A CIESE Realtime Data Project

Ground Level Ozone Primer

What is Ground Level Ozone? How Does Ground Level Ozone Form?
Where is Ozone Found? Is Ozone a Problem?
How do Scientists Know? Why Don't They Mix?
What Can I Do?  

Educators and learners can use this curriculum to acquire the knowledge and skills to understand ground level ozone.  Many people have roles in dealing with ground level ozone including scientists that conduct research to understand ozone and its health effects, Government agencies that monitor ground level ozone and enforce regulations to control ozone, and citizens who need to understand ground level ozone and its impacts on human health.  This primer provides the necessary background information about ground level ozone and its potential health effects to successfully implement the curriculum.

What is Ground Level Ozone?  (back to top)
Ozone (O3) is a gas that occurs in two layers of the atmosphere, the stratosphere and the troposphere.  The stratospheric or "good" ozone layer, which extends upward from about 10 to 30 miles above the earth's surface, protects life on earth from the sun's harmful ultraviolet rays (UV-b).  However, ozone found in the troposphere, the layer of the atmosphere that extends from the earth's surface to about 10 miles up, is deemed ground level or "bad" ozone. At ground level, ozone is an air pollutant that damages human health, vegetation, many common materials, and is a key ingredient of smog. 

How Does Ground Level Ozone Form?  (back to top)
Ozone has the same chemical structure (O3) whether it occurs miles above the earth or at ground level.  At ground level, "bad" ozone is formed when certain compounds react in the presence of direct sunlight. 

            VOCs + NOx + Sunlight = Ozone

VOCs, (volatile organic compounds) are widely used as ingredients in household products including; paints, varnishes, wax, fuels, cleaning, disinfecting, cosmetic, degreasing, and hobby products.  Some VOCs are safe to handle and have little known health effects, while other VOCs are highly toxic.  In addition to all of the man made sources of VOCs, natural sources of VOCs exist.  For example, trees naturally release small amounts of VOCs. 

NOx, (nitrogen oxide gases) is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts.  Many of the nitrogen oxides are colorless and odorless.  The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.

When high levels of VOCs and NOx are present in the air, they can react.  When they react in the presence of sunlight and hot weather, ground level ozone forms.  

There are other factors involved with the formation of "bad" or ground level ozone, including; cloud cover, wind direction, and low wind speeds.  If the weather conditions are conducive, and there are ample amounts of NOx and VOCs, harmful concentrations of ground level ozone can form in the air. 

Often industry is blamed entirely for emissions that cause ground level ozone air pollution, but actually private citizens are responsible for a significant percentage of the air pollutants that lead to ground level ozone production. Motor vehicle emissions are the single greatest contributor to ground level ozone pollution.   

Where is Ground Level Ozone Found?  (back to top)
Many urban areas tend to have high levels of ground level ozone. But even rural areas are subject to increased ozone levels because winds can carry ozone, and the pollutants that form it, hundreds of miles away from the original sources.  The process of prevailing winds carrying ground level ozone away from the original source is called transport.  For example, if large amounts of NOx are transported to a rural area, filled with trees that are naturally releasing VOCs, a large amount of ground level ozone can form in the rural area.  Regardless of how the ground level ozone gets to the cities, it can pose health threats to all the inhabitants, people, animals and plants. 

Is it a Problem?  (back to top)
Ground level ozone is a common and widespread air pollutant that causes injury to the environment and human health.  Exposure to ground level ozone can:

- irritate your respiratory system
- reduce lung function
- aggravate asthma
- inflame and damage cells that line your lungs
- aggravate chronic lung diseases
- cause permanent lung damage 

It is important to know that lung damage caused by ground level ozone can occur without any noticeable signs.  People who live in areas where ozone levels are frequently high may find that their initial symptoms go away over time, but ozone continues to cause lung damage even when the symptoms have disappeared. The best way to protect your health is to find out when ozone levels are elevated in your area and take simple precautions to minimize exposure to unhealthy levels of ozone, even when you dont feel obvious symptoms.

What is considered to be an unhealthy level of ground level ozone?  In an effort to determine unhealthy levels of air pollutants, National Ambient Air Quality Standards were created for ground level ozone.  Two standards have been set, a one hour (120 ppb) and an eight hour (80 ppb) standard.  When ozone levels surpass the set standards, unhealthy conditions are present. 

To ease understanding of the standards, the EPA developed the AQI, or Air Quality Index.  The AQI is a scale used to report ground level ozone and other common pollutants in the air. The higher the AQI value, the greater the health concern.  There are five categories, with a specific color assigned to each AQI category. The color scheme can help citizens quickly determine if air pollutants are reaching unhealthy levels.

In an effort to provide citizens with information about ozone, the United States Environmental Protection Agency (USEPA) coordinates the collection of real-time ground level ozone data from around the country and posts the information on the AIRNow web site.  The information is compiled into animations and forecast maps, allowing citizens to determine if ground level ozone is reaching unhealthy levels in their area. The location of monitoring stations across the country reflects the quantity and accuracy of the data collected.  Although ozone monitors represent ozone measurements over a large area, there are areas of the country that may not appear to have an ozone problem simply because there may not be any monitoring stations to report ground level ozone that may be present.  Citizens should keep this fact in mind when attempting to determine their potential exposure to ground level ozone.

