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Dust Mite Allergen: An Indoor Problem | Cladosporium species

Dust Mite Allergen: An Indoor Problem

Dr. Kamash Ramanathan

Dust mites are microscopic creatures that are not visible to the naked eye. Female dust mites are generally between 200 to 500µm in size, with males being smaller. They prefer warm, moist surroundings close to sources of food. Since one of their food sources is human skin scales, they are most prevalent in high-use areas such as living rooms, bedrooms, and areas with upholstered furnishings where shed skin scales can collect and serve as food. It has been estimated that the skin one human sheds each day (~1 gm) could feed several thousand mites for up to three months. Pillows and mattresses are also primary locations for dust mites. These are also key areas for exposure since in these locations a person's face is in very close proximity to the allergens, which are present in the dust mite feces and dried body fragments.

Dust mites belong to the kingdom of animals; phylum, Arthropoda; class, Arachnida; and group, Astigmata; with three genera (Dermatophagoides, Euroglyphus, and Blomia) important for humans indoors. The mites most commonly found in house-dust in homes worldwide are D. farinae, D. pteronyssinus, E. maynei, and B. tropicalis. In the Unites States, all of these dust mites may be found indoors, but D. farinae and D. pteronyssinus are found most frequently and are most widely distributed geographically.

Both food and water are critical for mite survival. Since adequate food (e.g., human skin flakes) is generally available, it is the relative humidity of a place that determines mite prevalence in a location. In humid regions, usually all homes and many other buildings contain breeding populations of dust mites. In dry climates, fewer homes contain dust mites and they are usually at low levels. However, by raising indoor humidity through the usage of evaporative coolers may alter the indoor environment that may lead to conditions that facilitate mite survival. Ambient relative humidity may influence the rate at which feeding mites produce allergens and its accumulation in dust. It has been shown that by lowering the relative humidity in a place may significantly drop the production of fecal allergen even though mites may continue to survive. However, lowering indoor humidity can reduce mite population density overtime because mites gradually dehydrate and die below 50% relative humidity.

Health Effects
Hypersensitivity diseases caused by allergens from mites that live indoors constitute a major health problem in the U.S. and elsewhere. House dust mites are primarily a concern in human dwellings, but dust mites and mite allergen have also been identified in office buildings in association with and without health complaints. Dust mite allergens are considered to be the major biological agent to have sufficient evidence for the casual relationship of their exposure to the development of asthma in susceptible children. They not only aggravate the problem in susceptible individuals but also may cause susceptible children to develop asthma. House dust mites are also known to play a major role in triggering asthmatic attacks in susceptible individuals. House dust mite allergen is the inhaled substance that actually triggers an attack by causing an allergic reaction. Dust mite allergens are proteins, which come from the digestive tract of mites and are found at high levels in mite feces. A dust mite fecal pellet, containing partially digested food and digestive enzymes, is ~10 to 35µm in diameter and contain allergens (protein) called Der p 1, Der f 1 and mite group 2. These allergens (proteins), when inhaled, attach to sensitized cells in the air passages causing hay fever and asthma, and aggravate atopic dermatitis in people who are susceptible to this problem. Approximately 85% of asthmatics are allergic to dust mite allergens.

Volumetric sampling of the air has repeatedly shown that concentrations of mite allergens are generally very low to undetectable. The International Association of Allergology and Clinical Immunology (IAACI) has recommended sampling settled dust to evaluate exposure to dust mite allergens. The ACGIH has stated that it is important to sample multiple indoor locations to obtain a reasonable determination of mite prevalence. Dust samples for dust mite allergens can be collected using a dust cassette or by using a specially designed dust trap attached to a vacuum cleaner. A filter or a 9 square inch bed linen can otherwise be placed between the hose and the attachment of vacuum cleaner to collect the dust. Keep samples cool (place in refrigerator until shipped to the lab) and ship the samples to the laboratory for allergen testing using a cold pack or in a cooler. By not cooling the samples, the dust mites present in the sample may grow in numbers and produce more fecal pellets than actually present at the time of sampling and so may produce inaccurate test results.

Sample Analysis
Detection and measurement of dust-mite antigen (allergen) is performed using an immunoassay. An Enzyme Linked Immuno-Sorbant Assay (ELISA) is used to quantify mite allergen concentration in dust samples. Using monoclonal antibodies specific for dust mite allergens Der p1, Der f1 and mite group 2, the level of dust mite allergens present in the sample are analyzed. The level of dust mite allergen present in the sample is reported in micrograms per gram (µg/g) of dust.

