Watertown, SD Air Quality
EPA monitor data through 2025 · Updated September 02, 2026 · Methods
In 2025, Watertown, SD recorded 4 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 364 days with monitoring data. The rest of the year was 284 Good days and 76 Moderate days. Most of the bad days were driven by fine particles. Under the AQI rules that applied before 2013, the same air would have scored 3 days.
Unhealthy days by year, scored two ways
EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those changes is not comparable to one published after. Each panel below applies a single fixed rulebook to every year, so each is internally consistent. Why this matters.
Scored by today's rules
Scored by the pre-2013 rules
What 2025 looked like
| Air quality category | Days | Share of days with an AQI |
|---|---|---|
| Good | 284 | 78% |
| Moderate | 76 | 21% |
| Unhealthy for sensitive groups | 3 | 1% |
| Unhealthy or worse | 1 | 0% |
Median AQI 36 · 90th-percentile AQI 58 · worst day 154. Scored under current rules, out of 364 days with an AQI.
What drove the unhealthy days
Of the 4 days above 100 in 2025, 0 were driven by ozone and 4 by fine particles. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke.
The unhealthy days in 2025
Every day the index passed 100, what it reached, and which pollutant drove it. 1 of these 4 would not have counted as unhealthy under the pre-2013 rules.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| May 31 | 139 | Fine particles | 121 |
| Jun 1 | 154 | Fine particles | 138 |
| Jul 31 | 108 | Fine particles | 96 (not unhealthy) |
| Aug 1 | 126 | Fine particles | 111 |
Statewide context
Watertown, SD sits in South Dakota.
Days in each air quality category, by year
How many days fell in each AQI category, for every year with an AQI on record. Categories are scored under EPA's current breakpoints. A year with fewer than 300 days is marked, because a short year lowers every count for reasons that have nothing to do with the air.
| Year | Good | Moderate | Unhealthy for sensitive groups | Unhealthy or worse | Days with an AQI |
|---|---|---|---|---|---|
| 2003 (partial) | 66 | 55 | 0 | 0 | 121 |
| 2004 (partial) | 62 | 59 | 1 | 0 | 122 |
| 2005 (partial) | 66 | 57 | 0 | 0 | 123 |
| 2006 | 275 | 65 | 0 | 0 | 340 |
| 2007 | 307 | 53 | 0 | 0 | 360 |
| 2008 | 296 | 65 | 0 | 1 | 362 |
| 2009 | 317 | 45 | 1 | 0 | 363 |
| 2010 | 310 | 54 | 0 | 0 | 364 |
| 2011 | 311 | 44 | 1 | 0 | 356 |
| 2012 | 151 | 206 | 0 | 0 | 357 |
| 2013 | 198 | 163 | 0 | 0 | 361 |
| 2014 | 327 | 38 | 0 | 0 | 365 |
| 2015 | 276 | 79 | 1 | 0 | 356 |
| 2016 | 295 | 71 | 0 | 0 | 366 |
| 2017 | 276 | 88 | 0 | 0 | 364 |
| 2018 | 260 | 104 | 1 | 0 | 365 |
| 2019 | 295 | 66 | 0 | 0 | 361 |
| 2020 | 293 | 72 | 1 | 0 | 366 |
| 2021 | 246 | 113 | 3 | 3 | 365 |
| 2022 | 285 | 79 | 1 | 0 | 365 |
| 2023 | 215 | 136 | 12 | 1 | 364 |
| 2024 | 270 | 91 | 1 | 0 | 362 |
| 2025 | 284 | 76 | 3 | 1 | 364 |
Good is AQI 0–50, Moderate 51–100, Unhealthy for sensitive groups 101–150. “Unhealthy or worse” combines the Unhealthy, Very Unhealthy and Hazardous categories, which are shown together because most areas record too few of each to report separately. The four columns sum to the last one by construction. These counts exclude sulphur dioxide, following EPA, which drops SO₂ from county and metro summaries because it is too localized to describe a whole area.
The full series
The same years scored two ways, with the pollutant that drove the days above 100. EPA's files record which pollutant set each day's AQI; they do not attribute days to wildfire smoke, so no column here claims to.
| Year | Unhealthy days, current rules | Of which ozone | Of which fine particles | Pre-2013 rules | Days monitored |
|---|---|---|---|---|---|
| 2003 | 0 | 0 | 0 | 0 | 121 |
| 2004 | 1 | 0 | 1 | 0 | 122 |
| 2005 | 0 | 0 | 0 | 0 | 123 |
| 2006 | 0 | 0 | 0 | 0 | 340 |
| 2007 | 0 | 0 | 0 | 0 | 360 |
| 2008 | 1 | 0 | 1 | 1 | 362 |
| 2009 | 1 | 0 | 1 | 0 | 363 |
| 2010 | 0 | 0 | 0 | 0 | 364 |
| 2011 | 1 | 0 | 0 | 1 | 356 |
| 2012 | 0 | 0 | 0 | 0 | 357 |
| 2013 | 0 | 0 | 0 | 0 | 361 |
| 2014 | 0 | 0 | 0 | 0 | 365 |
| 2015 | 1 | 0 | 1 | 1 | 356 |
| 2016 | 0 | 0 | 0 | 0 | 366 |
| 2017 | 0 | 0 | 0 | 0 | 364 |
| 2018 | 1 | 0 | 1 | 0 | 365 |
| 2019 | 0 | 0 | 0 | 0 | 361 |
| 2020 | 1 | 0 | 0 | 1 | 366 |
| 2021 | 6 | 1 | 5 | 4 | 365 |
| 2022 | 1 | 1 | 0 | 0 | 365 |
| 2023 | 13 | 9 | 4 | 5 | 364 |
| 2024 | 1 | 0 | 0 | 1 | 362 |
| 2025 | 4 | 0 | 4 | 3 | 364 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.