How do Scientists Know what levels are harmful? (back to top)
EPA has gathered a great deal of information about the health effects of ozone. This information comes from a number of sources including; studies that examine health statistics and ozone levels within communities, computer modeling and controlled testing of human volunteers to determine how ozone affects lung function. 

Though scientists' understanding of ozone's effects has increased substantially in recent years, many important questions still remain to be investigated.  For example, does repeated short-term exposure to high levels of ozone cause permanent lung damage?  Does repeated exposure during childhood to high levels of ozone cause reduced lung function in adults?  Scientists are continuing to study these and other questions to gain a better understanding of ozone's effects and make necessary revisions of safety guidelines to better protect public health and the environment.

Why Don't Stratospheric (good) and Tropospheric (bad) Ozone Mix?(back to top)
At first, the answer to the problem might seem easy.  Why don't we mix the layers and use the "bad" ground level ozone to plug the holes in the "good" ozone layer?  Wouldn't that solve both problems?  Yes, theoretically that would solve both problems, but unfortunately that cannot be done. 

First, we simply cannot mix the air in the troposphere and the stratosphere.  The troposphere is the layer of the atmosphere at the earth's surface.  The troposphere contains 75% of all the air found in our atmosphere and 99% of the water vapor.  The air in the troposphere is in constant motion, with both horizontal and vertical air currents.  The combination of vigorous air movement and water vapor creates weather.  The troposphere is capped by a thin layer known as the tropopause, which is a region of stable temperature that helps to confine most weather phenomena and "bad" ozone to the troposphere. 

The stratosphere is the second layer in the atmosphere from the earth's surface.  The lower part of the stratosphere contains the ozone layer.  The ozone layer prevents harmful ultraviolet radiation from reaching the earth's surface by absorbing the rays, causing the ozone layer and the air above it to warm.  The warm air tends to remain in the upper stratosphere, and cool air remains lower.  The layering of warm and cool air prevents vertical mixing, so the air moves only in a horizontal direction, making the stratosphere very stable, but also creating a kind of giant lid.  This is helpful to commercial airlines that often fly in the lower stratosphere because the air is relatively warm and stable, but not helpful to be able to mix ozone between the stratosphere and the troposphere.

Second, the "good" ozone that occurs naturally in the stratosphere is gradually being destroyed by man-made chemicals. The ozone depleting substances can remain intact for years while moving through the troposphere until they reach the stratosphere. There they are broken down by the intensity of the sun's ultraviolet rays and release chlorine and bromine molecules, which destroy "good" ozone. One chlorine or bromine molecule can destroy 100,000 ozone molecules, causing ozone to disappear much faster than nature (or man) could ever replace it.   We can't make enough ozone to replace what's been destroyed, but provided that we stop producing ozone depleting substances, natural ozone production reactions could return the ozone layer to normal levels.  Delays in ending production of ozone depleting substances could result in additional damage and prolong the ozone layer's recovery.

What Can I Do?(back to top)
Be alert.  If exceptionally high concentrations of ground level ozone are predicted, Ozone Action Days can be declared by the state environmental protection agencies.  When an Ozone Action Day is declared it serves two purposes, to alert citizens of possible exposure to unhealthy levels of ozone, and to offer an opportunity for citizens to curtail activities that contribute to the ozone problem.  An Ozone Action Day prompts participating businesses to notify their employees so they can telecommute, share rides to work, use mass transit, and take other steps to help reduce smog.  The press is also notified to alert the public so they can pitch in too.

But don't wait until an Ozone Action Day is declared.  Avoid unhealthy exposure to ozone by monitoring the amount of your outdoor activity and the strenuous nature of the activity on days when ozone is present. If an activity requires heavy exertion, you could either reduce the time spent on the activity or substitute another activity that requires more moderate exertion (e.g., go for a walk rather than a jog). In addition, you could plan outdoor activities when ozone levels are lower, usually in the morning or evening.

Avoid contributing to the ozone problem.  There are many simple steps citizens can take to help decrease the production of compounds that create ground-level ozone:

  • Conserve energy at home, at work, everywhere. In the long term, it helps to reduce the emissions associated with energy production.
  • Pick one day a week to leave your car at home. If only 1 percent of America's car owners did this, it would save millions of gallons of gasoline a year and keep a good deal of pollutants out of the atmosphere.
  • Try telecommuting. 
  • Consider manual or electric-powered lawn and garden maintenance equipment when replacing a gasoline-powered mower.

One of the most important steps to take is to tell friends, family and co-workers what you are doing and why. Education and small modification of activities will do wonders for keeping the air cleaner in your corner of the world. 

EPA | NESCAUM | CIESE | Stevens Institute of Technology