灰尘是大气中一种固态悬浮物，常态存在于空气之中，易伴随风的吹拂而四散至各处，包括家中的每一个角落。其粒径大小有所差异，不一定能以肉眼见到。

灰尘颗粒的直径通常小于500微米。小于10微米的悬浮粒子（PM₁₀），即被认定有害于人体；小于2.5微米的细颗粒物（PM_2.5），更可穿透肺泡直达血液。

Dust consists of particles in the atmosphere that come from various sources such as soil, dust lifted by weather (an aeolian process), volcanic eruptions, and pollution. Dust in homes, offices, and other human environments contains small amounts of plant pollen, human and animal hairs, textile fibers, paper fibers, minerals from outdoor soil, human skin cells, burnt meteorite particles, and many other materials which may be found in the local environment.