Bad Air Days in Worcester County, Massachusetts: 2 a Year

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 540 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (28% particle days).

The bad-air season here

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

What actually helps here

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 15 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2016, with 4 bad days on 366 monitored days.

Date	AQI	Main pollutant
2023-06-06	148	PM2.5
2023-07-01	145	PM2.5
2016-07-22	143	Ozone
2017-05-17	143	Ozone
2018-05-02	129	Ozone

An ordinary day in Worcester

Across 3,653 monitored days from 2016 to 2025, the air in Worcester County rated Good on 73% of them, close to the 72% typical of monitored counties, and Moderate on 27%. The median day sat at AQI 41; one day in ten went above 58.

The pollutant setting the AQI on an ordinary day here:

ozone50%1,842 days
fine particles (PM2.5)48%1,751 days
nitrogen dioxide2%56 days

Worcester County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	298	67	0	0	39	100
2024	366	321	45	0	0	37	90
2023	365	268	93	4	0	39	148
2022	365	228	137	0	0	45	100
2021	365	218	144	3	0	45	125
2020	366	257	109	0	0	43	90
2019	365	238	127	0	0	45	93
2018	365	265	96	4	0	40	129
2017	365	288	74	3	0	40	143
2016	366	285	77	4	0	41	143
2015	365	266	99	0	0	40	93
2014	365	242	122	1	0	43	115
2013	365	228	135	2	0	45	112
2012	366	228	135	3	1	45	166
2011	365	191	170	4	1	48	159
2010	365	190	170	5	0	49	143

*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.