Which particle characteristic is identified as the greatest threat to the respiratory system?

• A. Particles larger than 100 microns
• B. 0.5-1 micron
• C. Single fiber particles 3-5 microns in diameter, and a length less than 0.1 MM
• D. 20-50 microns

Answer: (C)

The most concerning particle traits are those that are small enough to reach deep into the lungs and have a shape that makes them hard to clear. Particles that are large (like hundreds of microns) get filtered out by the nose and upper airways, and mid-sized particles without a fibrous shape don’t persist as long in the lungs. A slender, fiber-like particle with a small diameter allows it to penetrate deep into the alveoli, and its high aspect ratio makes it difficult for immune cells to engulf and remove it—so it can stay in the lung tissue and cause chronic irritation and damage. A single fiber about 3–5 microns in diameter and shorter than about 100 microns fits this high-risk profile, making it the greatest threat among the options.