Particle Sizes

Overview

Engineering reference data for Particle Sizes in miscellaneous.

Key Formulas

Unit Conversion

$y = x \cdot k$

Multiply by conversion factor.

Linear Interpolation

$y = y_1 + \frac{(x - x_1)(y_2 - y_1)}{x_2 - x_1}$

Estimate between two known points.

Percentage

$p = \frac{\text{part}}{\text{whole}} \times 100\%$

Part as fraction of whole.

Variables

Symbol	Description	Unit
$x$	Input value	—
$y$	Output value	—
$k$	Conversion factor	—

Particle Size Reference Table

A comprehensive reference of common particles and their typical size ranges in microns (μm).

Unit Conversions

Fundamental conversions for particle size measurements.

$$1 \ \mu m = 1 \times 10^{-6} \ m = 0.001 \ mm$$ $$1 \ \mu m = 1000 \ nm$$ $$1 \ inch = 25,400 \ \mu m$$ $$1 \ mm = 1000 \ \mu m$$

Airborne Particle Classification

Particles suspended in air are commonly classified by their size and behavior.

• Large Particles (> 100 μm): Terminal velocities > 0.5 m/s, settle out quickly (e.g., hail, snow, coarse sand, gravel).
• Medium Particles (1 - 100 μm): Sedimentation velocities > 0.2 m/s, settle out slowly (e.g., pollen, fly ash, fine sand).
• Small Particles (< 1 μm): Remain suspended for days to years in quiet air; may never settle in turbulent conditions. Includes viruses, smoke, and metallurgical fumes.

Hazardous Dust Fractions

Regulatory and health-based classifications for respirable particulates.

• Inhalable Dust: Particles capable of entering the nose and mouth during normal breathing. Size ≤ 100 μm.
• Thoracic Dust (PM10): Particles that pass the nose and throat to reach the lungs. Size ≤ 10 μm.
• Respirable Dust (PM2.5): Particles that penetrate deep into the gas exchange region of the lungs. Size < 5 μm (often specified as ≤ 2.5 μm in the US).

The U.S. EPA specifies a total allowable concentration for particles < 10 μm (from building materials, combustion, fibers) of 50 μg/m³ (0.000022 grain/ft³).

References

• Original Source