Move workouts to the morning
Ozone peaks on hot, sunny afternoons. Mornings are usually cleaner.
Ozone guide →Air quality in Boulder County, CO
Includes Boulder and Longmont · EPA monitor readings, 2021 to 2025 · 331,000 people
For today's air and tomorrow's forecast, check AirNow by ZIP code.
Bad-air days a year
12
days with an AQI over 100
About 1 day in 30. In the early 2000s it was about 9 a year. Boulder County has the 8th most bad-air days of the 24 Colorado counties with monitors.
1day unhealthy for everyoneAQI over 150, a year
49AQI on a typical dayGood: fine for everyone
Jul to Sepworst monthsOzone on hot afternoons
181worst day since 2016May 19, 2023, fine particles
How to read the AQIThe EPA's air score runs from 0 to 500. Higher is worse. Over 100 counts as a bad-air day.
Typical day 49 (Good)Worst day 181 (Unhealthy)
Good0 to 50Moderate51 to 100Sensitive groups101 to 150Unhealthy151 to 200Very unhealthy201 to 300Hazardous301 to 500
When bad air happens in Boulder County
Average bad-air days in each month, from 10 years of daily readings since 2016. Most fall in July, August and September.
Jan0
Feb0.1
Mar0.5
Apr0.2
May0.6
Jun1.1
Jul4.3
Aug4.1
Sep1.3
Oct0.5
Nov0
Dec0
Fine particles (smoke, soot)Ozone (smog)Bars stack in that order, from the bottom.
See the numbers by month
| Month | Ozone | Fine particles | All |
|---|---|---|---|
| January | 0 | 0 | 0 |
| February | 0 | 0.1 | 0.1 |
| March | 0 | 0.5 | 0.5 |
| April | 0.2 | 0 | 0.2 |
| May | 0.4 | 0.2 | 0.6 |
| June | 1.1 | 0 | 1.1 |
| July | 4.3 | 0 | 4.3 |
| August | 3.3 | 0.8 | 4.1 |
| September | 0.7 | 0.6 | 1.3 |
| October | 0 | 0.5 | 0.5 |
| November | 0 | 0 | 0 |
| December | 0 | 0 | 0 |
The worst day since 2016 was May 19, 2023: AQI 181, from fine particles.
What causes it
The pollutant that pushed the AQI over 100 on each of the 127 bad-air days since 2016.
79%Ozone: smog that forms in sunlight on hot days
21%Fine particles: smoke and soot
Most bad days here are ozone days: smog that builds on hot, sunny afternoons.
CDC estimates by month, 2001 to 2022
The CDC's model counts bad-air days at the county's worst spot, from monitors and satellites. Particles (brown) fill the inside of the ring and ozone (blue) the outside.
See the numbers by month
| Month | Fine particles | Ozone |
|---|---|---|
| January | 0 | 0 |
| February | 0.2 | 0 |
| March | 0.2 | 0 |
| April | 0 | 0.2 |
| May | 0 | 0.9 |
| June | 0 | 4.9 |
| July | 0 | 8.0 |
| August | 0.3 | 4.4 |
| September | 0.3 | 0.6 |
| October | 0.1 | 0 |
| November | 0 | 0 |
| December | 0 | 0 |
What helps in Boulder County
Most bad days here are ozone, which builds on hot afternoons. Timing helps most; filters don't remove ozone.
Check tomorrow's forecast
AirNow posts an air quality forecast by ZIP code, usually the day before.
AirNow →Keep one room clean on smoky days
Shut the windows and run an air purifier sized for the room.
Choosing a purifier →For today's reading and tomorrow's forecast, check AirNow by ZIP code. Its fire and smoke map adds smoke plumes.
Parents: what schools do at each AQI level, and how to plan for smoke and ozone days.
Purifiers with a verified smoke rating
For a clean room on smoky days. Each has a smoke CADR tested by AHAM's independent program and is certified by California as a mechanical cleaner, so it makes no ozone.
