Cook County, Illinois air quality history

Daily Air Quality Index summaries from EPA monitors in Cook, Illinois, 1980–2025.

41%

Good days in 2025

Median daily AQI, 2025

Days above 100 in 2025

365

Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Cook, Illinois on 365 days. 149 of those days (41%) were in the Good range, and the median daily AQI was 53. That median is higher than the typical monitored county in Illinois (median 42 across 23 counties), and higher than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 212 days. Across the years with fuller monitoring, the share of Good days rose: an average of 20% in 1980–1982 versus 35% in 2023–2025.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median daily AQI
90th-percentile daily AQI

Main pollutant by day, 2025

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

Pollutant	Days as main pollutant	Share of monitored days
Fine particles (PM2.5)	212	58%
Ozone	109	30%
Nitrogen dioxide (NO₂)	33	9%
Coarse particles (PM10)	11	3%

Year-by-year data

Annual AQI summary by year
Year	Days with AQI	Good	Moderate	Unhealthy for Sensitive Groups	Unhealthy	Very Unhealthy	Median AQI	90th pct.	Max AQI	Main pollutant
2025	365	149	191	20	5	0	53	84	168	PM2.5
2024	366	131	224	10	1	0	54	84	166	PM2.5
2023	365	104	227	27	5	2	57	93	223	PM2.5
2022	365	117	240	7	1	0	54	76	151	PM2.5
2021	365	87	257	20	1	0	59	89	156	PM2.5
2020	366	86	260	16	4	0	58	84	164	PM2.5
2019	365	80	276	9	0	0	58	79	133	PM2.5
2018	365	86	260	15	4	0	59	86	177	PM2.5
2017	365	114	232	18	1	0	55	87	177	PM2.5
2016	366	115	233	16	2	0	56	80	169	PM2.5
2015	365	73	285	5	2	0	57	79	195	PM2.5
2014	365	76	281	8	0	0	59	80	143	PM2.5
2013	365	76	278	11	0	0	58	80	136	PM2.5
2012	366	53	271	32	10	0	61	105	195	PM2.5
2011	365	52	297	12	4	0	62	83	159	PM2.5
2010	365	43	298	24	0	0	62	90	140	PM2.5
2009	365	39	307	17	1	1	63	89	202	PM2.5
2008	366	30	299	37	0	0	70	101	117	PM2.5
2007	365	26	288	44	6	1	73	108	203	PM2.5
2006	365	43	302	18	2	0	66	90	164	PM2.5
2005	365	57	257	38	11	2	67	108	208	PM2.5
2004	366	44	301	20	1	0	67	90	156	PM2.5
2003	365	55	267	38	5	0	65	101	185	PM2.5
2002	365	32	281	35	13	4	69	115	212	PM2.5
2001	365	31	275	48	11	0	71	115	195	PM2.5
2000	366	31	300	34	1	0	72	100	157	PM2.5
1999	365	118	204	30	12	1	58	102	202	NO2
1998	365	94	234	28	9	0	63	101	185	PM2.5
1997	365	162	182	14	5	2	52	87	214	NO2
1996	366	169	169	21	4	3	51	83	215	NO2
1995	365	165	164	14	18	4	52	100	222	NO2
1994	365	140	202	18	3	2	56	89	214	NO2
1993	365	180	170	12	2	1	51	80	202	NO2
1992	366	196	149	17	0	4	50	84	212	NO2
1991	365	184	133	29	16	3	50	108	203	NO2
1990	365	162	173	28	2	0	53	99	172	NO2
1989	365	125	187	38	13	2	57	110	216	NO2
1988	366	137	145	46	21	17	57	151	285	NO2
1987	365	103	211	34	10	7	62	115	238	NO2
1986	365	82	248	31	3	1	62	100	205	NO2
1985	365	81	242	34	4	4	62	102	215	NO2
1984	366	82	229	41	9	5	64	106	227	NO2
1983	365	94	201	44	18	8	66	133	230	CO
1982	365	82	223	45	14	1	69	108	201	NO2
1981	365	96	228	31	6	4	62	103	235	CO
1980	366	37	232	65	27	5	77	136	217	NO2

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

Counties in Illinois with a similar median AQI in 2025

All monitored counties in Illinois

Source: U.S. EPA AirData, “Annual AQI by County” files for 1980–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem