In the context of air pollution, PM 2.5 refers to

PM 2.5 refers to particulate matter of the size 2.5 micrometres or less in diameter. According to Central Pollution Control Board (CPCB), PM 2.5 is responsible for causing the greatest harm to human health. These fine particulates can be inhaled deep into the lungs and can cause breathing and respiratory symptoms, irritation, inflammations and damage to the lungs and premature deaths.