Data Interpretation
The IAACI proposed dust-mite allergen limits for residential dwellings considers exposure to dust containing 2µg/g of mite allergen to increase the risk of dust-mite sensitization as well as the development of asthma and bronchial hyperreactivity in affected persons. Exposure to dust containing 10µg/g of mite allergen represents a higher risk level and an increased chance of acute asthma attacks.

Prevention and Control
A few suggestions on how to minimize mite reproduction and allergen release following confirmation that dust mites are present indoors are as follows:

  1. Where mites may breed, maintain relative humidity below 50%.
  2. Vacuum regularly and discard vacuum bags immediately.
  3. Wash bedding every week in hot water (54°C).
  4. Replace carpet with hard flooring if possible.
  5. Use allergen free mattress and pillow covers to minimize contact with the dust mites.
  6. Keep upholstered furniture to a minimum.
  7. Use HEPA air filters to keep mite particles out of air. (Note that since mite particles settle rapidly due to their relatively large size, this may be of limited value.)
  8. Minimize the use of curtains or drapes by using shades.

Bioaerosols: Assessment and Control, Janet Macher, Sc.D., M.P.H., Editor. 1999. ACGIH, 1330 Kemper Meadow Drive, Cincinnati, OH 45240-1634.
Clinical Reviews in Allergy and Immunology, Martin D. Chapman, Editor. June 2000, Volume 18; Number 3.


Fungi of the Month: Cladosporium species

Dr. Payam Fallah and Dr. Kamash Ramanathan

Cladosporium is considered by many to be one of the most prevalent fungal genera in the world. Its Latin name is derived from Greek, which means spores in branched chains. The genus contains about 60 or so species. Almost all species are either plant pathogenic or saprophytic on plant debris or man-made plant product such as paper based material. Only one known species so far, C. carrionii, has been known to be pathogenic in humans, causing skin lesions.

Spores of Cladosporium can easily be airborne and travel long distances. The fungus is capable of growth in a wide range of temperatures (18-28 C° and as low as -6 C°). It is one of the first fungi to be found in indoor environments. Under ideal environmental conditions the fungus can grow and multiply on variety of surfaces including metals. This is especially true when condensation occurs on metal surfaces in electrical outlets. Once spores settle on a wet surface (if still viable) they can germinate and form a network of hyphal fragments that keeps on growing, and if conditions are suitable it will sporulate.

Generally, there are three species that are commonly encountered outdoors, C. herbarum, C. cladosporiodes, and C. sphaerospermum. While all three species can be allergenic, C. herbarum is most prevalent outdoors and considered, by many, to be the number one cause of asthma and hay fever (type I allergy) in the western hemisphere. The other two species are mostly prevalent in indoor environments. Type III hypersensitivity pneumonitis can also occur by Cladosporium species. Cladosporium species can be found on a variety of substrata such as textiles, wood, moist windowsills as well as moist wallboard papers just to name a few.

MoldRANGE™ of Cladosporium by Month

The MoldRANGE™ chart of Cladosporium by month show the levels of Cladosporium spores detected in the outside air during the different months of the year in the United States. The numbers on the left of the chart (0.0 -1.0) correspond to the grey bar on the chart and indicate the frequency of occurrence of the Cladosporium spores at different months of the year in the United States. It can be observed (looking at the bars) that Cladosporium spores seem to occur more than 95% (0.95) of the time during all the months of the year indicating their common presence in the outside air at all times of the year.

The purple, green and red lines on the chart correspondingly represent the 5, 50, and 95 percentile levels of spores/m3 value for the Cladosporium spores when it is detected during the different months of the year in the United States (correspond to the numbers (spores/m3) on the right of the chart). It can be observed that whenever detected, 50% of the time the Cladosporium spore were present in levels above the detection limit and approximately below 500 spores/m3 during all the months of the year. Based on the chart it can also be inferred that the 95 percentile value (purple line) fluctuates from low levels of ~ 400 spores/m3 to high levels of ~ 6900 spores/m3 during the different months of the year, indicating that the levels of Cladosporium spores in the outside air are at the lowest levels during the winter time (December to April) and are at the highest levels during the Summer time (June to November) through out the country.

So whenever high levels of Cladosporium spores are observed indoors they must be carefully evaluated with the levels present in the outside air (due to their common occurrence outside in high levels) before making a judgement of their indoor source.

Cladosporium MoldRANGE Chart


This article was originally published on April 2004.