There are around 10,000 known varieties of mite. Mites are arachnids, and are a part of every intact ecosystem.

House dust mites cannot be seen with the bare eye: they are only 0,1-0,5 mm long. They prefer a warm, damp climate; they cannot drink, and so need a moist environment to survive (perfect for dust mites: 20 degrees Celsius, 80% humidity).

They also prefer dark, quiet spaces, especially in the bedroom, and feel right at home in blankets, matresses and pillows. They can also be found in stuffed animals, upholstered furniture, carpets and curtains. There is no such thing as a mite-free house. Mites live on skin flakes from human and animals, molds, and feathers. The average person sheds about 1,5 grams of skin each day – enough to feed 1,5 million dust mites.

The mites themselves are not allergens; but their excrement contains allergens. The severity of the allergic reaction depends on the concentration of dust mites and their excrement. A quantity of 100 milben per gram of dust is enough to raise the risk of an allergic reaction. A concentration of more than 500 mites per gram of dust can lead to allergic asthma. One gram of dust can contain up to 10,000 dust mites.

Other common mites include those found in food: cheese mites and flour mites.

Cockroaches can also cause allergies through allergens in their bodies and saliva. Cockroaches are often found in food-preparation facilities, and in air shafts.  They feed on food remains and live in dark, enclosed spaces. Even with regular extermination efforts, it is difficult to be fully rid of them. Even small amounts of cockroach allergens can cause an allergic reaction; this risk is often underestimated.

The common book louse can also cause allergies. The book louse lives on paper, but can also live on molds, food products, plants and animal by-products. There are 2,000 known types of book louse; 93 of these are found in central Europe.