Division Links

Visibility and Air Quality

Causes of Haze in the Valleys

Very small particles cause haze because they are about the same size as the wavelength of light in the visible spectrum and can either scatter or absorb light. Generally, scattered light causes a bright haze, while absorbed light causes a dark look to the sky. Furthermore, the chemical makeup of the particles can give a color to the haze.

Particles in the Air

Some particles occur naturally and some are manmade. Natural particles include salt particles from the Great Salt Lake. Natural sources also include emissions from biological processes that create small particles known as sulfates and nitrates. Fog and water vapor can add to the haze problem by enhancing particle formation and particle size.

Another major source of haze is manmade pollution. Pollution from combustion is the greatest source of particulate. Combustion pollution comes from vehicles, wood burning, and industry. Another source of manmade particles is very fine, nearly invisible dust pulled into the air from roads. Along the Wasatch Front, the biggest source of all fine particles is vehicles.

Harmful Particulate Pollution

The national air quality health standards that apply to particles are based on the mass or weight of particles in a cubic meter (approximately a cubic yard) of air. The standards are 150 micrograms (about 5 millionths of an ounce) of particles that are 10 microns or less in diameter (PM10). These particles are about a tenth of the diameter of a human hair. There is also a standard for smaller particles that are 2.5 microns or smaller (PM2.5); it is 65 micrograms per cubic meter.

The Department of Environmental Quality, Division of Air Quality issues health advisories whenever particulate pollution approaches these health standards.

Hazy Air

Is the air unhealthy when it gets hazy? It depends. Along the Wasatch Front, the most common situation is for particles of just the right size to cause haze to accumulate first. When this happens, the visibility deteriorates more quickly than the air quality. As a result, the air looks bad with a gray or brown haze, but the amounts of PM10 and PM2.5 do not exceed the health standard. It is not uncommon for air quality to comply with health standards, even though the valleys are very hazy.

Some people, especially those with respiratory problems, feel the effects of pollution at levels that do not violate health standards. Also, the severity of haze and pollution can sometimes be similar. It is always best to know the specific health condition of ourselves and family members and respond to health advisories and increased haze accordingly.

Getting Rid of the Haze

Haze per se is not a health issue, but it does affect our quality of life. The Clean Air Act requires the U.S. Environmental Protection Agency and states to take action to eliminate manmade haze from impacting visibility in most national parks and monuments. Utah has been a leader in this effort for over a decade.

During the last several years, the Western Regional Air Partnership (WRAP), consisting of western states and tribes along with stakeholders from business and environmental interests, has forged a plan to reduce man-made haze. The locations of greatest concern for our state are places known for their scenic vistas: Grand Canyon National Park, Arches National Park, Zion National Park, Bryce Canyon National Park, Cedar Breaks National Monument, and Canyonlands National Park.

The principal focus of WRAP’s effort so far has been on reducing emissions from coal-fired utilities, vehicles, and manmade fires. These anticipated reductions, as well as steps to meet more stringent health-based air quality standards, should also reduce haze in urban areas.

More Information

Division of Air Quality
Phone: (801) 536-4000

Last Updated:


Back to top