Move workouts to the morning
Ozone peaks on hot, sunny afternoons. Mornings are usually cleaner.
Ozone guide →Air quality in Montgomery County, MD
Includes Gaithersburg, Rockville, Bethesda and North Bethesda · EPA monitor readings, 2021 to 2025 · 1.1 million people
For today's air and tomorrow's forecast, check AirNow by ZIP code.
Bad-air days a year
2
days with an AQI over 100
9 bad-air days in 5 years of readings, 2021 to 2025. That's down from about 24 a year in the early 2000s. Montgomery County is tied with 1 other Maryland county for the 7th most bad-air days of the 16 with monitors.
<1day unhealthy for everyoneAQI over 150, a year
41AQI on a typical dayGood: fine for everyone
Apr to Junworst monthsOzone on hot afternoons
165worst day since 2016Jun 29, 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 41 (Good)Worst day 165 (Unhealthy)
Good0 to 50Moderate51 to 100Sensitive groups101 to 150Unhealthy151 to 200Very unhealthy201 to 300Hazardous301 to 500
When bad air happens in Montgomery County
Average bad-air days in each month, from 10 years of daily readings since 2016. Most fall in April, May and June.
Jan0
Feb0
Mar0
Apr0.1
May0.4
Jun0.5
Jul0.1
Aug0.1
Sep0
Oct0
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 | 0 |
| March | 0 | 0 | 0 |
| April | 0.1 | 0 | 0.1 |
| May | 0.4 | 0 | 0.4 |
| June | 0.2 | 0.3 | 0.5 |
| July | 0.1 | 0 | 0.1 |
| August | 0.1 | 0 | 0.1 |
| September | 0 | 0 | 0 |
| October | 0 | 0 | 0 |
| November | 0 | 0 | 0 |
| December | 0 | 0 | 0 |
The worst day since 2016 was June 29, 2023: AQI 165, from fine particles.
What causes it
The pollutant that pushed the AQI over 100 on each of the 12 bad-air days since 2016.
75%Ozone: smog that forms in sunlight on hot days
25%Fine particles: smoke and soot
The rare bad days here are mostly 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 | 0.4 | 0 |
| February | 0 | 0 |
| March | 0 | 0 |
| April | 0 | 0.5 |
| May | 0.2 | 1.8 |
| June | 0.6 | 4.1 |
| July | 0.8 | 4.3 |
| August | 1.0 | 4.5 |
| September | 0 | 1.6 |
| October | 0.1 | 0 |
| November | 0.1 | 0 |
| December | 0.1 | 0 |
What helps in Montgomery 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.
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 2023, with 5 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 | 17 | 2 |
| 2001 | 36 | 9 |
| 2002 | 36 | 11 |
| 2003 | 11 | 3 |
| 2004 | 22 | 2 |
| 2005 | 16 | 2 |
| 2006 | 22 | 4 |
| 2007 | 21 | 4 |
| 2008 | 19 | 1 |
| 2009 | 3 | 0 |
| 2010 | 24 | 0 |
| 2011 | 14 | 1 |
| 2012 | 7 | 1 |
| 2013 | 1 | 0 |
| 2014 | 0 | 0 |
| 2015 | 5 | 0 |
| 2016 | 0 | 0 |
| 2017 | 2 | 0 |
| 2018 | 1 | 0 |
| 2019 | 0 | 0 |
| 2020 | 0 | 0 |
| 2021 | 3 | 0 |
| 2022 | 1 | 0 |
| 2023 | 5 | 3 |
| 2024 | 0 | 0 |
| 2025 | 0 | 0 |
Montgomery County and its neighbors
Bad-air days a year, 2021 to 2025. Maryland counties with monitors averaged about 3 bad-air days a year, weighted by population.
Questions people ask about air in Montgomery County
How often is the air bad in Montgomery County?
From 2021 to 2025, monitors in Montgomery County recorded about 2 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 2023, with 5 bad-air days.
What causes bad air in Montgomery County?
The rare bad days here are mostly ozone days: smog that builds on hot, sunny afternoons. Bad days cluster in April, May and June.
Where can I see today's air quality in Montgomery 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 Montgomery County is about 41, which is good, fine for everyone. Check it most in April, May and June, 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 358 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 Montgomery County almost no high-particle days and about 2 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.