Levoit Vital 100S-P
143 smoke CADR
- Everyday
- up to 222 sq ft
- Heavy smoke
- up to 143 sq ft
- Filters
- about $40 a year
The cheapest unit we found with a verified smoke rating. Enough for a bedroom clean room.
$119.99 at LevoitLevoit Vital 200S-P
250 smoke CADR
- Everyday
- up to 388 sq ft
- Heavy smoke
- up to 250 sq ft
- Filters
- about $50 a year
The most verified smoke cleaning per dollar of any qualifying model: 75% more than the 100S-P for $50 more.
$169.99 at LevoitLevoit Core 600S-P
391 smoke CADR
- Everyday
- up to 606 sq ft
- Heavy smoke
- up to 391 sq ft
- Filters
- $60 to $120 a year
For a big room or a small apartment. Buy the LAP-C601S-WUS version, which CARB lists as mechanical.
$299.99 at LevoitWhy all 3 are Levoit: of the models we checked that pass both tests, these give the most verified smoke cleaning per dollar at each room size. Medify's MA-25 and MA-40 also pass but cost more for the same cleaning.
Ratings from the AHAM Verifide directory and CARB's certified list. Prices from Levoit's site, checked November 7, 2026; they change. We don't earn anything from these links. How to pick a purifier
Every year since 2000
Bad-air days each year. The striped part is days unhealthy for everyone, over AQI 150. The worst recent year was 2021, with 35 bad-air days.
Unhealthy for sensitive groups (101 to 150)Unhealthy or worse (over 150)
See the numbers by year
| Year | Bad-air days | Over 150 |
|---|---|---|
| 2000 | 5 | 0 |
| 2001 | 5 | 0 |
| 2002 | 12 | 1 |
| 2003 | 23 | 1 |
| 2004 | 0 | 0 |
| 2005 | 8 | 0 |
| 2006 | 19 | 3 |
| 2007 | 22 | 3 |
| 2008 | 16 | 0 |
| 2009 | 10 | 0 |
| 2010 | 12 | 0 |
| 2011 | 16 | 0 |
| 2012 | 35 | 4 |
| 2013 | 16 | 1 |
| 2014 | 5 | 0 |
| 2015 | 7 | 0 |
| 2016 | 0 | 0 |
| 2017 | 11 | 0 |
| 2018 | 29 | 1 |
| 2019 | 5 | 0 |
| 2020 | 21 | 2 |
| 2021 | 35 | 3 |
| 2022 | 7 | 0 |
| 2023 | 6 | 2 |
| 2024 | 10 | 1 |
| 2025 | 3 | 0 |
Boulder County and its neighbors
Bad-air days a year, 2021 to 2025. Colorado counties with monitors averaged about 15 bad-air days a year, weighted by population.
Questions people ask about air in Boulder County
How often is the air bad in Boulder County?
From 2021 to 2025, monitors in Boulder County recorded about 12 bad-air days a year with an AQI above 100, which is unhealthy for sensitive groups or worse. The worst year in that stretch was 2021, with 35 bad-air days.
What causes bad air in Boulder County?
Most bad days here are ozone days: smog that builds on hot, sunny afternoons. Bad days cluster in July, August and September.
Where can I see today's air quality in Boulder County?
AirNow, run by the EPA and its partners, shows the current AQI and tomorrow's forecast for your ZIP code. Its fire and smoke map adds smoke plumes. On a typical day the AQI in Boulder County is about 49, which is good, fine for everyone. Check it most in July, August and September, when bad days here cluster.
About these numbers
Day counts come from the EPA's county air quality summaries, which take the worst reading across a county's monitors each day. Monitors here reported on about 365 days a year from 2021 to 2025. A county is large, and your street can be cleaner or worse. CDC's model, which estimates every county from monitors and satellites, gives the worst spot in Boulder County about 4 high-particle days and about 19 high-ozone days a year from 2018 to 2022.
Sources: EPA AirData county AQI summaries (2000 to 2025) and daily county AQI (2016 to 2025); CDC Environmental Public Health Tracking Network modeled PM2.5 and ozone (2001 to 2022). Updated November 8, 2026.