House dust mite

The house dust mite (sometimes abbreviated by allergists to HDM), is a cosmopolitan guest in human habitation. Dust mites feed on organic detritus such as flakes of shed human skin and flourish in the stable environment of dwellings. In nature they are killed by micro-predators and by exposure to direct sun rays. Dust mites are the most common cause of asthma and allergic symptoms worldwide|May 2008. The enzymes they produce can be smelled most strongly in full vacuum cleaner bags. The European house dust mite (Dermatophagoides pteronyssinus) and the American house dust mite (Dermatophagoides farinae) are two different species, but are not necessarily confined to Europe or North America.

Size
It is possible to see a dust mite under a magnifying glass, when the subject is well lit and placed on a black background. A typical house dust mite measures 420 µm in length (almost 0.5 mm) and 250 to 320 µm in width. Both male and female adult house dust mites are globular in shape, creamy white and have a striated cuticle. A member of the class Arachnida, larval and post-larval stages of house dust mites have eight legs. Dust mites can be transported airborne by the minor air currents generated by normal household activities.

Life cycle
The average life cycle for a male dust mite is 20 to 30 days, while a mated female dust mite can live for 10 weeks, laying 60 to 100 eggs in the last 5 weeks of her life. In a 10 week life span, a dust mite will produce approximately 2000 fecal particles and an even larger number of partially digested enzyme-infested dust particles. 

A simple washing will remove most of the waste matter. Temperatures of over 60 degrees Celsius (140 degrees Fahrenheit) for a period of one hour, and freezing is usually fatal to dust mites; a relative humidity less than 50 may also be fatal. Dust mites reproduce quickly enough that their effect on human health can be significant.

Habitat and food
The dust mite survives in all climates, except at high altitudes where reproduction is halted. If trying to control dust mites and other indoor pathogens for which water is a gating factor, it is relative humidity that is important. This is because water condenses out of air onto a surface only when the air at that surface contains more water than it can hold at that temperature. When warm, moist air contacts a cool surface, the air touching that surface may cool and give up some of its moisture to condense on the surface. When humidity is less than optimal, dust mites function more slowly, eventually become dormant and may die. Dust mites thrive in the environment provided by beds, kitchens and homes in general, where the sun's rays do not reach them. Mites remain in mattresses, carpets, furniture and bedding, since they can climb lower down through the fabric to avoid sun, vacuum cleaners, and other hazards, and climb higher up to the surface if necessary to get another skin cell to feed on, when humidity is high. Even in dry climates, dust mites survive and reproduce easily in bedding (especially in pillows) because of the humidity generated by the human body during several hours of breathing and perspiring.

Dust mites consume minute particles of organic matter. Some species of mites prefer to eat skin cells, a large component of household dust; others prefer flour dust. Dust mites have a rudimentary alimentary system (no stomach) and require most digestion to occur outside their body. For this reason they secrete enzymes and deposit the fungus Aspergillus repens on dust particles, to enable the fungus to pre-digest the organic matter with its enzymes. Dust mites eat the same particle several times, only partially digesting it each time. Between feedings dust mites leave particles to decompose further. Ultimately a fully digested particle, which a dust mite will not eat, is deemed by scientists to constitute fecal matter. On average, a person sheds about 1.5 grams of skin cells and flakes every day (approximately 0.3-0.45 kg per year), which is enough to feed roughly a million dust mites under ideal conditions.. Dust mites in bedding derive moisture from human breathing, perspiration, and saliva.

Asthma and allergies
Dust mites are one of the most common allergens that trigger asthma.

Some main signs of dust mite bites are itchiness, sneezing, inflamed/infected eczema, watering eyes, runny nose, (if asthma), lungs clogging up and hay fever.