House Dust Mites are very small insects measuring about 0.4 mm and in a practical way, they are invisible to human eyes.
They feed on shaded human skin particles and concentrate on mattresses, bedding and clothing of humans. House dust mite allergy is very common.
The cause of allergy to these mites are particles of their bodies and certain proteins called proteases which are shed in their feces. All these allergenic particles are found in mattresses, bedding, in our clothing and in all our environment because due to their small size they "fly" through the air. Mites and their particles fly through the air and become suspended in the air, then precipitate and concentrate in the dust in our environment. That's why dust should be avoided, because the dust concentrates airborne substances that precipitated and can be easily resuspended in air. House dust contains a significant amount of dust mites and particles of their bodies and their feces. Dust is not the real cause of allergy by itself. Therefore dust of nature (for example dust of an open field) as opposed to house dust (dust from house, warehouses, offices) is exempt from dust mites and is not a cause of allergy. People with house dust mite allergy do not need to avoid exposure to natural dust or sand.