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

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 587 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (20% particle days).

When the bad days come in Plymouth

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

Is a purifier worth it 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 record for Plymouth

The worst recent year was 2017, with 4 bad days on 365 monitored days.

Date	AQI	Main pollutant
2017-05-18	147	Ozone
2018-05-02	126	Ozone
2023-06-06	114	PM2.5
2023-07-01	114	PM2.5
2017-06-11	112	Ozone

The air on a normal day

Across 3,645 monitored days from 2016 to 2025, the air in Plymouth County rated Good on 77% of them, more often than the 72% typical of monitored counties, and Moderate on 22%. The median day sat at AQI 40; one day in ten went above 56.

The pollutant setting the AQI on an ordinary day here:

ozone53%1,946 days
fine particles (PM2.5)47%1,699 days

Plymouth County by year

Year	Days monitored	Good	Moderate	Over 100	Median AQI	Highest AQI
2025*	365	308	56	1	37	108
2024	366	319	47	0	37	97
2023	365	278	83	4	40	114
2022	360	235	125	0	45	93
2021	365	257	106	2	39	112
2020	366	267	99	0	42	80
2019	363	289	74	0	40	90
2018	364	279	82	3	39	126
2017	365	281	80	4	39	147
2016	366	298	67	1	39	105
2015	363	246	116	1	40	126
2014	363	229	134	0	44	93
2013	192	139	53	0	37	75
2012	121	82	39	0	39	78
2011	121	73	48	0	40	74
2010	122	85	37	0	39	87

*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.