Is the Air Quality Good in Jefferson, Colorado?
No — air quality in Jefferson, Colorado is below the U.S. average. The city earns a Grade of D (poor) on a 5-year median AQI of 47 (Good), with 147 unhealthy-air days over five years (about 29 per year). Residents with asthma, heart disease, or young children should treat daily AQI forecasts as a real input.
Who Can Safely Breathe the Air in Jefferson, Colorado?
Treat daily AQI forecasts as essential input. On flagged days, sensitive groups (asthma, COPD, heart disease, pregnancy, young children, older adults) should limit outdoor exertion and keep windows closed. A HEPA air cleaner sized to a bedroom or family room can cut indoor PM2.5 by 80%+ during smoke or pollution events. Because ozone peaks in the afternoon on hot sunny days, plan outdoor exercise for early morning or after sunset on bad-air days.
Across the past five years, this area has logged 147 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 29 days per year, or roughly one every five to seven days. That is well above the national norm and explains the D grade.
Jefferson, Colorado Air Quality Snapshot
| Air Quality Grade | D46/100 |
| 5-Year Median AQI | 47 (Good) |
| Most Recent Median AQI (2023) | 49 (Good) |
| Dominant Pollutant | Ground-Level Ozone |
| 10-Year Trend | Stable (+0.26 AQI/yr) |
| Unhealthy Days (last 5 yr) | 147 |
| National Rank (cleanest = #1) | #847 of 1,020 (83th most polluted percentile) |
| Colorado Rank | #22 of 32 |
What Does the D Grade Mean?
Jefferson, Colorado earns a D — air quality falls below the U.S. average, with a 5-year median AQI of 47. Residents with asthma, COPD, heart disease, or young children should watch daily AQI forecasts and limit outdoor exertion when alerts go out.
Jefferson, Colorado's 5-year median AQI of 47 is 6 points above the national average of 41 — meaningfully more polluted than the typical U.S. metro tracked here. Within Colorado, Jefferson, Colorado runs more polluted than the state average of 39 — local sources or geography are concentrating pollution above the state's typical reading.
For context within Colorado: Alamosa, Colorado currently holds the state's cleanest grade (A, AQI 14), while Weld, Colorado sits at the bottom (D, AQI 53).
What's in Jefferson, Colorado's Air?
The dominant pollutant in Jefferson, Colorado is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle exhaust and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs, triggers asthma attacks, and reduces lung function — even healthy adults can feel chest tightness and shortness of breath after exercising in elevated ozone.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Ground-Level Ozone | 363 | 99% |
| Nitrogen Dioxide | 2 | 1% |
Is the Air Getting Better or Worse?
Air quality in Jefferson, Colorado has held roughly steady over the past decade, with year-to-year shifts in median AQI of less than half a point. That stability makes the city's long-run grade a reliable signal of what residents can expect day-to-day.
In 2014, Jefferson, Colorado posted a median AQI of 46. By 2023 that figure was 49 — a rise of 3 AQI points dirtier across 10 years of EPA records.
Year-by-Year AQI in Jefferson, Colorado
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 46 | 228 | 16 | Ozone |
| 2015 | 45 | 244 | 17 | Ozone |
| 2016 | 46 | 245 | 20 | Ozone |
| 2017 | 47 | 214 | 28 | Ozone |
| 2018 | 46 | 229 | 37 | Ozone |
| 2019 | 46 | 238 | 16 | Ozone |
| 2020 | 46 | 231 | 27 | Ozone |
| 2021 | 47 | 222 | 55 | Ozone |
| 2022 | 47 | 234 | 31 | Ozone |
| 2023 | 49 | 206 | 18 | Ozone |
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
More about Jefferson, Colorado
Jefferson, Colorado has an Air Quality Grade of D (poor) with a 5-year median AQI of 47. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
The data source behind this answer is the EPA Air Quality System (AQS). Every figure on the page traces back to that source; the methodology page describes the inputs and the refresh cadence in full detail.
For readers turning this answer into action: cross-reference against the underlying the EPA Air Quality System (AQS) record before acting on time-sensitive decisions. The site renders the data as it was published; subsequent revisions can shift the picture, and the live federal data is always the authoritative current reference.
Source: EPA Outdoor Air Quality Data, 2026.