Ozone is a highly reactive gas that consists of three oxygen atoms - one with a double bond and the other with a single bond. It has the same chemical structure whether it occurs miles above the earth or at ground level. Depending on its location in the atmosphere, ozone is considered either good or bad.

Stratospheric Ozone :   
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. The common phrase, "hole in the ozone," refers to the ozone in this layer. Various refrigerants contain chlorofluorocarbons or hydrochlorofluorocarbons which, once released, migrate into the upper atmosphere where a complex series of chemical reactions occur destroying ozone molecules and thinning the protective ozone layer.

Ground-Level Ozone :   
In the earth's lower atmosphere (also known as the troposphere - the layer of the atmosphere nearest the earth's surface), ground level ozone is considered bad. Ground-level ozone pollution causes human health problems, damages crops and other vegetation, and is a key ingredient of urban smog. Ground level ozone is created by photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) or hydrocarbons in the presence of sunlight. These reactions usually occur during the hot summer months as ultraviolet radiation from the sun initiates a sequence of photolytical reactions. Ground level ozone can also be transported hundreds of miles under favorable meteorological conditions. Ozone levels are often higher in rural areas than in cities due to transport to regions downwind from the actual emissions of ozone forming air pollutants. Shoreline monitors along Lake Michigan often measure high ozone concentrations due to transport from upwind states.

Health Effects: 
Ground-level ozone is unhealthy to breathe. It can narrow a person's airways, forcing their lungs to work harder to provide oxygen to the body. Individuals most susceptible to the effects of ozone exposure include individuals with a pre-existing or chronic respiratory disease, children, and adults who actively exercise or work outdoors. Human exposure to elevated concentrations of ozone can include the following effects:

• Eye irritation.
• Difficulty in breathing, shortness of breath.
• Aggravated/prolonged coughing and chest pain.
• Increased aggravation of asthma.
• Increased susceptibility to respiratory infection resulting in increased hospital admissions and emergency room visits.
• Repeated exposures could result in chronic inflammation and irreversible structural changes in the lungs, which can lead to premature aging of the lungs and illness such as bronchitis and emphysema.
• Growing evidence suggests association with premature death.

Other Effects :   
Ozone also impacts vegetation and materials. Vegetation and forest ecosystem changes that occur include:

• Agricultural crop and forest yield reductions.
• Leaf injury.
• Diminished resistance to pests and disease.
• Reduced tree seedling survival.
• Decreased numbers in species sensitive to ozone.

Other Resources Regarding the Impacts of Ozone  :  

• Ozone Action! In Michigan - Near real-time ambient ozone monitor values (including daily maximum values)
• Ozone Monitoring Network - Location of ozone monitors.
• Ozone - Is It Good or Bad
• National Air Quality Status and Trends: Ozone (O3)-
• Health and Environmental Impacts of Ground-level Ozone
• AIRNow - National and Regional ozone maps

Designation of Ozone Nonattainment Areas:
The ozone National Ambient Air Quality Standard was revised by the EPA on March 12, 2008 to 0.075 ppm and became effective on May 27, 2008.  To attain this 2008 standard, the 3-year average of the 4th highest daily maximum 8-hour average concentration within an area must not exceed 0.075 ppm.

The EPA announced on September 16, 2009 that it would reconsider its 2008 decision setting national standards for gournd-level ozone. EPA is reconsidering the standards to ensure they are clearly grounded in science, protect the public health with an adequate margin of safety, and are sufficienct to protect the environment,  The EPA will propose any needed revisions by December 2009 and issue a final decision by August 2010.  In the interium period of reconsideration the EPA will propose to stay the 2008 standards for the purpose of attainment and nonattainment designations.  The EPA will continue to require permitting of new and modified air pollution sources under the Prevention of Significant Deterioration program for the 2008 ozone standard.

The 1997 ozone National Ambient Air Quality Standard is 0.08 ppm.  To attain this 1997 standard, the 3-year average of the 4th highest daily maximum 8-hour average concentration within an area must not exceed 0.08 ppm. At this time both ozone standards are in effect.

Attainment and nonattainment designations for the 0.08 ppm standard were made in 2004. Twenty-five counties were designated nonattainment.  On May 16, 2007, EPA redesignated 16 counties as attainment.  These counties are Benzie, Mason, Muskegon, Kent, Ottawa, Van Buren, Kalamazoo, Calhoun, Berrien, Cass, Eaton, Ingham, Clinton, Genesee, Lapeer, and Huron. On June 29, 2009, the Southeast Michigan area was redesignated to attainment.  These counties are St. Clair, Livingston, Washtenaw, Lenawee, Monroe, Oakland, Macomb, and Wayne.

One county in Michigan, Allegan, retains an ozone nonattainment designation.  Allegan County is not classified.
The old 1-hour ozone standard was revoked for Michigan on July 15, 2005.

Other Resources Regarding Designations :   
For more information about ozone or for older files, please contact Mary Maupin at 517‑373‑7039, or e-mail: maupinm@michigan.gov

• MDEQ Ozone Designation Recommendations (3/2007)
• EPA's Approval of Ozone Redesignation Request(5/2007)
• 8-hour Ozone Classification Map
• 8-hour Ozone Designation Map
• 8-hour Ozone Counties Table
• Information regarding new source review (NSR) and potential non-attainment issues
• Clean Air Lawn Care
• EPA Brochure: "Ozone - Good Up High Bad Nearby"