EnviroNearby

Union County, South Dakota air quality history

Daily Air Quality Index summaries from EPA monitors in Union, South Dakota, 2009–2021.

66%
Good days in 2021
44
Median daily AQI, 2021
4
Days above 100 in 2021
273
Days with an AQI in 2021 (of 365)

In 2021, EPA reported a daily Air Quality Index (AQI) for Union, South Dakota on 273 days. 179 of those days (66%) were in the Good range, and the median daily AQI was 44. That median is higher than the typical monitored county in South Dakota (median 39 across 10 counties), and higher than the typical monitored U.S. county (41). The main pollutant was ozone on 155 days. Across the years with fuller monitoring, the share of Good days rose: an average of 56% in 2009–2011 versus 78% in 2019–2021.

EPA did not report an AQI for every day of 2021 here (273 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

The most recent year with data for this county is 2021; EPA's files for 2022–2025 contain no row for it, which usually means its monitors stopped reporting.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%2009: Good 221 of 365 days2009: Moderate 143 of 365 days2009: Unhealthy for Sensitive Groups 1 of 365 days20092010: Good 207 of 365 days2010: Moderate 156 of 365 days2010: Unhealthy for Sensitive Groups 2 of 365 days20102011: Good 188 of 365 days2011: Moderate 177 of 365 days20112012: Good 152 of 366 days2012: Moderate 211 of 366 days2012: Unhealthy for Sensitive Groups 3 of 366 days20122013: Good 170 of 365 days2013: Moderate 194 of 365 days2013: Unhealthy for Sensitive Groups 1 of 365 days20132014: Good 244 of 365 days2014: Moderate 121 of 365 days20142015: Good 243 of 365 days2015: Moderate 120 of 365 days2015: Unhealthy for Sensitive Groups 2 of 365 days20152016: Good 295 of 366 days2016: Moderate 69 of 366 days2016: Unhealthy for Sensitive Groups 2 of 366 days20162017: Good 278 of 365 days2017: Moderate 87 of 365 days20172018: Good 256 of 365 days2018: Moderate 106 of 365 days2018: Unhealthy for Sensitive Groups 3 of 365 days20182019: Good 306 of 364 days2019: Moderate 58 of 364 days20192020: Good 304 of 366 days2020: Moderate 61 of 366 days2020: Unhealthy for Sensitive Groups 1 of 366 days20202021: Good 179 of 273 days2021: Moderate 90 of 273 days2021: Unhealthy for Sensitive Groups 2 of 273 days2021: Unhealthy 2 of 273 days2021
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501002009: 90th percentile AQI 642010: 90th percentile AQI 732011: 90th percentile AQI 662012: 90th percentile AQI 712013: 90th percentile AQI 682014: 90th percentile AQI 612015: 90th percentile AQI 612016: 90th percentile AQI 572017: 90th percentile AQI 582018: 90th percentile AQI 622019: 90th percentile AQI 562020: 90th percentile AQI 562021: 90th percentile AQI 642009: Median AQI 452010: Median AQI 472011: Median AQI 492012: Median AQI 522013: Median AQI 512014: Median AQI 432015: Median AQI 432016: Median AQI 382017: Median AQI 412018: Median AQI 432019: Median AQI 372020: Median AQI 392021: Median AQI 442009201020112012201320142015201620172018201920202021
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2021

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Ozone15557%
Fine particles (PM2.5)11743%
Coarse particles (PM10)1<1%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20212731799022004464167Ozone
20203663046110003956141Ozone
2019364306580000375697Ozone
201836525610630004362112Ozone
20173652788700004158100Ozone
20163662956920003857112Ozone
201536524312020004361108PM2.5
201436524412100004361100PM2.5
201336517019410005168114PM2.5
201236615221130005271111PM2.5
201136518817700004966100PM2.5
201036520715620004773127PM2.5
200936522114310004564117PM2.5

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

All monitored counties in South Dakota

Source: U.S. EPA AirData, “Annual AQI by County” files for 2009–2021 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem