PM2.5 refers to atmospheric particulate matter (PM) that have a diameter of less than 2.5 micrometers, which is about 3% the diameter of a human hair.
Commonly written as PM2.5, particles in this category are so small that they can only be detected with an electron microscope. They are even smaller than their counterparts PM10, which are particles that are 10 micrometres or less, and are also called fine particles.
Fine particles can come from various sources. They include power plants, motor vehicles, airplanes, residential wood burning, forest fires, agricultural burning, volcanic eruptions and dust storms.
Some are emitted directly into the air, while others are formed when gases and particles interact with one another in the atmosphere.
For instance, gaseous sulfur dioxide emitted from power plants reacts with oxygen and water droplets in the air to form sulfuric acid as a secondary particle.
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