The air we breathe sustains our life, bringing oxygen and moisture, by screening the sun’s radiation, and by moderating the temperatures we experience as weather.

Its predominant constituents are Nitrogen, Oxygen, Water Vapor, Argon, and Carbon Dioxide. The water vapor content, the humidity, varies from hour to hour but even in our dry climate is a significant part of the total. Minor components of air include the ‘Noble Gases’ Neon (1.82 x 10^-3 %, Helium (5.24 x 10^-4 %), Krypton (1.14 x 10^-4 %), and Xenon (8.7 x 10^-6 %). Additional trace constituents make up less than 0.001% in total. The levels of their presence are usually expressed in parts per million or parts per billion, representing the proportion they occupy of the total volume of the air.

Among the trace constituents of the atmosphere are compounds that can be deleterious to health when their levels rise. Most of these pollutants, including sulfur dioxide (SO₂), nitrogen oxides (N₂O, NO, and NO₂), carbon monoxide (CO) and ozone (O₃), have both natural and industrial sources. The sources of these that are under human control are those we can regulate and manage in order to maintain the healthfulness of the air that we breathe.

When the levels are excessive, these substances can cause health problems. Some individuals, such as those suffering from asthma are more sensitive than average.

Strict regulations have been placed on the emissions from the major sources and also on the cumulative effect of all sources in the region on the quality of the ambient air. These regulations have been devised to protect the health of sensitive as well as average individuals, and to protect the general welfare.

These regulated pollutants include:

• Sulfur Dioxide, a colorless gas with a sharp, penetrating odor that can be an eye and lung irritant.

• Carbon Monoxide, a colorless, odorless gas that interferes with oxygen utilization by the body.
  
  
• Nitrogen Oxides, include colorless and brown-colored gases that can be eye and lung irritants.
  
  
• Ozone, a colorless gas that can be an eye and lung irritant, yet also screens out ultraviolet light.
  
  
• Particulate Matter, dust, that can darken the sky and irritate the nose and lungs.
  
  

In Yellowstone County, the industrial sources of air pollutants are managed by their corporate owners to comply with regulations imposed by several authorities, including the Yellowstone County Air Pollution Control, the State of Montana Department of Environmental Quality, and the US Environmental Protection Agency.

Several of these industries have organized as the Billings – Laurel Air Quality Technical Committee (BLAQTC) to conduct monitoring of the air pollutant levels in Yellowstone County by operating three monitoring stations, which augment four operated for the Montana Department of Environmental Quality (DEQ) by the Yellowstone County Air Pollution Control, and two stations operated by Rosebud Operating Services, Inc