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

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 539 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (6% particle days).

The bad-air season here

Most bad days here fall in June and May and July. 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 record for Norfolk

In 2000 to 2009 this county averaged 19 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2017, with 4 bad days on 364 monitored days.

Date	AQI	Main pollutant
2017-05-18	147	Ozone
2016-07-22	143	Ozone
2017-05-17	143	Ozone
2023-06-02	136	Ozone
2021-07-26	133	PM2.5

The air on a normal day

Across 3,636 monitored days from 2016 to 2025, the air in Norfolk County rated Good on 87% of them, more often than the 72% typical of monitored counties, and Moderate on 12%. The median day sat at AQI 37; one day in ten went above 52.

The pollutant setting the AQI on an ordinary day here:

ozone79%2,874 days
fine particles (PM2.5)18%672 days
nitrogen dioxide2%90 days

Norfolk County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	308	55	2	0	39	126
2024	366	319	47	0	0	39	90
2023	365	276	85	4	0	41	136
2022	365	304	60	1	0	40	105
2021	365	280	82	3	0	37	133
2020	366	330	36	0	0	34	74
2019	362	352	10	0	0	34	100
2018	353	337	15	1	0	32	105
2017	364	337	23	4	0	36	147
2016	365	326	36	3	0	37	143
2015	365	287	76	2	1	40	156
2014	365	286	77	2	0	42	105
2013	365	291	70	4	0	39	115
2012	363	268	89	6	0	41	133
2011	360	253	102	5	0	43	119
2010	365	281	75	9	0	40	143

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

Elsewhere in Massachusetts

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.