What is particulate matter and how does it get into the air?

Particulate matter is a mixture of solid particles and liquid droplets found in the air. It is one of the six criteria pollutants. Some particles, like 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 an electron microscope. Sources such as construction sites, unpaved roads, fields, smokestacks, or fires directly emit particulate matter. Other particles form because of complex chemical reactions in the atmosphere after pollutants are emitted from power plants, industries, and automobiles. EPA does not regulate particles of sand and large dust, greater than 10 micrometers. They do, however, regulate inhalable particles that have a diameter less than that (generally 10 micrometers or less).