Airborne particles are very fine particles made up of either solid or liquid matter that can stay suspended in the air and spread with the wind. Common examples of such particles are: fog, which is made up of tiny water droplets; dust, which is made up of very fine particles of solid matter; and smoke, which is made up of both solid matter and liquid. Airborne particle size varies greatly, and they are often measured in microns, meaning it is so small that it cannot be seen with the naked eye. Sources of airborne particles can be natural, such as the dust and smoke created by volcanic eruptions and forest fires, or man-made, such as the soot from the burning of coal in a power plant or the residual oil particles in vehicle exhaust fumes. Scientific studies show that these particle pollution can cause health problems in humans and affect the Earth's climate.
A common measurement used for an airborne particle is micron, also called a micrometer. One micron is one-millionth of a meter (39.37 inches). Airborne dust particles are often at least 1 micron in diameter. The particles that make up fumes can be as small as 0.1 micrometers, while the water droplets in mist can vary from 2-50 micrometers in size. Very small airborne particles can stay suspended in the air for years and spread over great distances, while large particles usually settle on the ground after a short time.
Naturally formed airborne particles make up about 90% of the particles suspended in Earth's atmosphere, and this includes ocean salt from sea spray and dust made up of mineral particles from the Earth's crust. Man-made particles can come from traffic, factory emissions, the burning of fossil fuel like oil, and many other sources. These man-made particles vary in composition. Some examples are carbon particles in diesel exhaust, metallic particles from smelters, and sulfur dioxide released from the burning of coal. At high concentrations, airborne sulfur dioxide particles can contribute to cooling the Earth's climate.
Airborne particle counters can be used to measure the particle content of indoor and outdoor air. Such instruments commonly detect particles with a diameter of 0.2-25 microns. Scientists believe that airborne particles with a diameter less than 2.5 microns can be especially harmful to humans. The small size of the particles means they can penetrate deep into lung tissue or even the blood stream, causing serious lung and heart disease.