A house dust mite is a tiny eight-legged animal that lives in the dust of people's homes. An adult is about 0.3 millimeters long, and adults range from about 0.2 to 0.4 millimeters, barely big enough to see with the naked eye. Mites and ticks are arachnids, the eight-legged group of animals that also includes spiders.

Size

The two species most common in homes in temperate parts of the United States are the American house dust mite, Dermatophagoides farinae, and the European house dust mite, Dermatophagoides pteronyssinus. A 2013 guide for allergy doctors describes adults as oval, with creamy to see-through white bodies that measure 0.2–0.4 millimeters.

A University of Florida guide gives a larger figure: about 0.42 millimeters long for both females and males. Females are wider, about 0.32 millimeters across the body, while males are about 0.25 millimeters. The size shown here, 0.3 millimeters, is in the middle of the range from the first guide.

Size comparisons

A dust mite is about three times as long as a human hair is wide.
About three dust mites end to end would span 1 millimeter, the smallest mark on a metric ruler.
A dust mite is about one and a half times as long as a paramecium of a large species, which is about 0.2 millimeters long.
A mite's droppings are 20–40 micrometers across, so a mite is about 8–15 times as long as one of them. A micrometer is a thousandth of a millimeter.
If a dust mite were as long as a person 1.7 meters (5 feet 7 inches) is tall, its droppings would be 11–23 centimeters (4–9 inches) across. The largest would be about the size of a basketball.

How dust mites live

Dust mites feed on flakes of shed human and pet skin, and on the fungi, yeasts and bacteria that grow on them. They have no eyes but avoid light, so they live deep inside pillows, mattresses and carpets. A single gram of house dust can hold thousands of mites.

A mite's body is about 75 percent water by weight, and mites do not drink. Instead, they absorb water vapor from the air, which works only when the relative humidity is at least about 65 percent. Relative humidity compares the water vapor in the air with the most the air could hold. Mites do best at 75–80 percent humidity and 25–30 degrees Celsius (77–86 degrees Fahrenheit).

A mite hatches from its egg as a six-legged larva, then passes through two eight-legged stages before becoming an adult. Adults live about 4–6 weeks. A female lays one or two eggs a day, 50–80 in her lifetime.

Dust mites and allergies

Allergy specialists have recognized dust mites as the major source of allergens in house dust since 1967. An allergen is a substance that causes an allergic reaction. Most mite allergens are in the droppings, which carry digestive enzymes from the mite's gut. Each mite produces about 20 droppings a day.

When someone makes a bed or walks on a carpet, droppings rise into the air and can be breathed in before they settle again 20–30 minutes later. They can trigger asthma, a runny or blocked nose, and an itchy skin rash called atopic dermatitis. Allergy doctors advise keeping the relative humidity at home at 35–50 percent to slow the growth of mite populations.

Sources

Environmental assessment and exposure control of dust mites: a practice parameter. Portnoy et al., Annals of Allergy, Asthma and Immunology, 2013.
House Dust Mites, Dermatophagoides spp.. Denmark and Cromroy, University of Florida IFAS Extension, 1998.
Just How Small Is "Nano"?. National Nanotechnology Coordination Office.
Integrative Neuroscience of Paramecium, a "Swimming Neuron". Brette, eNeuro, 2021.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.

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