Gaseous Pollutants, Ozone, and Smog

Stippling of Yellow (Tulip) Poplar leaves caused by ozone.
Stippling of Yellow (Tulip) Poplar leaves caused by ozone.

US Forest Service Ozone Biomonitoring website

What are Gaseous Pollutants?

The major gaseous pollutants include sulfur dioxide (SO2), carbon monoxide (CO), and nitrogen oxides (NOx) as well as ozone (O3). The primary source of these gases is the combustion of fossil fuels in power plants, various industrial processes, and motor vehicles and equipment. Each of these pollutants, in their gaseous form, can cause harm to human health and the environment. They are also essential ingredients in chemical and physical transformations which can result in further damages (see sections on acid deposition and visibility).

What is Ozone?

Ozone (O3) is a gas composed of three oxygen atoms. The oxygen we need to live contains two oxygen atoms per molecule. A natural layer of ozone in the upper atmosphere, known as the stratosphere, is created by reactions of oxygen, nitrogen oxides, and ultra-violet radiation.Ozone has the same chemical structure whether it occurs miles above the earth or at ground level and can be "good" or "bad," depending on its location in the atmosphere. "Good" ozone occurs naturally in the stratosphere approximately 10 to 30 miles above the earth's surface and forms a layer that protects life on earth from the sun's harmful rays. In the earth's lower atmosphere, which is known as the troposphere, ground-level ozone is considered "bad."

Where Does Ground-Level Ozone Come From?

Ozone forms near the earth’s surface when other chemicals in the air -- nitrogen oxides and volatile organic compounds (VOCs) -- react in the presence of sunlight. Nitrogen oxides and VOCs are emitted by mobile sources, most notably cars and trucks.

What are the Effects of Too Much Ozone?

High ozone concentrations at ground-level are considered a serious threat in our cities and rural areas, such as Shenandoah National Park.

Ozone injures the lungs of humans and other animals, decreasing their ability to fight infection and remove inhaled particles. Ozone can cause permanent lung damage, coughing, sinus inflammation, chest pains, scratchy throat, stinging eyes and general malaise. High concentrations are hazardous for people with heart and respiratory ailments. Most vulnerable are children, the elderly, and those with existing health problems.

Ground-level ozone also interferes with the ability of plants to produce and store food, which makes them more susceptible to disease, insects, other pollutants, and harsh weather.

Plants that are reliable indicators of phytotoxic (i.e., harmful) levels of pollutants are known as bioindicators. Examples of bioindicators for ozone include black cherry, blackberry, common milkweed and yellow-poplar. Forty ozone sensitive plant species have been identified at Shenandoah.

Ozone damages the appearance of leaves of trees and other plants. The most common visible symptom of ozone injury on broad-leaved bioindicator species is uniform interveinal leaf stippling or speckling on the upper surface of the leaf. As a gaseous pollutant, ozone enters plant leaves through the normal process of gas exchange, damaging the plant tissue.

Ozone at ShenandoahNational Park

Elevated levels of ozone have been documented at Shenandoah as has damage to park vegetation. Park staff members are concerned with this situation and therefore work on a variety of programs related to monitoring, research, and emissions reduction. Park staff members have also instituted an Ozone Advisory program aimed at educating employees and park visitors about the risks of exposure to ozone and precautions that can be taken.

Related Information

Helpful literature related to ozone includes:

Anon. 1999. Ozone monitoring, mapping, and public outreach – delivering real-time ozone information to your community. EPA/625/R-99/007. Office of Research and Development, Environmental Protection Agency, Washington, DC.

Garner, J.H.B., T. Pagano, and E.B. Cowling. 1989. An evaluation of the role of ozone, acid deposition, and other airborne pollutants in the forests of eastern North America. General Technical Report SE-59. Southeastern Forest Experiment Station, Forest Service, U.S. Department of Agriculture.

Hakkarinen, C. editor. 1987. Forest health and ozone. Electric Power Research Institute, Palo Alto, California.

Good Up High, Bad Nearby – EPA publication

Ozone and Your Health – EPA publication

Smog – Does It Hurt You? – EPA publication

The following websites provide helpful information related to ozone:

EPA Air Quality Index – AIRNOW website

EPA Ozone information during the previous 12 hours

Recent Ozone Data from Shenandoah National Park

Recent Ozone Data from other National Parks across the country

Ozone Standard Exceedences during the Previous Year

Historic and Archived Gaseous Pollutant Data from Shenandoah National Park

Listing of these websites does not and is not intended to imply endorsement by the National Park Service of commercial services or products associated with the sites.

Last updated: February 26, 2015

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Shenandoah National Park
3655 U.S. Highway 211 East

Luray, VA 22835


(540) 999-3500

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