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Connecticut Air Quality

EPA monitor data through 2025 · Updated August 13, 2026 · Methods

In 2025, Connecticut recorded 24 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. Most of those bad days were driven by ozone. Under the AQI rules that applied before 2013, the same air would have scored 18 days. Since 2000, unhealthy days are 33% lower on a consistent definition.

Unhealthy days, 2025

AQI above 100

Same air, pre-2013 rules

for comparison with older reports

Good-air days

214

AQI 50 or below

Days monitored

365

full year

How to read a statewide figure. The Air Quality Index describes one place at one time. A statewide number is the worst monitor anywhere in the state on each day, so it answers “was the air unhealthy somewhere in Connecticut?” — not “what did a typical resident breathe?” In a large state with many monitors it will read high even if most of the state was clean. EPA publishes the AQI for metropolitan areas rather than states for exactly this reason. For what people actually breathed, use the metro pages below.

Unhealthy days by year, scored two ways

EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those changes is not comparable to one published after. Each panel below applies a single fixed rulebook to every year, so each is internally consistent. Why this matters.

Scored by today's rules

Scored by the pre-2013 rules

What 2025 looked like

Air quality category	Days	Share of monitored days
Good	214	59%
Moderate	127	35%
Unhealthy for sensitive groups	20	5%
Unhealthy or worse	4	1%

Median AQI 46 · 90th-percentile AQI 84 · worst day 174. Scored under current rules.

What drove the unhealthy days

Of the 24 days above 100 in 2025, 22 were driven by ozone and 2 by fine particles. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke. In the eastern United States, ozone has stopped responding to emission cuts: nitrogen oxide emissions kept falling after 2014 and ambient ozone barely moved.

The unhealthy days in 2025

Every day the index passed 100, what it reached, and which pollutant drove it. 6 of these 24 would not have counted as unhealthy under the pre-2013 rules.

Date	Peak AQI	Driven by	Pre-2013 score
Apr 24	119	Ozone	101
Apr 25	105	Ozone	90 (not unhealthy)
May 12	101	Ozone	87 (not unhealthy)
Jun 4	140	Ozone	116
Jun 5	166	Ozone	142
Jun 6	119	Ozone	101
Jun 11	108	Ozone	93 (not unhealthy)
Jun 12	151	Ozone	127
Jun 23	119	Ozone	101
Jun 24	147	Ozone	122
Jun 25	122	Ozone	104
Jul 1	119	Ozone	101
Jul 6	119	Ozone	101
Jul 8	115	Ozone	100 (not unhealthy)
Jul 16	115	Ozone	100 (not unhealthy)
Jul 25	147	Ozone	122
Jul 26	134	Fine particles	118
Jul 29	174	Ozone	150
Aug 4	129	Ozone	109
Aug 5	127	Fine particles	112
Aug 10	133	Ozone	111
Aug 11	140	Ozone	116
Aug 12	164	Ozone	140
Oct 5	108	Ozone	93 (not unhealthy)

Metro areas in Connecticut

Ranked by unhealthy days in their latest year. These are the geographies EPA actually reports the AQI for.

Metro area	Unhealthy days	Pre-2013 rules	Main driver
Bridgeport-Stamford-Norwalk, CT	21	13	ozone
New Haven-Milford, CT	13	7	ozone
Hartford-West Hartford-East Hartford, CT	11	5	ozone
Torrington, CT	9	5	ozone
Norwich-New London, CT	8	4	ozone
Worcester, MA-CT	1	0	ozone

The full series

Year	Unhealthy days, current rules	Pre-2013 rules	Days monitored
2000	40	27	366
2001	52	42	365
2002	66	55	365
2003	48	32	365
2004	29	23	366
2005	51	36	365
2006	41	31	365
2007	57	44	365
2008	39	25	366
2009	19	6	365
2010	40	25	365
2011	23	17	365
2012	36	27	366
2013	28	20	365
2014	19	10	365
2015	34	23	365
2016	32	22	366
2017	20	10	365
2018	24	18	365
2019	22	12	365
2020	19	10	366
2021	23	19	365
2022	24	12	365
2023	23	15	365
2024	23	14	366
2025	24	18	365

Sources

EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
Download the full dataset for every state and metro.