Ninety-five percent of pollution
related sulfur oxide emissions are in the form of sulfur dioxide (SO2),
a heavy, colorless gas with an odor like just struck matches. This gas
combines easily with water vapor, forming aerosols of sulfurous acid
(H2SO3), a colorless, mildly corrosive liquid.
This liquid may then combine with oxygen in the air, forming the
even more irritating and corrosive sulfuric acid (H2SO4).
Emission Sources of
Sulfur is present in all raw materials,
including crude oil, coal, and ore that contains common metals like
aluminum, copper, zinc, lead, and iron. Sulfur oxides are formed
when fuel containing sulfur, such as coal or oil, is burned, and
when gasoline is extracted from oil, or metals are extracted from ore.
Some sources are:
- from coal-burning electrical
utilities (over 65% of SO2 released to the air, or more than 13
million tons per year, comes from electric utilities, especially
those that burn coal).
- from pulp and paper mills.
- from natural sources.
- from other human-generated
Note: The amount of SO2
released depends on the sulfur content of coal, normally 0.7% to 2%
by weight. High sulfur coal sometimes contains as much as 6% sulfur
Other Sulfur Sources
Other sulfur-containing compounds,
with their typical unpleasant odors, are pollutants as well. Several
- Hydrogen sulfide (H2S)
gas - rotten eggs.
- Mercaptans - skunk spray or
decayed garbage. (Added in trace amounts to natural gas, providing
a leak-detecting warning odor.)
Sulfur dioxide not only has a bad
odor, it can irritate the respiratory system. Exposure to high
concentrations for short periods of time can constrict the bronchi
and increase mucous flow, making breathing difficult. SO2 can also
aggravate existing heart and lung diseases. Children, the
elderly, those with chronic lung disease, and asthmatics are
especially susceptible to these effects. Sulfur dioxide can also:
- Immediately irritate the lung and
throat at concentrations greater than 6 parts per million (ppm) in
- Impair the respiratory system's
defenses against foreign particles and bacteria, when exposed to
concentrations less than 6 ppm for longer time periods.
- Apparently enhance the harmful
effects of ozone. (Combinations of the two gases at concentrations
occasionally found in the ambient air appear to increase airway
resistance to breathing.)
Sulfur dioxide tends to have more
toxic effects when acidic pollutants, liquid or solid aerosols, and
particulates are also present. (In the 1950s and 1960s, thousands of
excess deaths occurred in areas where SO2 concentrations
exceeded 1 ppm for a few days and other pollutants were also high.)
Effects are more pronounced among mouth breathers, e.g., people who
are exercising or who have head colds. These effects include:
- Health problems, such as episodes
of bronchitis requiring hospitalization associated with
lower-level acid concentrations.
- Self-reported respiratory
conditions, such as chronic cough and difficult breathing,
associated with acid aerosol concentrations. (Asthmatic
individuals are especially susceptible to these effects. The
elderly and those with chronic respiratory conditions may also be
affected at lower concentrations than the general population.)
- Increased respiratory tract
infections, associated with longer term, lower-level exposures to
SO2 and acid aerosols.
- Subjective symptoms, such as
headaches and nausea, in the absence of pathological abnormalities,
due to long-term exposure.
Effects on Plants
Sulfur dioxide easily injures many
plant species and varieties, both native and cultivated. Some of the
most sensitive plants include various commercially valuable pines,
legumes, red and black oaks, white ash, alfalfa and blackberry. The
- Visible injury to the most
sensitive plants at exposures as low as 0.12 ppm for 8 hours.
- Visible injury to many other plant
types of intermediate sensitivity at exposures of 0.30 ppm for 8
- Positive benefits from low levels,
in a very few species growing on sulfur deficient soils.
Increases in sulfur dioxide
concentrations accelerate the corrosion of metals, probably through
the formation of acids. (SO2 is a major precursor to
acidic deposition usually known as acid rain.) Sulfur oxides may also damage stone and masonry,
paint, various fibers, paper, leather, and electrical components.
Increased SO2 also
contributes to impaired visibility. Particulate sulfate, much of
which is derived from sulfur dioxide emissions, is a major component
of the complex total suspended particulate mixture.
SO2 also accelerates the decay of
building materials and paints.