Particulates, alternately referred to as Particulate Matter (PM), aerosols or fine particles are tiny particles of solid or liquid suspended in the air. They range in size from 10 nanometers to 100 micrometers in diameter.

There are both natural and human sources of particulates. The biggest natural sources are wind-blown dust, volcanoes, and forest fires. Sea spray is also a large source of particles though most of these fall back to the ocean close to where they were emitted. The biggest human sources of particles are combustion sources, mainly the burning of fossil fuel in vehicles and power plants, and wind blown dust from construction sites and other land areas where the water or vegetation has been removed. Some of these particles are emitted directly to the atmosphere (primary emissions) and some are emitted as gases and form particles in the atmosphere (secondary emissions).

The composition of fine particles depends on the source. Wind-blown dust tends to be made of mineral salts and other crustal earth material. Primary emissions from combustion sources are made primarily of unburned fuel (hydrocarbons), elemental carbon, elemental sulfur, mineral salts, and often contain traces of toxic metals. Secondary emissions are a combination of ammonia with either sulfuric acid or nitric acid and water.

In general, the smaller and lighter a particulate is, the longer it will stay in the air. Larger particles (greater than 10 micrometers in diameter) tend to settle to the ground by gravity in a matter of hours whereas the smallest particles (less than 1 micrometer) can stay in the atmosphere for weeks and are mostly removed by precipitation.

The size of the particle also determines where in the body the particle may come to rest if inhaled. Larger particles are generally filtered by small hairs in the nose and throat and do not cause problems, but particulate matter smaller than about 10 micrometers, referred to as PM10, can settle in the lung and cause health problems. Particles smaller than 2.5 micrometers, PM2.5, can penetrate into the deep lung and tend to be the most hazardous when inhaled.

The health effects of inhaling particulate matter has been widely studied in humans and animals and include asthma, lung cancer, and premature death. Particulate matter pollution is estimated to cause thousands of deaths per year in the United States. For this reason, the Environmental Protection Agency (EPA) sets standards for PM10 and PM2.5 concentrations in urban air. EPA regulates primary particulate emissions and of precursors to secondary emissions (NOx, sulfur, and ammonia). Many urban areas in the United States still frequently violate the EPA particulate standards, though urban air has gotten cleaner, on average, with respect to particulates over the last quarter of the 20th century.

From 2005, the European Union has set limits for PM10 in the air:

¹ indicative value.