Allergens from house dust mites, Dermatophagoides farinae and Dermatophagoides pteronyssinus, can sensitize and trigger perennial allergic rhinitis, asthma, and atopic dermatitis. These two cosmopolitan mites are important sources of perennial indoor allergens in homes in humid climates of developed countries. They can be found in mattresses, carpets, curtains...practically everywhere in the house. In a home without dust mites, they can establish themselves in only a few months. They feed on human (and animal) skin flakes and on some fungi (Aspergillus). Their predators are other allergenic mites (Cheyletus), silver fish and pseudoscorpions.
Like every terrestrial animal, house dust mites lose water during egg production, waste excretion and through body secretions and transpiration. However in our houses, there is apparently no source of water for mites, no river, no rain, no dew. How do they obtain enough water to survive? Dust mites obtain water passively from humidity in the atmosphere by osmoregulation: when the haemolymph of mites is too concentrated and thus mites are dehydrated, the water molecules that lie on the cuticle pass through this cuticle and hydrate the mite. Moreover, mites are able to actively secrete a hygroscopic solution from the supracoxal glands that open just above the first pair of legs. The secretion flows through an open trough into their mouth, where it is ingested. The hygroscopic solution absorbs water as it flows from the gland opening to the mouth. Mites swallow the solution and therefore save water.
Mites prefer to live in our beds near their source of humidity. When we sleep, we breathe and sweat. During one night, we lose about 1 litre of water that is partially saved by mites. The sleeper in his bed plays an important role in dust mite ecology particularly by changing the hygrothermic conditions of the mattress. At bedtime, dust mites migrate towards us (but not on us, we are too hot) to absorb water from our breath and perspiration before taking refuge and aggregating in the depths of the mattress when we get up. As we supply moisture for 8 hours per day to the dust mites in our beds, high levels of dust mites are often associated with the mattress.
Whether we like it or not, they have been part of our environment for a long time: they have lived and will always live among us. However, they are not dangerous for 90% of the population. If you are not allergic to house-dust mites, forget them, these animals are totally harmless.
The excrement and exuviae of these animals, can, through dissemination in the air, reach our lungs and cause disease, such as asthma rhinitis or dermatitis, in hypersensitive people. It is impossible to totally eliminate all dust mites. But, it is possible to reduce their populations. In all the successful controlled trials of mite avoidance, controlling mites in the bed and bedroom is the primary measure. How?
Allergies and recommendations
Why do mites cause allergy?
House dust mites eat skin scales which continuously shed from our skin. They leave droppings everywhere they go. Their droppings contain the enzymes that they use to digest dead skin. These enzymes are an important component of dust, and are a major cause of asthma and other allergic diseases. When these enzymes are inhaled or touch the skin of hypersensitized people, their bodies produce antibodies. These antibodies cause the release of a chemical called histamine that leads to swelling and irritation of the upper respiratory passages – typical asthma and hay fever symptoms. The predisposition for allergy is often hereditary.
What are the symptoms of allergy to mites?
Nose and eye inflammation, perennial rhinitis.
Hay fever, runny nose, itching, sneezing.
Asthma, difficulty in breathing.
Infantile eczema (a skin disease that may get worse with age).
Are more people allergic to mites nowadays?
Yes, the number of allergies due to dust mites is increasing. One of the main causes is that nowadays, houses are warmer, more humid and less ventilated than before. These are conditions in which the dust mite species can thrive.
The average house temperature has increased with the rising incidence of central heating systems. Water vapour production has also increased with improving hygiene standards (more showers, more baths, internal clothes drying). The heating and insulation of our houses and the double-glazing of our windows have considerably reduced background ventilation.
Some houses contain huge numbers of dust mites and other houses contain almost none. This does not only depend on cleanliness, but also depends very much on the amount of moisture in the house; dry houses in very cold climates or on high mountains have few mites, but houses in temperate climates and normal altitudes have more.
Must we get rid of mites as a precaution?
No. It is not easy to get rid of mites. Be sure that you are allergic to mites before spending time and money controlling them! Demand evidence! Get the diagnosis made properly before you spend a lot of money and effort. Having a cold or being asthmatic is not always due to an allergy. Furthermore, allergies can be caused by something else other than mites (pollen, cockroaches, mice, mould…).
What are the recommendations for reducing mites and allergens in the home?
1) Reducing indoor humidity
Mites must obtain sufficient water from the air to survive and they die in dry environments. Therefore, maintaining humidity below 50% is one of the most common recommendations for reducing dust mites and their allergen levels in homes. In temperate climates, it is possible to maintain humidity under 50% by using high-efficiency dehumidifiers and air conditioning. In this environment, this intervention may be the only control measure needed. If the lower edge of the window is moist when you wake up in the morning, there is too much humidity in the air.
To compensate for dry air, the lungs increase mucus production. The additional, thick mucus in the airways can lead to asthma problems. If you are asthmatic, drinking plenty of water is the best and simplest way to thin the mucus.
2) Ventilate the rooms frequently and keep the bedroom temperature under 20 °C
At lower temperatures, mites are less active and breed more slowly. Keeping your house cooler will reduce mite populations, especially if ventilation keeps the humidity low. An inadequate air turnover in a house creates high humidity. Therefore, if you want to get rid of mites, dress more warmly and allow your house to be cooler.
