According to Central Pollution Control Board, which particulate matter size in diameter (in micrometers) are responsible for causing the greatest harm to human health?

According to the Central Pollution Control Board (CPCB), particulate size $2.5$ micrometers or less in diameter (PM $2.5$) are responsible for causing the greatest harm to human health.

These fine particulates can be breathed deep into the lungs and can cause breathing and respiratory symptoms, irritation, inflammations, and damage to the lungs and premature deaths.

Answer: 2.5 or less