According to the $\text{CPCB}$, which particulate size (in diameter) is responsible for the greatest harm to human health?

The Central Pollution Control Board ($\text{CPCB}$) identifies particulate matter of size $2.5\mu \text{m}$ or less (${\text{PM}}_{2.5}$) as the most hazardous because these particles can bypass the respiratory defense system and penetrate deep into the lungs.