## **Core Concept**
The question pertains to the size of respirable dust, which is crucial in occupational and environmental health. Respirable dust refers to particles that are small enough to be inhaled deep into the lungs. The size of these particles is typically measured in micrometers (μm).

## **Why the Correct Answer is Right**
The correct answer, **particles less than 5 μm**, is based on the definition of respirable dust. Particles in this size range can bypass the upper respiratory tract's defense mechanisms and reach the alveoli, where gas exchange occurs. This size range is critical for occupational exposure limits, as it directly correlates with the potential for lung damage and disease.

## **Why Each Wrong Option is Incorrect**
- **Option A:** **Particles greater than 10 μm** - These particles are generally too large to be considered respirable and are usually trapped in the upper respiratory tract.
- **Option B:** **Particles between 5-10 μm** - While some of these particles can be respirable, they are less likely to reach the deeper parts of the lung compared to smaller particles.
- **Option D:** **Particles less than 1 μm** - Although very small particles can be respirable, they may also be less likely to be retained in the lung due to their small size and potential for being exhaled back out.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **respirable fraction** of dust is defined by particles that are **less than 5 μm** in diameter. This is crucial for understanding occupational lung diseases such as silicosis, coal workers' pneumoconiosis, and asbestosis, where exposure to respirable dust is a significant risk factor.

## **Correct Answer:** . **particles less than 5 μm**