Scanning electron micrograph of a dust mite, Dermatophagoides sp. The picture shows a mite in dust from a domestic vacuum cleaner bag. It is covered with small dust particles, with a few larger flakes visible - human skin flakes. Species of dust mites are found worldwide; they feed on skin flakes shed by animals and humans, but may go unnoticed inside houses due to their very small size (here, about 0.4mm in length). Their faeces cause an allergic response in suspectible humans, manifesting as eczema, asthma or allergic rhinitis. Mites are found on soft furnishings, carpets and mattresses, especially where humidity is high. Their faeces can be detected in 50-80% of homes, but the number of mites is often small; a vacuum cleaner bag full of dust might hold none, or up to 100. The easiest and most effective deterrent to their presence in the home is to reduce humidity to below 50%.