About the air quality data

This page provides general information about the air quality data and measures developed by the Minnesota Environmental Public Health Tracking (MN EPHT) Program. For more information about these data, contact MNPH Data Access.

  • How many days or percent of days per year ozone or fine particle (PM2.5) monitors measured levels above the National Ambient Air Quality Standards (NAAQS).
  • The measured average annual concentration of PM2.5.
  • How the frequency of days violating the NAAQS has changed over time.
  • Location of ozone and PM2.5 air monitors in Minnesota.
  • The number of good, moderate, and air alert days measured via the Air Quality Index each year.
  • Explore county-level differences in ambient air quality for the selected pollutants.
  • Communicate to sensitive populations (such as those with asthma) the number of days county residents might be exposed to unhealthy levels of ozone or PM2.5.
  • Look at trends in the frequency of unhealthy levels of ozone and PM2.5.
  • Information about concentrations of other air pollutants of health concern, such as air toxics and mercury.
  • The concentration of air pollutants for short time periods.
  • The precise population or demographic distribution exposed to various pollutant levels.
  • These data cannot be used to estimate personal exposures, since personal exposure depends on a variety of factors such as specific location, time spent outdoors, and physical exertion.
  • Whether or not Minnesota is in regulatory compliance with the NAAQS.
  • The Minnesota Pollution Control Agency measures the level of pollutants in Minnesota's air and reports its findings to the U.S. Environmental Protection Agency (EPA).
  • The data used for MNPH Data Access is retrieved from EPA's Air Quality System (AQS) Database, the national database for air quality data.
  • Air Quality Index results are calculated and provided by the MPCA.

The MPCA monitors PM2.5 pollution at locations across Minnesota using a variety of monitoring techniques. These include:

The MPCA also collects air quality data from monitoring networks operated in collaboration with the EPA. These networks include:

Separate from these networks, a continuous air quality monitor is operated by the Grand Portage Band of Lake Superior Chippewa and is supported in part by the MPCA.

  • The Centers for Disease Control and Prevention's National Environmental Public Health Tracking Network (National Tracking Network) chose to focus on PM2.5 and ozone, as both are widespread air pollutants associated with serious adverse respiratory and cardiovascular health effects.
  • MN EPHT is working with the Minnesota Pollution Control Agency (MPCA) and other stakeholders to develop additional air quality data and measures of interest in Minnesota and nationally.

Modeled air data are predictions, or estimates, of the levels of PM2.5 and ozone in the air. These estimated predictions are applied to areas that do not have air quality monitors and fill in time gaps when monitors may not be recording data. EPA provided the modeled air data for PM2.5 and ozone by statistically combining air monitoring data from the Air Quality System (AQS) Database with data from EPA's Community Multiscale Air Quality (CMAQ) model, using a statistical method called Hierarchical Bayesian modeling. The CDC used the results from EPA's Hierarchical Bayesian modeling to derive the modeled PM2.5 and ozone estimates for each county in the US. Those county estimates are available on the air quality data query page.

The Air Quality Index, or AQI, was developed by the U.S. Environmental Protection Agency (EPA) to provide a uniform way to report daily air quality conditions. Minnesota AQI numbers are determined by hourly measurements of five pollutants, including PM2.5, ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide. Each hour, air pollution levels are ranked as good (green, AQI < 51), moderate (yellow, AQI 51-100), unhealthy for sensitive groups (orange, AQI 101-150), unhealthy (red, AQI 151-200), or very unhealthy (purple, AQI 201-300). The MPCA issues an air quality advisory when the AQI is approaching the orange category (AQI 90-100), while air quality alerts are issued when the AQI is expected to climb to orange levels or worse (>100).

Annual average concentrations of PM2.5 by county:

  • To calculate the annual average PM2.5 concentration, daily concentration data from PM2.5 monitors are averaged by calendar quarter, and the average of the quarters determines the annual average. In counties with more than one monitor, the monitor with the maximum annual mean determines the reported annual average for the entire county.
  • The completeness of the data used to calculate the measures is tested to ensure that data are available for at least eleven monitoring days in a quarter. To calculate a valid annual average, all four quarters must meet the completeness requirement. If the completeness requirement is not met, an annual average is not reported for the county.
  • The completeness test used by MNPH Data Access is more stringent than that used by the National Tracking Network. As a result, the National Tracking portal may report annual averages for counties that are not reported through MNPH Data Access.

