PM2.5 refers to atmospheric particulate matter (PM) that has a diameter of less than 2.5 micrometers, which is about 0.03 the diameter of a human hair. Fine particulars can come from various sources, such as power plants, motor vehicles, residential wood/leaves burning, forest fire, volcanic eruptions, agricultural burning, and dust storms.

Particulars in this category are so small that they can only be detected with an electron microscope. Since they are so small and light, fine particles tend to stay longer in the air than heavier particles. This increases the chances of humans inhaling them into the bodies. Particles smaller than 2.5 micrometers are able to bypass the nose and throat and penetrate deep into the lungs and some may even enter the circulatory system.

Studies have found a close link between exposure to fine particles and premature death from heart and lung disease. Fine particles are also known to trigger or worsen chronic disease such as asthma, heart attack, bronchitis and other respiratory problems.

On a very clear and non-hazy day, the PM2.5 concentration can be as low as 5 μg/m3 or below. The 24-hour concentration of PM2.5 is considered unhealthy when it rises above 35 μg/m3.