Ozone is a gas composed of three oxygen atoms.   This gas is created at ground-level by a chemical reaction between oxides of nitrogen and volatile organic compounds (VOC) in the presence of sunlight.  Ozone maintains the same chemical structure at miles above the earth or at ground-level.  It is categorized as “good” or “bad” depending on its location in the atmosphere.

Ozone found at the lower atmosphere and which is referred as “ground-level ozone” is considered as “bad” ozone.  Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents and other natural sources emit nitrogen oxide (NOx) and VOC leading to the formation of ozone.  Ground-level ozone is the primary constituent of smog.  Sunlight and hot weather contribute to the formation of harmful concentrations of ground-level ozone in the air.  Many urban areas have high levels of “bad” ozone.  Certain rural areas tend to have high “bad” ozone levels because wind carries ozone and pollutants that form it hundreds of miles away from their original sources.

Breathing ozone triggers a variety of health problems including chest pain, coughing, throat irritation, and congestion.  It can worsen bronchitis, emphysema, and asthma.  Ground-level ozone can also reduce lung function and inflame the linings of the lungs.  Repeated exposure may permanently scar lung tissue.

Ground level ozone not only poses a threat to human and animal life, it can also cause significant harm to sensitive vegetation and ecosystems.  The Clean Air Act requires Environmental Protection Agency (EPA) to set air quality standards to protect both public health and public welfare.

The Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for ground-level ozone and five other pollutants.  The NAAQS ensure the quality of outdoor air through out the country.  The Clean Air Act established two types of national air quality standards for ground-level ozone: the ‘primary’ and ‘secondary’ standards.  Primary standards set limits to protect public health, including the health of “sensitive” populations such as asthmatics, children, and the elderly; while secondary standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings.

The EPA is required under the Clean Air Act to review the latest scientific information and standards every five years to improve the outdoor air quality.  Before establishing new standards, policy decisions undergo rigorous review by the scientific community, industry, public interest groups, the general public and the Clean Air Scientific Advisory Committee (CASAC).

Throughout the country, additional programs are being employed to cut NOx and VOC emissions from vehicles, industrial facilities, and electric utilities.  These programs are also aimed at reducing pollution by reformulating fuels and consumer/commercial products, such as paints and chemical solvents that contain VOC.  Voluntary and innovative programs also encourage communities to adopt practices, such as carpooling, to reduce harmful emissions.

Under the Clean Air Act, the EPA has set protective health-based standards for ozone in the outdoor air.  The EPA and others also institute a variety of multi-faceted programs to meet these health-based standards.  The EPA also works with other Federal, State, local and tribal air quality agencies to achieve reductions in emissions of toxic air pollutants.  The National Air Toxics Assessments (NATA) is the EPA’s ongoing comprehensive evaluation of air toxics in the U.S.

The EPA formulates national and regional rules to reduce emissions of pollutants that form ground-level ozone.  These rules help the state and local governments to meet the Agency’s national air quality standards.  Some examples of the rules and programs are:

The 2005 Clean Air Interstate Rule (CAIR) by EPA aims at the reduction of ground-level ozone in the east by permanently capping emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx).  The Clean Air Visibility Rule (2005) requires emission controls for industrial facilities emitting air pollutants that reduce visibility.  The Regional Transport Rule (1998) reduces regional emissions of nitrogen oxides (NOx) in 22 states and the District of Columbia, thereby reducing the regional transport of ozone.  The Acid Rain Program employs a combination of traditional requirements and a market-based cap and trade program to reduce power plant emissions of nitrogen oxides (NOx) and sulfur dioxide (SO2), which contribute to ground-level ozone formation.  NOx SIP Call, another EPA program reduces the regional transport of ground-level ozone pollution in the East.  Then there is the 2004 Clean Air Nonroad Diesel Rule that sets emission standards for engines used in most construction, agricultural, and industrial equipment, and it also reduces the quantity of sulfur allowed in the fuel they use.  The 2007 Clean Diesel Trucks and Buses Rule (2000), that aims at building a fleet of vehicles that will be 95 percent cleaner than today’s trucks and buses.  Tier 2 Vehicle Emission Standards and Gasoline Sulfur Program, that treats all passenger vehicles and the fuels they use as a system, it also sets tailpipe emissions standards for all passenger vehicle beginning with the 2004 model year, and requires reduced levels of sulfur in gasoline they use.  Emissions standards are fixed for highway motorcycles along with standards for engines that power forklifts, electric generators, recreational boat engines, snowmobiles, all-terrain vehicles and offroad motorbikes.  EPA also intents to propose more stringent standards modeled after the Clean Air Nonroad Diesel Engines Program for locomotives and for all new commercial, recreational, and auxiliary marine diesel engines except the very large engines used for propulsion on deep-sea vessels.

The EPA also formulates several voluntary programs that are vital in reducing ground-level ozone formation such as the National Clean Diesel Campaign.  The voluntary aspects of this campaign reduce ground-level ozone pollution through programs such as the Voluntary Diesel Retrofit Program, the Smartway Transport Partnership and Clean School Buses USA.

Under the Clean Air Act, the EPA sets and reviews national air quality standards for ozone.  It tracks the progress made in reducing ground-level ozone in the ambient air by analyzing the national and local trends in ozone levels.  Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for ozone.  Air quality monitors measure concentrations of ozone throughout the country.  The EPA, state, tribal and local agencies use that data to ensure that ozone is at levels that protect public health and the environment.  Nationally, average ozone levels showed a notable decline after 2002.

Inside Ozone