Sulfur dioxide (SO₂) is formed whenever sulfur-containing substances are burned. Other oxides of sulfur are usually formed along with it, but sulfur dioxide is the predominant member of this class and both the State of Montana and the Federal Government regulate its levels in the air. Other sulfur oxides include sulfur trioxide, sulfuric acid, sulfurous acid, and various sulfates. Sulfur compounds, including the sulfates, are necessary for life, but are harmful in high concentrations.

Sulfur dioxide (SO₂) is a colorless gas with a sharp, acrid odor that most people can smell when concentrations are above 2 - 5 ppm. It also irritates the respiratory tract and can trigger breathing difficulties, it can cause damage to vegetation, and it contributes to acid rain. Federal regulations require that the annual average concentration of SO₂ be less than 0.03 ppm, around one hundredth the odor level.

The average levels of SO₂ measured in Yellowstone County have decreased over the past decade, from around 0.02 ppm in the early ‘90s to around 0.005 ppm in 2000. This can be attributed to substantial investments by the industrial SO₂ sources in emission control equipment and to refinements of their operating practices.

Sulfur oxides come from many sources. Major natural sources include volcanoes, forest fires, and the atmospheric oxidation of sulfur-containing gases such as hydrogen sulfide (H₂S) emitted by swamps and marshes. Volcanoes and related geological process are the largest of all sources, far exceeding human activities. Except in the immediate vicinity of the volcanism, though, their emissions are diluted by the vastness of the atmosphere and have low impacts on human health.

Among human activities that emit sulfur dioxide, combustion of fuels that contain sulfur is the greatest source. Practically all fuels contain at least a little sulfur, but coal and crude oil are among those containing high levels. Petroleum refining separates most of the sulfur in crude oil from the fuel products, and a small fraction of that sulfur is lost to the atmosphere as SO₂, though most of it is recovered for industrial uses. Industrial chemical processing, including production of sulfur chemicals, also loses a small fraction of its sulfur to the atmosphere. Most industrial sources, including coal-fired power plants, petroleum refineries, and chemical plants fall under state and federal emissions regulations, and emissions control devices are now employed at almost all such plants.

Metal production, such as copper smelting, also emits sulfur oxides into the air because many of the ores contain sulfur compounds. This was formerly a large contributor to sulfur dioxide in the air of western Montana, but has diminished with the closure of the Berkeley Pit and the smelters utilizing its ore.

Home coal burning (and to a far smaller extent, wood burning) also emit sulfur dioxide, and in some communities are also regulated because of air pollution problems. These are considered to have been the major factors in the infamous air pollution episodes reported in London as far back as the seventeenth century.

Gasoline, diesel fuel and heating oil contain only small amounts of sulfur because most of it is removed in the refining process, but these are under consideration for regulation of the sulfur content because of the large amounts burned each year.

Like the geological sources, human caused emissions of sulfur oxides are diluted by the vastness of the atmosphere, and have their most significant effects on health in the vicinity of the sources. There are occasions, though, when sulfur oxides are carried by the wind over long distances and add to the overall concentration in Yellowstone County.

Sulfur oxides are removed from the air by continuous natural processes. When there is sufficient moisture they are removed by rain, snow and fog, dissolving in the droplets and being carried to the ground. But, in the arid western climate this process is not as common as dry deposition processes.

Some of the sulfur oxides are removed from the air by adsorption, sticking to the surface of solids including soil, vegetation and buildings, and on airborne particles (such as prairie dust) that settles back to the ground.

Sulfur oxides are also transformed by further oxidation to sulfates, which form minuscule particles – often less than 2.5 microns in diameter -- that are irritating when breathed. These particles in the upper atmosphere contribute to the brilliant orange and red sunsets that follow for many months after major volcanic eruptions. They eventually are removed either by rain and snowfall or by aggregation into larger particles that fall to the ground.

Much of the sulfur required for the growth of plants is delivered naturally by the mechanisms just mentioned. Sulfur emitted by both natural and human activity is oxidized and transported by the atmosphere and deposited onto the soil. In regions of acidic soils, the acidity of this deposition (acid rain) may worsen the already marginal growing conditions in forest soils and lakes, causing damage to the trees and to aquatic habitats. In Yellowstone County, as in most of the West, the soils are predominantly alkaline and quickly neutralize the acidity of deposited sulfur compounds without harm.

Sulfur dioxide can cause irritation of the lungs and bronchi, leading to respiratory distress. For most healthy adults, such effects only occur at high exposure levels that seldom occur. However, those who suffer from chronic respiratory disease such as asthma and emphysema are more sensitive to the irritating effect of SO₂ as well as to other challenges, and can be affected at levels of 0.5 to 1 ppm, and in rare cases perhaps at levels down to 0.25 ppm. These effects are summarized in the table below.

The sensitivity of both normally healthy and asthmatic individuals to SO₂ dependends on the level of physical exertion because it increases the rate and depth of breathing. People are more sensitive when exercising than when at rest, therefore if high concentrations occur, sensitive individuals should reduce or delay outdoor exercise.

Sensitivity to sulfur dioxide is also influenced by the weather, with sensitivity being greatest when breathing cold, dry air, perhaps because cold, dry air itself is mildly irritating to the lungs. Moreover, it is influenced by the individual’s style of breathing, due to the nose’s function in cleaning and warming the air breathed. Nose breathers generally can tolerate more SO₂ than can mouth breathers.

Additional concern has been expressed over the effects of chronic exposure to SO₂, which can lead to or aggravate respiratory problems. Children are affected by lower levels than are adults, but the State and Federal standards for long-term concentrations have been set at levels well below those identified as causing chronic health problems in either children or adults.

Concentration	General Health Effects	Cautionary Statements
0.4 – 1 ppm	A substantial proportion of exercising asthmatics may experience chest tightness, cough and/or wheezing. No health effects for non-asthmatics or non-exercising asthmatics.	Individuals with asthma should reduce outdoor physical exertion. If such activity cannot be avoided they should consider pre-medicating.
0.3 – 0.39 ppm	A small percentage of exercising asthmatics may experience chest tightness, cough and/or wheezing. No health effects for non-asthmatics or non-exercising asthmatics.	Asthmatics with a history of sensitivity to low levels of SO2 should reduce outdoor physical exertion. If such activity cannot be avoided they should consider pre-medicating.
< 0.25 - 0.29 ppm	Health effects are few or none for the general population and most exercising asthmatics. *	None required.

Concentration	Exposure Interval	Effects
10 ppm	2 hour	Visible foliar injury in both wet and dry regions
1 ppm	5 minutes	Visible injury to sensitive vegetation in humid regions
0.2 ppm	3 hour	Visible injury to sensitive vegetation in humid regions
0.1 - 0.2 ppm	Long-term	Reduced growth in some species, without visible injury

The standards discussed below require that ambient air concentrations be kept well below the lowest of these levels.

By law, the total concentration of certain pollutants must not exceed allowed concentrations. This means that all significant anthropogenic sources of pollutants like SO₂ are regulated so that their collective emissions do not cause the concentration in the outdoor air to exceed that limit. This regulation is effected through the Operating Permits for industrial sources and through regulations of other activities that produce pollution through non-industrial activities (such as home heating and automobile usage). These regulations are enforced by monitoring of the actual concentrations in the air, with reporting of the results to the State of Montana Dept. of Environmental Quality and the US EPA. In Yellowstone County, about half the monitoring is conducted by the State of Montana, and half is conducted by the regulated industries.

Air quality standards include both short and long-term values in order to protect health and welfare from both acute and chronic effects of exposure to pollutants, as shown in the table below.

Regulatory Standards for Ambient Air Concentrations of Sulfur Dioxide (1)

1. In addition to the standards listed, which have been promulgated by law or formal rulemaking, the US EPA is examining the potential need for standards for 5-minute average SO₂ concentrations, which may include the following:

Federal 5-minute Endangerment Level (under discussion): 2.0 ppm

Federal 5-minute Level of Concern (under discussion): 0.60 ppm

At present, neither the 5-minute levels, nor the maximum allowable number of occurrences of these levels have been formally promulgated as rules or regulations.

2. To provide measurement values that properly represent the concentrations of pollutants in the air, measuring instruments are set to make numerous measurements at short intervals and average these over specified reporting intervals, generally one hour. The 3-hour, 24-hour, and annual averages are arithmetic means of all the validated reporting hours within the larger reporting interval.