Ozone is a gas that occurs in both the Earth’s upper atmosphere and at ground level. Ozone can be "good” or "bad” for your health and the environment, depending on its location in the atmosphere. While the ozone layer in the upper atmosphere protects you from the sun’s most damaging rays, ozone near the ground is a harmful pollutant and health hazard. Ozone pollution is especially a concern during the summer months because strong sunlight and hot weather result in harmful ozone concentrations in the air we breathe.

Ozone is the primary ingredient in smog and is formed by a chemical reaction between air pollutants and sunlight. Ground-level ozone affects the respiratory system and can be especially harmful to the very young, the very old, and people with chronic lung disease. Older adults are at an increased risk for ozone as it can aggravate pre-existing respiratory diseases.

Types of Ozone

Besides the Jefferson–Orange–Hardin county area, three other areas in Texas have also been cited for ozone nonattainment. El Paso and allas/Fort Worth are designated as serious nonattainment areas and Houston/Galveston is a severe nonattainment area.

Ozone occurs in two layers of the Earth’s atmosphere. The layer closest to the Earth’s surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant that is harmful to breathe and it damages crops, trees, and other vegetation. It is a main ingredient of urban smog and is both naturally occurring and produced by human activities.

The troposphere generally extends to a level about 6 miles up, where it meets the second layer of the atmosphere, the stratosphere. The stratosphere or "good” ozone layer extends upward about 6 to 30 miles, occurs naturally, and protects life on Earth from the sun’s harmful ultraviolet (UV) rays.

Ozone is a reactive form of an oxygen molecule made up of three oxygen atoms (O3) instead of two (O2)-the type of oxygen in the atmosphere that animals and humans require to stay alive.

Ozone Production

Ground-level, or tropospheric, ozone is not emitted directly to the atmosphere, but instead is formed by a series of complex atmospheric chemical reactions. This type of ozone is formed when sunlight reacts with nitrogen oxides (NOx) and volatile organic compounds (VOCs). Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.

In addition to sunlight being needed to form ozone, certain meteorological conditions are also usually required: clear skies, low wind speeds, elevated temperatures, and certain wind patterns. These variables make ozone one of the most difficult pollutants to control.

Sources of NOx and VOCs in Hardin, Jefferson, and Orange Counties include the following major categories: point, area, mobile, and biogenic. Point, or stationary sources, comprise the majority of NOx emissions and come from industrial operations. Area, or nonroad, sources come from engines, trains, planes, boilers, solvents, paints, dry cleaning facilities, and construction equipment. Mobile, or on-road, sources are based on car and truck emissions. Biogenic sources are emissions based on the natural result of plant photosynthesis and are based on the quantity and type of vegetation in an area. In southeast Texas, vegetation produces significant quantities of biogenic emissions that account for the majority of VOC emissions for the area.


Air pollution in southeast Texas also includes transported air pollutants that combine with locally produced emissions to produce high ozone levels. This phenomenon, called transport, makes it difficult for the area to maintain consistently low ozone levels.

In the southeast Texas area, ozone transport occurs in basically two ways. One situation conducive to transport arises when the area is dominated by high pressure over Texas and weak surface pressure gradients, which will favor light northerly or easterly winds. Along the coast, this scenario includes a phenomenon called the sea/land breeze circulation or flow reversal. The sea/land breeze transports nighttime/early morning emissions out to the Gulf of Mexico with the land breeze and brings them back during the afternoon with the sea breeze. This adds the nighttime/early morning emissions on top of the fresh afternoon emissions and has a doubling effect on emissions in the atmosphere

The other transport-related meteorological regime occurs when a high-pressure system in the Gulf of Mexico allows pollutants to transport from one area to another in a westerly and southwesterly direction.


The Clean Air Act of 1970, which was amended in 1990, directs the EPA to identify air pollutants that may reasonably be anticipated to endanger public health and welfare, and to establish appropriate ambient air quality standards. These federal standards are known as the National Ambient Air Quality Standards (NAAQS). Six pollutants, known as the criteria pollutants, must meet the NAAQS:

  • ozone
  • carbon monoxide
  • nitrogen dioxide
  • sulfur dioxide
  • lead
  • particulate matter

Ozone is the only criteria pollutant for which southeast Texas has exceeded the NAAQS.

Stay Air Aware

You can be part of the South East Texas Regional Planning Commission Ozone Action Day Program. We’ll notify you the afternoon before a broadcasted Ozone Action Day.