Dust Mites - Click to play Video
House dust is a mixture of various waste materials like dead human skin cells, animal dander, fabric fibres, dust mites, bacteria, cockroach parts, mould spores, food particles and other debris.
In fact, dead skin cells account for up to 80% of house dust. However, it is the the house dust mite allergen that is the most common contributor to allergic asthma, allergic rhinitis and atopic eczema.
So generally, when doctors say that patients are allergic to house dust, they usually refer to the house dust mite. Of course, patients could also be allergic to other elements of house dust, such as cockroach parts, mould, pet dander and so on.
Dust mites are eight-legged insects that belong to the spider and tick family (i.e. Arachnid family). At up to 0.3 mm length, they are barely visible without a microscope. They belong to the genus Dermatophagoides, which means “skin eater”.
It is so named because it feeds on animal materials with high protein content, especially dead human skin cells. It also feeds on mould. The two common species throughout the world that are also allergenic are the Dermatophagoides pteronyssinus (Der p), and Dermatophagoides farinae (Der f). In the context of tropical environments like Singapore, there is a third species called Blomia Tropicalis.
The female dust mite lays up to 50 eggs within 3 weeks, and each egg takes about three to four weeks to reach adulthood. The lifespan of a dust mite is about 10 weeks, and each female would have laid between 40 to 80 eggs within a period of about 6 weeks. So, it is easy to see that they multiply at a very fast rate indeed, especially if the conditions are right (see below).
The common misconception is that the dust mite itself is the allergen that causes the allergic reaction. Actually, there are many allergens, which are the many different enzymes produced by the dust mites to digest their food. Typically these are excreted in the faecal dropping of the dust mites. Some suggest that they are also deposited in the surroundings when they eat, to go airborne when these fibres and skin flakes become loose.
A dust mite can produce 20 faecal droppings per day, 200 times their body weight in their short lifetime. Some estimate that 10% of a two-year-old pillow’s weight can be made up of dust mites and their droppings.
A gramme of dust can contain as many as 1,000 dust mites. This same gramme can contain 250,000 faecal pellets that are much smaller and lighter than dust mites. When these faecal pellets decay and break down into smaller particles smaller than 2 microns, they can be easily inhaled deep into the lungs.
Research indicates that most people toss and turn up to 70 times during one night’s sleep. Every time the mattress is moved, the tiny particles containing allergens are expelled into the air, and are easily inhaled.
Dust mites thrive best where the relative humidity is above 70% RH, and temperatures above 23oC. As such, tropical climates like Singapore would have a higher dust mite population than temperate climates in Europe or North America. Conversely, dust mites are rare in cold climates, or during winters.
Within the home, most of the dust mite colonies can be found in the bed mattress, pillows and bolsters. This is because these areas have high concentrations of:
- food – we shed a lot of skin cells throughout the day,
- humidity – we perspire when we sleep, and there is vapour in our breath,
- warmth – our body heat.
Other areas which are particularly susceptible to dust mites are the carpet or rug, soft toys, upholstery and anything else that can trap dust – e.g. curtains and clothes.
Dust mites can be found in all homes, despite best efforts to keep them clean.