House Dust Mites (HDM) are a major source of allergen and found in temperate and humid areas. There is widely published data that shows how House Dust Mites avoidance can reduce exposure to allergens. This article looks at the properties of house dust mite and recommendations for reducing HDM in the home.
Biology of the house dust mite
House Dust Mites are arachnids not insects and related to ticks and spiders such as daddy long legs. Thirteen species of HDM have been found in dust however the two which are the most common and are the main source of the HDM allergen are Dermatophagoides farinae and Dermatophagoides pteronyssinus from the family Pyroglyphidae. The life cycle of the HDM Dermatophagoides pteronyssinus consists of 5 stages. An adult mated female will lay 40 – 80 eggs in her lifetime, when the egg hatches a six legged larva emerges. There are two nymphal stages (which feed and moult before an eight legged adult is developed. The adult HDM has a mouth-like appendage and no eyes or antennae. The duration of the cycle is usually one month and dependent on the climate, ideally 25°C and 75% RH. An adult HDM can live for one to three months under favourable conditions.
Their translucent body is 300µm – 400µm in length and only visible under a microscope with the faecal pellets ranging from 20µm – 50µm, it is estimated that the HDM can produce 20 pellets per day. HDM are ~75% water by weight and therefore need to absorb water from the water vapour in the air making relative humidity (RH) a critical factor for survival.
Dust Mite Habitat
HDM primarily feed on organic detritus such as flakes of shed skin, other nutrients are provided by animal dander, pollen, bacteria and mould. HDM survive and reproduce the best in soft furnishings such as carpets with long pile, bedding and plush toys, which contains a large supply of their food source, this stable environmental dwelling is best provided by homes. Indoor humidity is very important and when it is less than 50% they are unable to maintain their water balance and are more susceptible to desiccation.
The HDM selects food that has been pre decomposed by fungi which reduces the fat content of the skin cells. The fungi in turn are using the house dust mite faeces and skin cells as a source of nitrogen (Woodcock et al. 2006) which forms a minute ecosystem in their environment.
Dust Mite Allergies
Allergy is a response of the human immune system to a foreign protein substance (allergen). A HDM allergy is the result of a hypersensitive immune system response to the mite faces. Faecal pellets are easily airborne and when they become suspended in the air they can be inhaled, the allergic reaction will cause sneezing, wheezing, headaches, coughing, eye irritation, fatigue or dizziness and are confirmed as a major trigger for acute asthma attacks in sensitive individuals. Roughly 1-2 % of the world population (65-130 million people are allergic to house dust mites (Collof, 1999)
Dermatophagoides farinae (commonly found in drier regions) and Dermatophagoides pteronyssinus (commonly found in temperate to tropical regions), produce at least ninteen different allergens each. These allergens are further classified depending on their biochemical characteristic by group numbers:
Group 1 allergens (Der p 1, Der f 1), glycoproteins originating from the digestive tract of the mites.
Group 2 allergens (Der p 2, Der f 2), non-glycosylated proteins with a high IgE binding capacity and associated with the mite bodies.
Group 3 allergens are associated with digestive enzymes, Group 4 allergens are sugar associated and so on.
Around 80% of mite allergic patients are sensitive to Group 1 and Group 2 allergens from HDM (Thomas, WR et al. 2004). Examples of other allergens include pollen, pet dander and mould. Allergy is the 5th leading chronic disease in the U.S. among all ages, and the 3rd most common chronic disease among children under 18 years old as reported in “Chronic Conditions: A Challenge for the 21st Centrury,” National Academy on an Aging Society, 2000
How to control allergen levels in the home
There is no cure for allergies to dust mites. The only way to avoid allergy symptoms when it comes to dust mites is to prevent exposure. The use of a multifactor approach to allergen control has been recommended by, the Report on the Third International Workshop on Indoor Allergens and Asthma (1997) as well as the National Heart, Lung and Blood Institute’s (NHLBI) National Asthma Education and Prevention Program (2007).
Maintaining RH below 50% by using dehumidifiers in homes is effective in reducing RH and therefore reducing mite populations. Regular vacuuming and steam cleaning carpets can kill mites and remove surface allergens. Encasing mattresses and pillows with products that have verified allergen barrier properties are effective in reducing exposure to HDM and their allergens.
Many products claim to reduce allergens without independent evaluation of their effectiveness making it difficult for the consumer to make the right choice; however Allergy Standards Limited (ASL) an international standards and certification body helps consumers and retailers identify toys, bedding, vacuum cleaners, dehumidifiers etc. for people concerned about allergies, asthma and indoor air quality. ASL’s independent certification standards have been written in consultation with industry, retailer and health care professionals and are the only standards that are accepted by the Asthma and Allergy Foundation of America. Reputable retailers should ensure that all allergen claims have been conducted in a meaningful way to avoid falling foul of labelling guidelines. Concerned consumers should also seek that all labelling where allergies are concerned have been validated by a third party independent testing authority.
ASL believes that a home allergen reduction plan which incorporates a number of changes is critical for reducing exposure to allergens.
The independent testing and research for the Asthma and Allergy Foundation of America certification program is conducted by the life sciences company airmid healthgroup ltd (AHG). The airmid healthgroup ltd research spans all disciplines relevant to indoor particulate contaminants, ranging from virology, microbiology, mycology, allergen, dust mite and bed bug research. The airmid healthgroup ltd laboratory offers a complete range of advanced Allergen Testing Services specialising in Allergen Barrier Services, Dust Mite related Testing, Claim verification services and Allergen detection.