Annual percent of monitored days and number of person days above the National Ambient Air Quality Standard (NAAQS) for PM2.5:

  • The monitoring data used to calculate the PM2.5 measures are generally collected on a 1 in 3 day schedule. Reporting the number of days above the NAAQS based on this monitoring schedule would likely underestimate the actual number of days above the NAAQS. As a result, the National Tracking Network reports the annual percent of monitored days above the NAAQS.
  • The annual percent of monitored days above the NAAQS is calculated by dividing the number of daily exceedances over the year by the total number of days monitored, resulting in an ‘exceedance fraction.' Then the exceedance fraction is multiplied times 100 to make the number a percentage. The daily dataset is initially created by retaining the maximum concentration among all monitors within the county for each monitored day. So data from all monitors in a county are examined for this calculation.
  • The number of person-days above the NAAQS is calculated by multiplying the 'exceedance fraction' times 365 to get annual days, then this result is multiplied by the total county population.
  • The county population estimate data for years 2000-2009 are based on U.S. Census Bureau population estimates updated in 2009. The county population data for years 2010 to 2014 are based on U.S. Census Bureau 2010 data.

Number of days and person days with maximum 8-hour ozone concentration above the NAAQS:

  • For calculating Number of Ozone days above NAAQS, any monitored day between April through October with an 8-hour average ozone concentration above 70 parts per billion (ppb) is counted as a day above the NAAQS. In counties with more than one monitor, the monitor with the daily maximum value is used. Number of days with maximum 8-hour greater than 70 ppb are added up and reported.
  • For each county with ozone monitors, the Number of Person-Days with Ozone levels above NAAQS is calculated by using the daily maximum value from all monitors in a county, adding up the number of days over 70 ppb then multiplying that result times the total county population.
  • The completeness of the data used to calculate the measures is tested to ensure that data are available for at least 75% of required monitoring days during the ozone season. The ozone season in Minnesota is March through October.
  • The county population estimate data for years 2000-2009 are based on U.S. Census Bureau population estimates updated in 2009. The county population data for years 2010 to 2014 are based on U.S. Census Bureau 2010 data.

Number of days in each Air Quality Index category:

  • This measure is an annual count of the number of days in each AQI category (good, moderate, or alert days) for available AQI reporting areas).
  • This measure is based on ozone and PM2.5 AQI values. The pollutant with the highest AQI is used to represent the overall index for the day.
  • The AQI is calculated by converting measured pollutant concentrations to a uniform index which is based on peer-reviewed scientific evidence of the health effects associated with a pollutant. For more information on how the AQI is calculated, visit the MPCA's AQI about the data page

We use "person-days" to estimate the exposure to high levels of PM2.5 or ozone at the county level. The number of "person-days above the standard" is calculated:

  1. for PM2.5: the number of days that were above the PM2.5 NAAQS for a particular county are divided by the total number of monitoring days, then multiplied times 365 to get annual days, then multiplied by the total county population. 
  2. For ozone: the number of people in that county.

For these reasons, person-days are highly influenced by county populations. If two counties have the same number of days above the NAAQS, but one of the counties has a much larger population, then that county will have more person-days above the NAAQS as well.

  • A monitoring site meets the regulatory requirements of the annual PM2.5 NAAQS if the 3-year average of the annual average PM2.5 concentration is less than or equal to 12 micrograms per cubic meter (μg/m3).
  • A monitoring site meets the regulatory requirements of the daily PM2.5 NAAQS if the 98th percentile of the 24-hour PM2.5 concentrations in a year, averaged over three years, is less than or equal to 35 μg/m3.
  • A monitoring site meets the regulatory requirements of the ozone NAAQS if the 3-year average of the fourth highest daily maximum 8-hour concentrations is less than or equal to 70 ppb.
  • While the measures highlight concentrations exceeding the NAAQS, pollutant concentrations below the standard may still result in adverse health effects, particularly for susceptible populations.
  • The measures calculated before 2012 include only PM2.5 data collected using the Federal Reference Method. They do not currently use other available monitoring data such as continuous monitoring measurements.
  • MNPH Data Access' air quality measures may differ from data reported by other sources because of alternative data analysis techniques.
  • Monitored air quality data do not represent actual exposure. Therefore, monitored air quality data cannot be used to determine individual exposure levels.

To learn more about regulatory and monitoring requirements related to air quality, contact the Minnesota Pollution Control Agency (MPCA). For more information about the data and measures developed by the MN Environmental Public Health Tracking Program, contact MN Data Access.