A particle smaller than about 2.5 micrometers and larger than about 0.1 micrometers is called:

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Particles smaller than 2.5 micrometers are generally referred to as fine particles or PM2.5. These particles are smaller than the eye can see and can be harmful to human health as they can penetrate deep into the lungs. Nanoparticles are generally smaller than 0.1 micrometer, and ultrafine particles refer to particles smaller than 0.1 micrometers. Ionic particles are charged atoms or molecules that are not necessarily related to particle size.

A Particle Smaller Than About 2