Air Quality

Climate Changers                Air Pollutants

Air Days

There were 343 “good air days” statewide in 2024, which was greater than the previous year 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 2024, there were 23 days when the AQI for ozone* exceeded 100.⁵ When air quality is “unhealthy”, some members of the general public may experience health effects and members of sensitive groups may experience more serious health effects.⁶ Outdoor activities may be restricted or advised against for all populations.

The image (below) illustrates a bad-air day in 2024 for ozone that was more intense than average but followed the typical pattern of Connecticut having the worst ozone pollution in New England.⁷ The yellow areas indicate moderate air quality that meets the standard for ground-level ozone*, while the orange and red areas did not. Some residents in the yellow areas, who are unusually sensitive to air pollution, might have also been adversely affected. 

High ozone concentrations typically occur on hot summer days when winds from the southwest and west transport ozone precursors from the large metropolitan areas to the south and west, especially from the New York City area. Air emissions from mobile, industrial and commercial sources in Connecticut then enhance the production of ground-level ozone.⁸ 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.⁹

On July 25, 2024, the Environmental Protection Agency (EPA) published a final rule, granting a request from the States of New York, New Jersey, and Connecticut to reclassify the New York-Northern New Jersey-Long Island, (NY-NJ-CT) ozone nonattainment area from ‘‘Moderate’’ to ‘‘Serious’’ for the 2015 8-hour ozone NAAQS. On July 29, 2024, EPA published a final rule reclassifying Greater Connecticut from “Moderate” to “Serious” nonattainment for the 2015 ozone NAAQS. The result of these changes means the attainment dates move to August 2027, based on certified 2024-26 monitoring data; however, reclassification does not change the submission requirement or implementation deadlines for elements of the State Implementation Plan (SIP) that were due for the Moderate classification for Southwest Connecticut and Greater Connecticut areas.¹⁰

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 Clean Air Act Amendments of 1990 are achieving large health benefits that will grow further over time as programs take full effect. The chart above, “Air Pollutants”, depicts that in the 1980’s, exceedances for sulfur dioxide (SO2) and nitrogen dioxide (NO2) were more common, but not so recently. Statewide exceedances of pollutants, except for ozone, are rarely seen, due to federal restrictions on emitters, mostly to Connecticut’s west and southwest.**

_{—————}

⁵ _{EPA, Air Data: Air Quality Data Collected at Outdoor Monitors Across the US; www.epa.gov/outdoor-air-quality-data.}

⁶ _{EPA, AirNow, Air Quality Index (AQI) Basics; www.airnow.gov/aqi/aqi-basics/}

⁷ _{EPA, AirNow, Interactive Map of Air Quality, Archive – June 20, 2024; gispub.epa.gov/airnow/index.html.}

⁸ _{DEEP, Air Quality Trends – Ozone; portal.ct.gov/DEEP/Air/Monitoring/Trends/Ozone-Trends.}

⁹ _{DEEP, Source Contribution to Connecticut’s Ozone Problem; portal.ct.gov/deep/air/planning/ozone/source-contribution-to-connecticut-ozone.}

¹⁰ _{DEEP, Presentation by Paul Farrell “Update on ozone reclassifications of CT Nonattainment areas for the 2008 & 2015 Ozone NAAQS”, September 25, 2025.}