Bad Air Days in Middlesex County, Massachusetts: under 1 a Year

under 1 bad air day a year

Rare: among the cleanest-air counties EPA monitors. Ranked 821 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (40% particle days).

The bad-air season here

Most bad days here fall in July and April and May and August. Averaged over 2016 to 2025.

What to buy, and what not to

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

The worst days on file

In 2000 to 2009 this county averaged 11 bad days a year; in 2016 to 2025, under 1. The air has improved. The worst recent year was 2021, with 2 bad days on 365 monitored days.

Date	AQI	Main pollutant
2021-07-26	153	PM2.5
2017-05-17	105	Ozone
2025-08-12	105	Ozone
2021-07-20	101	PM2.5
2023-04-14	101	Ozone

An ordinary day in Middlesex

Across 3,639 monitored days from 2016 to 2025, the air in Middlesex County rated Good on 85% of them, more often than the 72% typical of monitored counties, and Moderate on 15%. The median day sat at AQI 37; one day in ten went above 53.

The pollutant setting the AQI on an ordinary day here:

ozone64%2,337 days
fine particles (PM2.5)34%1,254 days
nitrogen dioxide1%48 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	299	65	1	0	38	105
2024	366	314	52	0	0	37	93
2023	365	281	83	1	0	39	101
2022	365	310	55	0	0	38	90
2021	365	299	64	2	1	37	153
2020	366	306	60	0	0	39	93
2019	364	289	75	0	0	41	80
2018	365	308	57	0	0	38	97
2017	356	341	14	1	0	33	105
2016	362	341	21	0	0	33	100
2015	362	345	16	1	0	33	119
2014	357	338	19	0	0	32	97
2013	358	335	21	2	0	34	101
2012	365	324	37	4	1	34	159
2011	363	324	37	2	1	34	151
2010	365	313	50	2	0	35	129

*2025 may be a partial year in EPA's file.

Massachusetts compared

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.