PM refers to Particulate Matter, and PM2.5 is one of the terms for fine particulate matter. In February 2013, the China National Committee for Terminology in Science and Technology officially named PM2.5 as fine particulate matter.

PM2.5 refers to particulate matter in the ambient air with an aerodynamic equivalent diameter less than or equal to 2.5 micrometers.

There are two main sources of PM2.5 pollution: anthropogenic (man-made) and natural.

Anthropogenic sources include stationary and mobile sources. Stationary sources encompass various fuel combustion sources, such as power generation, metallurgy, petroleum, chemicals, textile printing and dyeing, and other industrial processes, as well as soot emitted from coal, gas, or oil during heating and cooking. Mobile sources are mainly exhaust gases emitted into the atmosphere by various vehicles using fuel during operation.

PM2.5 can be formed from the transformation of sulfur and nitrogen oxides. These gaseous pollutants are often caused by human combustion of fossil fuels (coal, petroleum, etc.) and waste. In developing countries, coal combustion is the primary method for home heating and energy supply. Diesel vehicles without advanced exhaust treatment devices are also a source of particulate matter. Diesel-burning trucks produce more particulate matter due to impurities in their emissions.

Indoors, secondhand smoke is the most significant source of particulate matter. Particulate matter originates from incomplete combustion; therefore, any tobacco product that relies on burning will produce highly harmful particulate matter. Using higher-quality cigarettes is merely self-consolation for smokers and may even cause greater harm due to lower odor levels. The same applies to burning joss paper, incense, and mosquito coils. However, the claim that PM2.5 increases 20-fold after 5 minutes of stir-frying is a misconception.

Natural sources include soil dust (containing oxide minerals and other components), sea salt (the second largest source of particulate matter, with a composition similar to seawater), plant pollen, spores, bacteria, etc. Natural disasters, such as volcanic eruptions, emit large amounts of volcanic ash into the atmosphere. Forest fires, exposed coal field fires, and dust storms also transport vast amounts of fine particulate matter into the atmosphere.