Ozone standard in Minnesota

Levels of ozone in Minnesota are regulated through National Ambient Air Quality Standards set by the U.S. Environmental Protection Agency. The EPA sets distinct standards to protect public health (primary standard) and the environment/welfare (secondary standard) from the negative effects associated with exposure to elevated levels of ozone in the air. The Clean Air Act requires the EPA to evaluate whether the current standards continue to protect public health, the environment, and welfare every five years.

On October 1, 2015, the EPA strengthened the ozone standard to 70 parts per billion (ppb), down from the previous standard of 75 ppb, which was set in 2008. States and tribes with ozone measured above the revised ozone standard must notify EPA by October 2016. Thus, ozone measured in 2013, 2014, 2015, and possibly 2016 will be used to determine whether an area of the state meets the revised federal standard.

For more information on the ozone standard, visit the EPA's ground-level ozone website.

Does Minnesota meet the ozone standard?

Yes, all areas of Minnesota currently meet the strengthened ozone standard of 70 parts per billion (ppb). The map and chart below compares ozone levels in Minnesota to the 2015 and 2008 ozone standards.


Through efforts at the federal, state, and local levels we are continuing to see improvements in ozone pollution levels in Minnesota. However, there is uncertainty as to whether Minnesota will continue to meet the ozone standard in the future. EPA modeling indicates that existing rules that require new air pollution emissions reductions for power plants, vehicles, and other industries should result in continued improvements in ozone pollution levels in Minnesota. However, the impacts of climate change, which may result in hotter summers and more frequent wildfire smoke events, economic growth and increased global impacts, may result in higher ozone levels despite new emission reductions. In addition, as science continues to show health impacts at lower and lower levels of ozone, to protect public health, air quality standards continue to become more stringent over time.

Air monitoring data is used to determine whether an area meets the ozone standard. When the EPA sets an ambient air quality standard, they also establish a method to determine whether an area is meeting (in attainment) or not meeting (in nonattainment) the standard. If a monitor is found to be in nonattainment with the standard, the area of nonattainment includes surrounding areas that have contributed to the violation. Because ozone is formed in the air through chemical reactions from many diverse and often small emissions sources and can travel over many miles, the nonattainment area typically includes an entire metropolitan area, not just the counties in which a violation has been measured. Determining whether monitored data meets the standard or not requires a complicated series of calculations that use several years of data.

To meet the 8-hour ozone standard the 3-year average of the annual 4th-highest daily maximum 8-hour ozone concentration must be less than or equal to the level of the standard. A detailed description of the method used to determine compliance with the ozone standard is detailed in 40 CFR Part 50 Appendix I.

Basic steps to determine compliance with the ozone standard

  • Measure hourly ozone concentrations during the ozone season. In Minnesota, the ozone season runs from April 1 - October 31.
  • Calculate rolling 8-hour ozone concentration averages for all hours measured.
  • Extract the maximum 8-hour rolling average ozone concentration for each day.
  • Rank the daily maximum 8-hour rolling average ozone concentrations for the year.
  • Extract the 4th-highest daily maximum 8-hour rolling average ozone concentration for the year.
  • Repeat the above steps for 3-consecutive years.
  • Average the 4th-highest daily maximum 8-hour rolling average ozone concentration from each of the three years.
  • The resulting value is assigned to the last year included in the 3-year average.
  • A monitoring site meets the standard if the calculated value is less than or equal to the level of the standard.

Determining compliance with the ozone standard requires 3-years of complete ozone monitoring data.

Ozone, and the pollutants that react to create it, can travel long distances. This means that pollution created in Minnesota can contribute to air quality problems in other "downwind" states. Even if all areas of Minnesota comply with the new standard, we may still have responsibilities to cut emissions here in Minnesota to help areas in other states come into compliance with the standard. This is because we may be responsible for a portion of the emissions that contribute to ozone problems in other states and those areas may not be able to come into compliance without our help. These are called "good neighbor requirements."

Determining whether Minnesota contributes to ozone problems in other states requires advanced computer modeling of chemical reactions, meteorology, and movement of pollutants. This type of analysis takes a great deal of effort and time. We therefore will not know for some time whether Minnesota is responsible for ozone violations in other states. If we do contribute to another state's ozone problems, Minnesota will work with its neighbors and the EPA to comply with its good neighbor requirements and develop a plan to reduce necessary emissions.