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Ozone
What is Ozone?
Health Effects
Environmental Effects
National Ambient Air Quality Standards
Reducing Transported Ozone
Ozone Action Days
Definitions
What is Ozone?
Ozone is a photochemical oxidant that forms in the atmosphere when three atoms of oxygen are combined (O3). In the upper atmosphere, ozone occurs naturally and shields the Earth from the Sun's harmful ultraviolet radiation. At ground level, ozone adversely affects human health and damages vegetation and many common materials. It is a major component of urban smog.
Ground level ozone is not emitted directly into the air, but rather is formed by complex chemical reactions between volatile organic compounds (VOC) and oxides of nitrogen (NOx) in the presence of sunlight. (What are VOCs? What is NOx?) These reactions are stimulated by sunlight and temperature, so that peak ozone levels typically occur during hot weather.
There are numerous sources of VOC emissions, including automobiles, gasoline vapors, chemical solvents, and consumer products like paints. Emissions of NOx are produced primarily when fossil fuels are burned in motor vehicle engines, power plants and industrial boilers. Ozone "precursors" (NOx and VOCs), as well as ozone itself, can be carried hundreds of miles from their origins, causing air pollution over wide regions.
The strong seasonality of ozone levels makes it possible for areas to limit their ozone monitoring to a certain portion of the year, termed the ozone season. Peak ozone concentrations typically occur during hot, dry, stagnant summertime conditions, i.e., high temperature and strong solar insolation. The length of the ozone season varies from one area of the country to another. April through October is typical, but states in the south and southwest may monitor the entire year. Northern states have shorter ozone seasons, e.g., April through September for Michigan.
Health Effects
The reactivity of ozone causes health problems because it damages lung tissue, reduces lung function and sensitizes the lungs to other irritants. When inhaled, even at low levels, ozone can: cause acute respiratory problems such as shortness of breath, chest pain, wheezing, and coughing; aggravate asthma; cause significant temporary decreases in lung capacity; cause inflammation of lung tissue; lead to hospital admissions and emergency room visits; and impair the body's immune system defenses, making people more susceptible to respiratory illness, including bronchitis and pneumonia. Repeated exposure to ozone pollution for several months may cause permanent structural damage to the lungs.
Because ozone pollution usually forms in hot weather, anyone who spends time outdoors in the summer is at risk, particularly children, moderate exercisers, and outdoor workers. Children are at greatest risk from exposure to ozone because their respiratory systems are still developing and are more susceptible to environmental threats. Children also breathe more air per pound of body weight than adults, thus increasing their exposure. People with existing lung disease, including asthma, chronic bronchitis, and emphysema, are at particular risk from high ozone levels. Since they already suffer from reduced ability to breathe, these individuals are often greatly affected by the increased impairment that can result from exposure to ozone.
Environmental Effects
Ground-level ozone interferes with the ability of plants to produce and store food, so that growth, reproduction and overall plant health are compromised. By weakening sensitive vegetation, ozone makes plants more susceptible to disease, pests and environmental stresses. Ozone can kill or damage leaves so that they fall off the plants too soon or become spotted or brown, thus detrimentally affecting the natural beauty of many areas. The effects of ozone on long-lived species such as trees are believed to add up over many years so that whole forests or ecosystems can be affected. Additionally, ozone has been shown to reduce agricultural yields for many economically important crops such as soybeans, kidney beans, wheat, and cotton.
National Ambient Air Quality Standards
Reducing Transported Ozone
Ozone "precursors," such as NOx emissions, as well as ozone itself, can be carried hundreds of miles from their origins, causing air pollution over wide regions. EPA issued a new regulation in September 1998 to reduce the regional transport of ozone through reductions in nitrogen oxides (NOx). This rulemaking action is commonly referred to as the NOx SIP Call. (What is a SIP?) Click here to learn more about the NOx SIP call and EPA's strategy for reducing transported ozone.
Ozone Action Days
- Ozone Action Days Information: an all-in-one fact sheet explaining ozone, voluntary actions to reduce ozone, health information, the pollution standard index, and the contribution of automobiles
- Oxides of Nitrogen, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. These gases form when fuel is burned at high temperatures. The primary sources of nitrogen oxides are fossil fuel fired electric utilities, motor vehicles and other industrial, commercial and residential combustion sources. Emissions of NOx have increased approximately 10 percent since 1970.
- A Volatile Organic Compound, or VOC, is any organic compound that, when released to the atmosphere, participates in photochemical reactions. There are numerous sources of VOC emissions, including automobiles, gasoline vapors, chemical solvents, and consumer products like paints.
- A State Implementation Plan, or SIP, is a collection of regulations that explain how a State will clean up polluted areas, as required by the Clean Air Act. States must adopt the plan and then submit it to EPA for approval. A SIP adopted by the state government and approved by EPA is legally binding under both state and federal law and may be enforced by either government.