According to Central Pollution Central Board particulate size 2.5 `mu m` or less in diameter are responsible for causing greatest harm to human health.

Answer

### Step-by-Step Solution:

1. **Understanding Particulate Matter**: Particulate matter (PM) refers to tiny particles or droplets in the air that can be inhaled into the lungs. These particles can vary in size and composition.

2. **Focus on Size**: The question specifically mentions particulate matter that is 2.5 micrometers (µm) or less in diameter. This size range is crucial because it can penetrate deep into the respiratory system.

3. **Health Impacts**: Particles that are 2.5 µm or smaller are known as PM2.5. These particles can cause significant health issues, including respiratory diseases, cardiovascular problems, and other serious health effects.

4. **Sources of PM2.5**: Common sources of PM2.5 include vehicle emissions, industrial discharges, burning of fossil fuels, and natural sources like wildfires and volcanic eruptions.

5. **Regulatory Standards**: The Central Pollution Control Board (CPCB) in India monitors air quality and sets standards for permissible levels of particulate matter in the air to protect public health.

6. **Conclusion**: Therefore, according to the Central Pollution Control Board, particulate matter with a diameter of 2.5 µm or less poses the greatest risk to human health due to its ability to penetrate the lungs and enter the bloodstream.