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The 1990 federal Clean Air Act sets forth air-quality standards for six pollutants: ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), respirable particulate matter (PM10), and lead (Pb). The EPA has determined an allowable ambient limit for each of these so-called "criteria" pollutants intended to protect the public from exposure to dangerous levels of pollution. These national standards are known as the national ambient air quality standards.
One set of standards, called primary standards, is designed to protect human health. Another set, called
secondary standards, is designed to protect the environment and limit property damage. Any area of a city that exceeds a standard a specified number of times causes the entire metropolitan
area to be in violation, also known as being in "nonattainment."
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POLLUTANT
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AVERAGING
PERIOD
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PRIMARY
NAAQS**
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NOTES
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Ozone*
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1-Hour
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0.125 ppm
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1
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8-Hour
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0.085 ppm
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2
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Carbon Monoxide
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1-Hour
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35.50 ppm
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3
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8-Hour
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9.50 ppm
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3
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Sulfur Dioxide
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Annual
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0.035 ppm
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4
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24-Hour
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0.145 ppm
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3
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Nitrogen Dioxide
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Annual
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0.054 ppm
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4
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Respirable Particulate Matter (PM10)
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24-Hour
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155.00 mg/m3
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5
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Annual
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51.00 mg/m3
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6
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Fine Particulate Matter (PM2.5)
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24-Hour
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65.00 mg/m3
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7
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Annual
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15.00 mg/m3
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6
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Lead
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Quarter
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1.55 mg/m3
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4
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ppm = parts per million
mg/m3 = micrograms per cubic meter
1. Not to be at
or above this level more than 3 days over 3 years. The 1-hour ozone standard is being phased out.
2. Average of yearly fourth highest 8-hour ozone level over
3 years not be at or above this level.
3. Not to be at or above this level more than once per calendar year.
4. Not to be at or above this level.
5. Three-year average of the 99th percentile of concentrations not to be at or above this level.
6. Three-year average not to be at or above this level.
7. Three-year average of the 98th percentile of concentrations not to be at or above this level.
*The
1-hour standard applies only to communities that did not meet the one-hour standard when the 8-hour standard was adopted in July, 1997.
**Primary NAAQS levels
reported here represent the concentrations required to exceed the standard. For example, standard for 24-hour respirable particulate matter is actually 150
mg/m3 but an exceedence is measured at 155 umg/m3 due to rounding conventions.
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Between 1983 and 2002, the most dramatic decline in both ambient air concentrations
(94 percent) and emissions (93 percent) has been lead.*
. This dramatic reduction is due
largely to the phase-out of leaded gasoline, which began in the 1970s and continued in the 1980s. Other criteria pollutants have witnessed a much more modest decline due
largely to a continued reliance on combustion of fossil fuels for transportation and power. Today, the most abundant single pollutant is carbon monoxide,
accounting for about 50 percent of total emissions by weight.
In 1997 the EPA, following a scientific assessment and public comment period that
elicited more than 50,000 comments, adopted a new standard for "fine" respirable particulate matter up to 2.5 microns (PM2.5), while relaxing the previous PM10
standard, and replaced the 1-hour ozone standard with a slightly more stringent 8-hour ozone standard. However, in May 1999 the U.S. Court of Appeals for the
District of Columbia Circuit ruled that the air standards revisions were unconstitutional because EPA did not precisely define their criteria for revising the standards. This
decision was appealed.* In 2001, the U.S. Supreme Court upheld the ability of EPA to adopt the new standard. EPA is at present
continuing to define exactly how they will implement the new standard and designate which areas are considered non-attainment. They are scheduled to designate
non-attainment areas for the 8-hour ozone standard in April of 2004*.
The new ozone standards is expected to cause three new metropolitan areas in Texas – San Antonio, Austin and
Longview-Marshall-Tyler -- to fall out of compliance. Nevertheless, the 1-hour standard will continue to apply to the
four existing nonattainment areas in Texas –- Houston, Beaumont/Pt. Arthur, El Paso and Dallas/Ft. Worth. Three of
these areas -- Houston/Galveston, Dallas/Ft. Worth and Beaumont/Pt. Arthur - have also been recommended as non-attainment for the 8-hour standard.
At monitoring stations around the state, the four gaseous criteria pollutants (nitrogen dioxide, sulfur dioxide, carbon
monoxide, and ozone) are monitored continuously, with 1-hour averages measured each hour, every day. PM10 and
lead are measured at least once every six days for a 24-hour averaging period, although some sites in Texas are
monitored more frequently. The TCEQ has also instituted a new continuous monitoring network of PM-2.5 monitors
around the state to measure compliance with the new PM-2.5 standard. Currently, there are 41 monitors throughout
the state which are continously monitoring hourly levels of PM-2.5. In addition, other monitors sample PM-2.5 at PM-10
sites. In all, the TCEQ wants to eventually expand the network to a total of 85 PM 2.5 monitors.*
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