PM stands for particulate matter, the term for a mixture of particles found in the air. These particles come from a variety of sources, both natural and human-caused: industrial activities, construction sites, roads, vehicles and traffic, residential and industrial chimneys, wildfires, etc. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using a microscope.
These particles come in many sizes and shapes and can be made up of hundreds of different materials. PM10 (generally 10 micrometers and smaller) and PM2.5 (generally 2.5 micrometers and smaller) are two common size fractions used to assess air quality.