The particles dealt with in this book refer to particles in the air medium, with particle sizes ranging from $10^{-7} sim 10^{-1}$ cm. As the particle size changes, its physical properties and behavior will all change accordingly. Particles in the air medium are generally called atmospheric dust, and are solid or liquid particles with sizes smaller than $100mu m$.

Suspended particles (Airborne particles)

Particles in the air medium; solid or liquid particles with sizes smaller than $10mu m$, also called total suspended particulates.

Ultrafine particles (Ultrafine particles)

Particles in the air medium; solid or liquid particles with sizes smaller than $0.1mu m$.

Viable particles (Viable particle)

Isolated, naturally occurring or accumulated microorganisms that are capable of reproducing and producing observable growth; also called living particles.

[Reference] Classification by the formation mode of particles.

(1) Dispersed particles: formed when solids or liquids become suspended under the action of splitting, fragmentation, air currents, vibration, etc.; among them, solid dispersed particles are particles of completely irregular shape, or fine particles that have become spherical again through the combination of loosely aggregated, loosely coalesced particles.

(2) Coagulated particles: formed through combustion, sublimation and vapor condensation, as well as gas reactions; among them, solid coagulated particles are generally loose aggregates formed by a large number of primary particles with regular crystal shapes or spherical shapes, while liquid coagulated particles are much smaller than liquid dispersed particles.

[Reference] Classification by the origin of particles.

(1) Inorganic particles: for example, metal dust particles, mineral dust particles, and building-material dust particles.

(2) Organic particles: for example, plant fibers; animal hair, fur, keratin, and skin flakes; chemical dyes and plastics.

(3) Living particles: for example, unicellular algae, fungi, protozoa, cells, and viruses.

[Reference] Classification by the visibility of particles.

(1) Visible particles: visible to the naked eye, with particle diameters greater than $10mu m$.

(2) Microscopic particles: visible under an ordinary microscope, with particle diameters of $0.25 sim 10mu m$.

(3) Ultramicroscopic particles: visible under an ultramicroscope or an electron microscope, with particle diameters smaller than $0.25mu m$.

[Reference] The effect of particles on human health is related to their properties, the amount inhaled, and their particle size. Particles with aerodynamic diameters greater than $10mu m$ are essentially trapped in the human nasal cavity; about 90% of particles greater than $2mu m$ and smaller than $10mu m$ can enter and deposit In the various parts of the respiratory tract, 10% can reach the deep regions of the lungs and deposit in the lungs; particles smaller than $2mu m$ can be 100% inhaled into the lungs, among which particles of $0.3sim 2mu m$ almost entirely deposit in the lungs and cannot be exhaled.

【Reference】 An aerosol refers to an air medium containing suspended particles of the dispersed phase, and is a dispersion system. Viewed from the perspective of aerosol mechanics, its size can range from $10^{-7}sim 10^{-1}$cm; but it usually refers to $10^{-7}sim 10^{-2}$cm.

【Reference】 A bioaerosol refers to biological media dispersed in the air environment (living particles, allergens, toxins, or microorganisms)

【Reference】 ISO definition: An aerosol refers to a suspension in a gas medium of solid particles, liquid particles, or solid and liquid particles whose settling velocities are negligible.