Regional Haze

Regional Haze

On most days, in many parts of the country, any time of the year, how far you can see is affected by air pollution that can obscure views of mountain ranges and scenic vistas. Here in Montana, we have some of the oldest and most treasured national parks and wilderness areas in the nation. However, a wonderful experience in Glacier Park can be affected by hazy skies. Haze, caused by emissions of air pollution, can have a serious impact on one of our most valuable assets – our big skies. As the Big Sky State, Montana’s scenery is valued not only by Montanans but by the millions of tourists who visit every year, supporting our economy.


The Federal Clean Air Act established a program of visibility protection in 156 national parks and wilderness areas across the United States. These are referred to as "Mandatory Class I Federal Areas." Montana is home to 12 Mandatory Class I Federal Areas, including Glacier National Park, the Bob Marshall Wilderness Area, and many others. These areas are managed by three different federal agencies, the U.S. Forest Service, Fish and Wildlife Service, and National Park Service.

The goal of understanding regional haze is not just to be able to measure it, but also to decrease haze and improve visibility. To do so, the U.S. Environmental Protection Agency (EPA) published a set of regulations in the late-1990s that was intended to reduce the amount of visibility-impairing particles in the air over time. This set of regulations is called the Regional Haze Rule. The rule calls for state and federal agencies to work together to improve visibility in Mandatory Class I Federal Areas.

The regulations set a goal of reducing haze in each Class I Area to a level that is considered natural, meaning no particles from man-made sources are contributing to haze.

In order to track changes in visibility, EPA had to set a starting point, or baseline, which was set as the level of visibility in the years 2000-2004, or before the regulations took effect. The regulations require gradual improvements in visibility over time such that each Class I Area will achieve its calculated natural visibility goal by the year 2064.

These projected improvements form a straight line, what is often referred to as a “glide path,” between the 2000-2004 baseline period and the 2064 natural conditions. Due to the unique conditions in different parts of the country, each Class I Area has a different baseline and a different estimation of natural visibility conditions.

Actions that Reduce Haze

In order to improve visibility, we have to control emissions of the pollutants that contribute to haze. As described above, the particles that cause haze come from a wide variety of places. Reducing haze means reducing the total amount of particulate matter being emitted or formed in the air.

The Regional Haze Rule directed states to develop and implement plans to reduce particulate pollution. These plans are expected to contain specific control measures for emission sources that were determined to be causing or contributing to haze in Class I Areas.

Such measures might take the form of direct limits on the amount of emissions allowed from a particular source. For example, a manufacturing facility might be required to reduce its total tons of nitrogen dioxide (NOx) emissions. The facility could achieve the limit in a number of ways, including installing new technology designed to remove NOx.

Other control measures may take the form of state or federal regulations that contribute to a general reduction of haze-causing pollutants across an entire sector. For example, when EPA revises a National Ambient Air Quality Standard, many facilities across the country must evaluate how best to reduce their emissions. Another such example is federal standards for automobile fuel efficiency. These fuel efficiency standards contribute to decreasing NOx emissions from cars and trucks.

In addition, like most technology, pollution control technology is always advancing and improving due to continual research and pressure from industries that need to comply with control measures or regulations.