The percentage of days for which air quality was monitored and found to be good, indicating healthy air both for sensitive groups, such as people with asthma, and for the population as a whole. The Air Quality Index (AQI) summarizes levels of ground-level ozone, particulate matter (soot and other particles), carbon monoxide, sulfur dioxide, and nitrogen dioxide into one measure.
Exposure to these pollutants can cause short-term health effects such as irritation to the eyes, nose and throat, coughing, sneezing, runny nose, and shortness of breath. Continued exposure can also worsen medical conditions such as asthma and heart disease.
In 2025, 81% of days with air quality measurements in Essex County were good air days, down one point from the rate in 2024, and a 20 percentage point increase from 2010.
Essex County had the lowest percentage of good air days among comparison counties in 2025. Lake County, IL had the highest at 85%, Middlesex County, MA and Westchester County, NY were similar to Essex County (both at 82%). Since 2010, all counties but Middlesex have had an increase in the number of good air days.
Comparable national and state data is not available due to the quantity and location of EPA monitoring stations.
| 2010 | 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Essex County, MA | 61% | 49% | 56% | 59% | 67% | 70% | 81% | 78% | 72% | 79% | 77% | 78% | 76% | 67% | 82% | 81% |
| Middlesex County, MA | 86% | 89% | 89% | 94% | 95% | 95% | 94% | 96% | 84% | 79% | 84% | 82% | 85% | 77% | 86% | 82% |
| Lake County, IL | 79% | 88% | 82% | 87% | 89% | 92% | 88% | 88% | 86% | 86% | 83% | 74% | 82% | 75% | 82% | 85% |
| Westchester County, NY | 65% | 57% | 67% | 68% | 65% | 63% | 73% | 80% | 81% | 85% | 84% | 84% | 82% | 79% | 83% | 82% |
| 2010 | 2011 | 2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Essex County, MA | 222 | 179 | 206 | 216 | 244 | 255 | 295 | 285 | 263 | 288 | 282 | 285 | 276 | 243 | 300 | 296 |
| Middlesex County, MA | 313 | 324 | 324 | 335 | 338 | 345 | 341 | 341 | 308 | 289 | 306 | 299 | 310 | 281 | 314 | 299 |
| Lake County, IL | 287 | 316 | 296 | 305 | 317 | 332 | 306 | 316 | 304 | 210 | 199 | 180 | 199 | 182 | 201 | 196 |
| Westchester County, NY | 238 | 207 | 244 | 249 | 237 | 231 | 269 | 292 | 295 | 309 | 305 | 307 | 300 | 287 | 302 | 301 |
| INDICATORS | TREND | ESSEX COUNTY |
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