Air Quality

Climate Changers                Air Pollutants

Air Days

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Connecticut residents breathed healthful air on 341 days in 2023; the same as last year and the previous ten-year average (341 days).

There were 341 “good air days” statewide in 2023, which was the same as 2022 and the previous 10-year average. A “good air day” is when every monitoring station in the state records “satisfactory air quality”, which is defined here as air that meets the health-based National Ambient Air Quality Standard (NAAQS) for all of the following six pollutants: sulfur dioxide, carbon monoxide, particulate matter (PM2.5 and PM10), nitrogen dioxide, and ground-level ozone. 

An Air Quality Index (AQI) above 100 is considered “unhealthy for sensitive groups”, which includes people with heart or lung disease, older adults, and children. In 2023, there were 19 days that exceeded the AQI for ozone* and six days that exceeded the AQI for particulate matter (PM 2.5), including one day (July 1) that had an AQI above 100 for both ozone and PM2.5.5  When air quality is “unhealthy”, there is an increased likelihood for all individuals for aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly. There are also increased respiratory effects for the general population.6

In 2023, the Department of Energy and Environmental Protection (DEEP) provided notice of its intent to revise the State Implementation Plan (SIP) to comply with the 2015 National Ambient Air Quality Standards (NAAQS) for Ozone. The revision to the SIP addressed Connecticut’s two nonattainment areas:1) Southwest Connecticut, which is part of the New York- Northern New Jersey- Long Island (NY-NJ-CT) Nonattainment Area and includes Fairfield, New Haven, and Middlesex counties, and 2) the Greater Connecticut Nonattainment Area, comprised of Connecticut’s remaining five counties. The SIP revision also included motor vehicle emissions budgets for both Greater Connecticut and Southwest Connecticut.7 

There has been a long term trend of improved air quality, in part, due to the air pollution controls that were put in place after the enactment of the 1971 Clean Air Act. The chart below, “Air Pollutants”, shows that in the 1980’s, exceedances for sulfur dioxide (SO2) and nitrogen dioxide (NO2) were more common, but not so recently.8  Statewide exceedances of pollutants, except for ozone, are rarely seen, due to federal restrictions on emitters, mostly to Connecticut’s west and southwest.**

Air Pollutants

Connecticut experienced six days during the summer of 2023 when particulate matter (PM2.5) levels were elevated, primarily as a result of forest fires in Canada, making the air unhealthy. Exposure to particle pollution is linked to a variety of significant health problems, ranging from aggravated asthma to premature death in people with heart and lung disease. Fine particles are a health concern because fine particles can easily reach the deepest parts of the lungs.9

The image (below) illustrates a bad-air day in 2023 for ozone that was more intense than average but followed the typical pattern of Connecticut having the worst ozone pollution in New England.10 The yellow areas indicate moderate air quality that meets the standard for ground-level ozone*, while the orange and red areas did not. Connecticut had 19 days in 2023 that exceeded the 8-hour ozone standard of 0.070 parts per million (ppm); Massachusetts had the next highest with ten days.11

Map depicting bad air day in CT July 12, 2023

Some residents in the yellow areas, who are unusually sensitive to air pollution, might have been adversely affected. Much of Connecticut's ground-level ozone originates in states to the west and southwest. Unless emissions in those states are reduced substantially, Connecticut residents are likely to continue to breathe unhealthful air. Past ozone control strategies for nitrogen oxides (NOx) have centered around point source electrical generating units, which have been effective in reducing long-range air pollutant transport into Connecticut. Increasingly, area sources and on-road / non-road mobile sources have become the dominant source of nitrogen oxides (NOx) production.12

Goal: While not formally stated, the goal is for Connecticut residents to have a “good air day”, every day. 
Technical Note: *The federal air quality standard for ozone was revised prior to the 2016 ozone season. The new standard (0.070 PPM over eight hours) is slightly more protective of human health than the older standard (0.075 ppm). **Lead (Pb) is not shown. Connecticut’s lead levels have been below the national standard (NAAQS) since 1994. 




5 EPA, Air Data: Air Quality Data Collected at Outdoor Monitors Across the US;

6 DEEP, New Release, June 7, 2023, DEEP Forecasts Unhealthy Levels of PM2.5 Wednesday for the Entire State from Canadian Wildfire Smoke;

DEEP, Notice of Intent to Revise the State Implementation Plan for Air Quality: 2015 Ozone National Ambient Air Quality Standards, December 6, 2023;

EPA, Outdoor Air Quality Data, Air Data – Multiyear Tile Plot, accessed 1-3-2023;

Connecticut Department of Health (DPH), Air Pollution;

10 EPA, AirNow, Interactive Map of Air Quality, Archive – July 12, 2023;

11DEEP, Annual Summary Information for Ozone;

12 DEEP, Source Contribution to Connecticut’s Ozone Problem;