Also consider that only 1% of total carpet dust mass can be found on the surface of which mite allergen comprises only a very small percentage of the total dust mass.
This further questions the procedure for vacuuming carpet dust and evaluating risk potential based on the amount of allergen contained in carpet dust.
From a carpet cleaning standpoint, it has long been known of the effectiveness of hot water extraction carpet cleaning in removing mite allergen through water transport (water injection and extraction). Numerous studies evaluating carpet cleaning methods and mite allergen removal have been presented for peer review and each study indicates the success in reducing allergen levels. Some of the results of these studies are reported in the graph at the left.
The following should be considered in evaluating mite allergen impact:
Dust mite allergen in carpet can be removed through steam cleaning. In numerous studies, 99% of dust mite allergen in carpet can be removed with carpet cleaning. Carpet traps dust mite allergen and does not allow the allergen to become airborne where it can be inhaled.
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.