Which one of the following statements correctly defines PM1 particles in air?

Particulate matter (PM) is categorized by the aerodynamic diameter of the particles. PM10 refers to inhalable particles with diameters generally 10 micrometers and smaller, while PM2.5 refers to fine particles with diameters of 2.5 micrometers and smaller [1][2]. PM1 specifically defines extremely fine or ultrafine particles with an aerodynamic diameter of less than 1 micron (1 μm). While PM10 particles are often trapped in the upper respiratory tract or larger airways, PM1 particles are so small they can travel deep into the lungs to the pulmonary alveoli. Because of their minute size, these particles can penetrate the alveolar cell membranes, entering the bloodstream directly and spreading to various organs, posing significant health risks. Consequently, statement 3 is the only accurate definition of PM1 particles.

Sources

1. [1] https://www.epa.gov/pm-pollution/particulate-matter-pm-basics
2. [2] https://ww2.arb.ca.gov/resources/inhalable-particulate-matter-and-health