Fine Particulate Pollution

Particle pollution (also called particulate matter or PM) is the term for a mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, and 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.

Particle pollution includes "inhalable course particles," with diameters larger than 2.5 micrometers and smaller than 10 micrometers, and "fine particles," with diameters of 2.5 micrometers and smaller.

How small is 2.5 micrometers? Think about a single strand of hair from your head. The average human hair is about 70 micrometers in diameter, making it 30 times larger than the largest fine particle.

These particles come in many sizes and shapes and can be made up of hundreds of different chemicals.

Some, known as "primary particles," are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks, or fires.

Others form in complicated reactions in the atmosphere from such chemicals as sulfur dioxides and nitrogen oxides emitted from power plants, industries, and automobiles. These particles, known as "secondary particles," make up most of the fine particle pollution in the country.

A study conducted by the University of Montana during the winter of 2007/2008 found that wood smoke was the major source of fine particulate pollution in the Helena Valley during the winter. It contributed 66 percent of the particulate pollution measured (see chart below).

For this reason, the Lewis and Clark County Outdoor Air Quality Regulations focus primarily on wood stoves.

Sources of Fine Particulate Pollution