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About Air Quality Data

about air quality

Tracking Outdoor Air Quality involves collecting monitoring data on specific pollutants, and modeled predictions of air quality in areas that do not have monitors.

This page provides general information about air quality data and measures developed by the Iowa Public Health Tracking program. Contact Us for more information about these data.

Information on this page:

What do these data tell us?

  • The average annual concentration of PM 2.5.
  • How often levels above the National Ambient Air Quality Standards (NAAQS) are measured.
    • Number of days for ozone
    • Percent of days for fine particulate matter (PM 2.5)

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How we can use these data?

  • 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 PM 2.5.
  • Look at trends in the frequency of unhealthy levels of ozone and PM 2.5.

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What these data cannot tell us?

  • 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 Iowa is in regulatory compliance with the NAAQS.

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The source of the data:

Monitored Air Quality Data

  • The Iowa Air Monitoring Network measures the level of pollutants in Iowa's air and reports its findings to the U.S. Environmental Protection Agency (EPA).
  • The data used for Iowa Public Health Tracking portal is retrieved from EPA's Air Quality System (AQS) Database, the national database for air quality data.

Modeled Air Quality Data

CDC and EPA have worked together to develop a statistical model to make predictions of air quality available in areas of the country that do not have monitors and to fill in the time gaps when monitors may not be recording data. There are two primary benefits to creating modeled air pollution data:

  • Approximately 20% of counties in the United States (16 of 99 counties in Iowa) have actual air monitors. With modeled data, the Tracking Network is able to create indicators for counties that do not have monitors;
  • Most PM 2.5 air monitors take samples every three days and many ozone monitors sample only during the ozone season. Modeled data helps to fill in these time gaps.

After careful study, EPA and CDC found that air pollution modeled predictions are very similar to actual monitor data in areas where the two can be compared. In some areas, the modeled data underestimates or overestimates the air pollutant concentration levels when compared to AQS monitoring data. Therefore, the best way to use modeled air data is in conjunction with actual monitoring data.

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Why the data are focused on PM 2.5 and ozone:

The Centers for Disease Control and Prevention's National Environmental Public Health Tracking Network (National Tracking Network) chose to focus on PM 2.5 and ozone, as both are widespread air pollutants associated with serious adverse respiratory and cardiovascular health effects.

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What time period of data is available?

  • The Iowa Public Health Tracking portal displays monitored and modeled air quality data from the year 2000 through the most recent year of data available.
  • Monitored measures are published on the IPHT portal annually following the release of population estimates. This creates a six-month lag before monitored air quality data are available.
  • Modeled measures are published on the IPHT portal when a new year of modeled data is released by the CDC. Currently the most recent year of modeled data is 2012.

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How the measures are calculated:

Annual average concentrations of PM 2.5 by county:

  • To calculate the annual average PM 2.5 concentration, daily concentration data from PM 2.5 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.

Annual percent of monitored days and number of person days above the National Ambient Air Quality Standard (NAAQS) for PM 2.5:

  • The monitoring data used to calculate the PM 2.5 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 Iowa 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.

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What are "person-days"?

We use "person-days" to estimate the exposure to high levels of PM 2.5 or ozone at the county level. The number of "person-days above the standard" is calculated:

  1. for PM 2.5: the number of days that were above the PM 2.5 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, and one of the counties has a much larger population, then that county will have more person-days above the NAAQS as well.

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What are the limitations of the data?

  • 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 PM 2.5 data collected using the Federal Reference Method. They do not currently use other available monitoring data such as continuous monitoring measurements.
  • Monitored air quality data do not represent actual exposure. Therefore, monitored air quality data cannot be used to determine individual exposure levels.

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Where can I find more technical information about the data?

More detailed documentation of the air quality data can be found in the following documents:

Indicator Profiles

Annual Average Particulate Matter (PM 2.5) Levels pdf
PM 2.5 Days Above Regulatory Standard pdf
Ozone Days Above Regulatory Standard pdf

Descriptive Metadata

Monitored Air Quality Data pdf
Modeled Air Quality Data pdf

Contact Us to learn more about air quality data, or any of the measures displayed on the Iowa Public Health Tracking Portal.

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