Check tomorrow's forecast
AirNow posts an air quality forecast by ZIP code, usually the day before.
AirNow →Air quality in Middlesex County, MA
Includes Cambridge, Lowell, Newton and Framingham · EPA monitor readings, 2021 to 2025 · 1.6 million people
For today's air and tomorrow's forecast, check AirNow by ZIP code.
Bad-air days a year
<1
day with an AQI over 100
4 bad-air days in 5 years of readings, 2021 to 2025. That's down from about 12 a year in the early 2000s. Middlesex County has the fewest bad-air days of the 13 Massachusetts counties with monitors.
<1day unhealthy for everyoneAQI over 150, a year
38AQI on a typical dayGood: fine for everyone
5bad-air days since 2016Too few to show a season
153worst day since 2016Jul 26, 2021, 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 38 (Good)Worst day 153 (Unhealthy)
Good0 to 50Moderate51 to 100Sensitive groups101 to 150Unhealthy151 to 200Very unhealthy201 to 300Hazardous301 to 500
Bad-air days by month in Middlesex County
Bad air is rare here: 5 bad-air days in 10 years of daily readings since 2016, too few to show a season. Each tile is the average for that month.
Jan0
Feb0
Mar0
Apr0.1
May0.1
Jun0
Jul0.2
Aug0.1
Sep0
Oct0
Nov0
Dec0
The worst day since 2016 was July 26, 2021: AQI 153, from fine particles.
What causes it
The 5 bad-air days since 2016 were 3 ozone days (in May 2017, April 2023 and August 2025) and 2 fine-particle days (in July 2021).
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 | 0.2 | 0 |
| February | 0 | 0 |
| March | 0 | 0 |
| April | 0 | 0.5 |
| May | 0.1 | 1.1 |
| June | 0.3 | 2.0 |
| July | 0.2 | 2.5 |
| August | 0.2 | 1.8 |
| September | 0 | 0.8 |
| October | 0 | 0.1 |
| November | 0 | 0 |
| December | 0.1 | 0 |
What helps in Middlesex County
The air here is clean most days: 5 bad-air days in 10 years of daily readings. Wildfire smoke can still drift in from far away, so it's worth being ready.
Keep a cheap backup
A box fan with a MERV 13 filter clamped to it cleans a room in a pinch.
Build one →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.
Smoky days are rare here, so a purifier is optional. If you want one for wildfire season anyway, see the 3 models we'd buy, each with a verified smoke rating.
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 2 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 | 8 | 1 |
| 2001 | 24 | 10 |
| 2002 | 22 | 9 |
| 2003 | 4 | 0 |
| 2004 | 3 | 0 |
| 2005 | 12 | 3 |
| 2006 | 9 | 0 |
| 2007 | 19 | 5 |
| 2008 | 6 | 1 |
| 2009 | 4 | 0 |
| 2010 | 2 | 0 |
| 2011 | 2 | 1 |
| 2012 | 4 | 1 |
| 2013 | 2 | 0 |
| 2014 | 0 | 0 |
| 2015 | 1 | 0 |
| 2016 | 0 | 0 |
| 2017 | 1 | 0 |
| 2018 | 0 | 0 |
| 2019 | 0 | 0 |
| 2020 | 0 | 0 |
| 2021 | 2 | 1 |
| 2022 | 0 | 0 |
| 2023 | 1 | 0 |
| 2024 | 0 | 0 |
| 2025 | 1 | 0 |
Middlesex County and its neighbors
Bad-air days a year, 2021 to 2025. Massachusetts counties with monitors averaged about 2 bad-air days a year, weighted by population.
Questions people ask about air in Middlesex County
How often is the air bad in Middlesex County?
From 2021 to 2025, monitors in Middlesex County recorded about 1 bad-air day a year with an AQI above 100, which is unhealthy for sensitive groups or worse. The worst year in that stretch was 2021, with 2 bad-air days.
What causes bad air in Middlesex County?
The 5 bad-air days since 2016 were 3 ozone days (in May 2017, April 2023 and August 2025) and 2 fine-particle days (in July 2021).
Where can I see today's air quality in Middlesex 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 Middlesex County is about 38, which is good, fine for everyone. Bad days are rare here, so check it when wildfire smoke is in the news.
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 Middlesex County almost no high-particle days and about 1 high-ozone day 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.