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U.S. Nuclear Regulatory Commission
Ozone: Good Up High, Bad Nearby
GOOD UP HIGH
Ozone is a gas that forms in the atmosphere when 3 atoms of oxygen are combined (03). It is not emitted directly into the air, but at ground level is created by a chemical reaction between oxides of nitrogen (NOx), and volatile organic compounds (VOC) in the presence of sunlight. Ozone has the same chemical structure whether it occurs high above the earth or at ground level and can be "good" or "bad," depending on its location in the atmosphere.
Ozone occurs in two layers of the atmosphere.
The layer surrounding the earth's surface is the troposphere. Here, ground-level
or "bad" ozone is an air pollutant that damages human health, vegetation,
and many common materials. It is a key ingredient of urban smog. The troposphere
extends to a level about 10 miles up, where it meets the second layer, the
stratosphere. The stratospheric or "good" ozone layer extends upward from
about 10 to 30 miles and protects life on earth from the sun's harmful ultraviolet
Ozone occurs naturally in the stratosphere and is produced and destroyed at a constant rate. But this "good" ozone is gradually being destroyed by manmade chemicals called chlorofluorocarbons (CFCs), halons, and other ozone depleting substances (used in coolants, foaming agents, fire extinguishers, and solvents). These ozone depleting substances degrade slowly and can remain intact for many years as they move 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 can replace it.
It can take years for ozone depleting chemicals to reach the stratosphere, and even though we have reduced or eliminated the use of many CFCs, their impact from years past is just starting to affect the ozone layer. Substances released into the air today will contribute to ozone destruction well into the future.
Satellite observations indicate a world-wide thinning
of the protective ozone layer. The most noticeable losses occur over the
North and South Poles because ozone depletion accelerates in extremely cold
As the stratospheric
ozone layer is depleted, higher UV-b levels reach the earth's surface. Increased
UV-b can lead to more cases of skin cancer, cataracts, and impaired immune
systems. Damage to UV-b sensitive crops, such as soybeans, reduces yield.
High altitude ozone depletion is suspected to cause decreases in phytoplankton,
a plant that grows in the ocean. Phytoplankton is an important link in the
marine food chain and, therefore, food populations could decline. Because
plants "breathe in" carbon dioxide and "breathe out" oxygen, carbon dioxide
levels in the air could also increase. Increased UV-b radiation can be instrumental
in forming more ground-level or "bad" ozone.
The Montreal Protocol, a series of international agreements on the reduction and eventual elimination of production and use of ozone depleting substances, became effective in 1989. Currently, 160 countries participate in the Protocol. Efforts will result in recovery of the ozone layer in about 50 years.
In the United States, the U.S.
Environmental Protection Agency (EPA) continues to establish regulations
to phase out these chemicals. The Clean Air Act requires warning labels on
all products containing CFCs or similar substances, prohibits nonessential
ozone depleting products, and prohibits the release of refrigerants used
in car and home air conditioning units and appliances into the air.
What Causes "Bad" Ozone?
Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC, also known as ozone precursors. Strong sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. Many urban areas tend to have high levels of "bad" ozone, but other areas are also subject to high ozone levels as winds carry NOx emissions hundreds of miles away from their original sources.
Ozone concentrations can vary
from year to year. Changing weather patterns (especially the number of hot,
sunny days), periods of air stagnation, and other factors that contribute
to ozone formation make long-term predictions difficult.
Repeated exposure to ozone pollution may cause permanent damage to the lungs. Even when ozone is present in low levels, inhaling it triggers a variety of health problems including chest pains, coughing, nausea, throat irritation, and congestion. It also can worsen bronchitis, heart disease, emphysema, and asthma, and reduce lung capacity.
Healthy people also experience difficulty in breathing when exposed to ozone pollution. Because ozone pollution usually forms in hot weather, anyone who spends time outdoors in the summer may be affected, particularly children, the elderly, outdoor workers and people exercising. Millions of Americans live in areas where the national ozone health standards are exceeded.
Ground-level ozone damages plant life and is responsible for 500 million dollars in reduced crop production in the United States each year. It interferes with the ability of plants to produce and store food, making them more susceptible to disease, insects, other pollutants, and harsh weather. "Bad" ozone damages the foliage of trees and other plants, ruining the landscape of cities, national parks and forests, and recreation areas.
The Clean Air Act Amendments of 1990 require EPA, states, and cities to implement programs to further reduce emissions of ozone precursors from sources such as cars, fuels, industrial facilities, power plants, and consumer/commercial products. Power plants will be reducing emissions, cleaner cars and fuels are being developed, many gas stations are using special nozzles at the pumps to recapture gasoline vapors, and vehicle inspection programs are being improved to reduce emissions.
The ultimate responsibility
for our environment is our own. Minor lifestyle changes can result in major
air quality improvements.
What Can You Do?
High-Altitude "Good" Ozone
Ground-Level "Bad" Ozone
live with ozone every day. It can protect life on earth
This document is not necessarily endorsed by the Almanac of Policy Issues. It is being preserved in the Policy Archive for historic reasons.