1.   But entrenched positions make less sense when it seems impossible to plant your feet at all and when one of the enemies is an invisible particle measured in microns.

2.   It can range in size from invisible microscopic particles to grains of sand.

3.   Metal dust and insulation fibers are typical examples of invisible particles.

4.   Perhaps most of the universe is composed of invisible exotic particles --WIMPs, for weakly interacting massive particles -- that supply much of the gravity shaping galactic structures.

5.   The Egis machine sniffs vapors of invisible particles.

6.   These invisible particles are powerful biological allergens that can trigger reactions, including hypersensitivity pneumonitis and allergic rhinitis.

7.   He used the level of invisible inhalable particles floating in the air to measure outdoor air pollution.