Ozone Data

The maps have been populated using 2013 EPA design values for all counties reporting sufficient annual ozone measurement data for years 2011, 2012, and 2013. Alaska and Hawaii are omitted because each state reported data for only one county.  In each case the reported ozone level was below the most stringent potential standard of 60 ppb.

Design Values

The EPA continually monitors county-level ozone concentrations and reports a “fourth daily max 8-hour” concentration each year. Using this data, the EPA calculates a three-year average ozone level for each county, which is referred to as the “design value”.  Design values serve as the analytical basis for evaluating whether a county or area is meeting the current ozone standard. For more information on design values, please see: http://www.epa.gov/airtrends/values.html.

Methodology

Counties shown in red have 2013 design values above the current 75 ppb standard. Counties shown in yellow have 2013 design values above the potential 70, 65, or 60 ppb standard. Thus, assuming that ozone levels remain unchanged, counties in yellow are said to be “at risk” of violating the revised ozone standard.

Non-Attainment Designation

Counties with design values that exceed the ozone standard are considered “in violation” of the standard. This is not the same as saying that a county is in “non-attainment”. Rather, non-attainment is a formal, legal designation in which the EPA places a county after a lengthy reporting, review, and investigation process. Thus, a county could have a design value that is higher than the standard, but not yet be formally designated as a non-attainment area. Conversely, even if a county that previously received a non-attainment designation has a design value that indicates ozone levels have come into compliance with the standard, it may retain its non-attainment designation for several more years.

Contextual Data

(1) U.S. population data is from Census; (2) Employment data is from the Bureau of Economic Analysis.