Open the window to ventilate your rooms only when the humidity outdoors is under 50%! Use a hygrometer (sometimes called a humidity sensor or relative humidity indicator) or listen to weather forecasts.
3)Using protective covers
Encasing mattresses and pillows in specially manufactured protective coverings is effective in reducing exposure to house dust mites and their allergens. The principle is to keep mites and dust mites in the bedding away from the air that we breathe at night. Covers may be made from plastic, vapour-permeable materials, finely woven fabrics, or non-woven synthetics. It is essential to cover the mattress, pillows and duvet (comforter) and to regularly launder all uncovered items, preferably at 60 °C.
Good protective covers are expensive. To be effective and good value they must be:
1. Dustproof. To keep mites and dust mites away from the air we breathe at night, they must enclose the mattress, pillow(s) and duvet (comforter) completely.
2. Permeable to water vapour. Otherwise sweat will accumulate next to the skin.
3. Comfortable to sleep on. Soft enough and not noisy when you turn over in bed.
4. Tough. Cheap non-woven fabric may soon tear and prove poor value.
There is no national or international standard by which consumers can judge the quality of these covers, and some are of inadequate construction or quality.
4) Washing, drying, and dry cleaning of bedding materials
Washing sheets, pillow-cases, blankets, and mattress pads at least weekly in hot water (55 °C or higher) kills mites and removes most allergens. Washing in warm (less than 55 °C) or cold water does not kill most mites but probably removes most allergens because allergens are water-soluble. Dry cleaning of fabrics is effective in killing mites, but it does not destroy all allergens.
5) Replacing carpets, draperies, and upholstery
Carpets, draperies, and upholstery fabrics collect detritus and hold moisture, providing an ideal habitat for mite breeding. In humid climates, carpets are usually heavily infested with dust mites and should be removed, especially in the bedroom, in favour of hard surfaces (vinyl, linoleum, tiled or purpose-made wooden floor). Likewise, draperies and curtains can be replaced with blinds or shades. To further reduce infestations, fabric upholstery can be replaced with vinyl or leather coverings on cushions. Wooden furniture with no fabric is also recommended.
The above is the dominant view among experts, but there is another view: Dust on smooth floors blows up into the air more easily than dust from carpets. Some specialists argue that carpets are therefore better than smooth floors, as they trap dust. Dust in the carpet is harmless as long as it stays there.
Unfortunately, there is no published research to prove which approach works best for allergic people. With or without carpets, you should vacuum clean regularly.
6) Vacuuming carpets
Regular vacuum cleaning removes allergens. It must be done at least weekly and frequent replacement of the vacuum bags is recommended. Vacuum bags with a high-efficiency particulate air (HEPA) filter are recommended to prevent allergens from being aerosolized during the vacuuming process. Vacuum cleaning removes surface mites and allergens, but it does not remove deeply embedded allergens or reduce the number of living mites.
Vacuuming always causes some degree of transient increase in aeroallergen levels. Patients with severe dust mite sensitivity should wear a mask while vacuuming or leave the house while the vacuuming is done.
7) Freezing soft toys and small items
Freezing (–17 °C to –20 °C) soft toys and small items (pillows) for at least 24 hours is an effective method of killing mite populations on these objects. After freezing in a domestic freezer, these items can then be washed to remove the dead mites and allergens. When it freezes outside, leaving rugs, mattresses and pillows outside for at least 24 hours is also a recommended method for killing mites.
8) No tobacco
Exposure to tobacco smoke is also a risk factor for the development of asthma in infancy and childhood and is a major trigger for an exacerbation of asthma in children and adults.
NOT Recommended: Air cleaning/filtration
Mite allergens and dust particles may become airborne from disturbances, but they settle quickly. Therefore, air cleaning or filtration in undisturbed spaces probably captures few mite allergens and is not generally recommended.
This procedure is probably ineffective at removing dust mites or mite allergens because mites do not normally reside in heating, ventilating and air conditioning systems.
Using ozone generators
Studies are also needed to determine the efficiency and safety of ozone generators in killing mites or denaturing mite allergens. At present, there are no published studies to show that ozone can transform mite allergens.
All warm-blooded animals produce potential allergens in their urine, faeces, and saliva. Exposure to pet allergens, especially cat allergens, can lead to sensitization. However, complete pet avoidance in infancy is not currently recommended: Some studies suggest that exposure to pets early in childhood can even protect against the subsequent development of allergic disease.
The use of chemicals for controlling dust mites and their allergens is not recommended: Some studies show a reduction in allergen concentrations, and others show insufficient reduction. Extensive studies have shown that treatment with acaricides (chemical agents that kill dust mites, such as benzyl benzoate, disodium octaborate tetrahydrate, sumethrin, and permethrin) or tannic acid (a chemical agent that denatures dust mite allergens) are only minimally effective and must be repeated frequently. Therefore, chemical treatment of carpets is not recommended for most patients and should only be used as an adjunct measure when carpet removal is not an option.
The text was inspired by the article:
- “The biology of dust mites and the remediation of mite allergens in allergic disease” Larry G. Arlian, PhD, and Thomas A. E. Platts-Mills, MD, PhD Dayton, Ohio, and Charlottesville, Va