Jefferson County, Missouri air quality history

Daily Air Quality Index summaries from EPA monitors in Jefferson, Missouri, 1981–2025.

73%

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 Jefferson, Missouri on 365 days. 268 of those days (73%) were in the Good range, and the median daily AQI was 42. That median is about the same as the typical monitored county in Missouri (median 41 across 19 counties), and about the same as 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 fell: an average of 77% in 1981–1988 versus 72% in 2023–2025.

Years with fewer than 120 monitored days (1982, 1985, 1986) are shown in the table but left out of comparisons and trend statements.

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	147	40%
Coarse particles (PM10)	5	1%
Nitrogen dioxide (NO₂)	1	<1%

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	268	93	4	0	0	42	60	150	PM2.5
2024	366	297	68	1	0	0	40	54	105	Ozone
2023	362	225	121	14	2	0	44	72	185	PM2.5
2022	360	236	121	3	0	0	44	65	108	PM2.5
2021	365	244	116	4	1	0	44	62	164	PM2.5
2020	365	250	113	2	0	0	43	59	141	PM2.5
2019	361	252	107	2	0	0	42	62	105	Ozone
2018	351	255	90	6	0	0	42	63	119	PM2.5
2017	365	231	133	1	0	0	42	63	102	PM2.5
2016	366	219	145	2	0	0	46	63	108	PM2.5
2015	365	142	220	3	0	0	53	70	136	PM2.5
2014	363	172	184	7	0	0	51	76	142	PM2.5
2013	357	172	182	3	0	0	51	70	150	PM2.5
2012	340	146	164	27	3	0	53	97	190	PM2.5
2011	365	158	195	10	2	0	53	82	192	PM2.5
2010	359	135	203	20	1	0	54	84	151	PM2.5
2009	365	131	232	2	0	0	54	73	122	PM2.5
2008	364	118	241	5	0	0	55	77	136	PM2.5
2007	365	95	240	26	4	0	58	97	169	PM2.5
2006	363	92	254	16	1	0	58	84	159	PM2.5
2005	365	80	248	33	4	0	61	101	187	PM2.5
2004	363	104	254	5	0	0	56	77	133	PM2.5
2003	358	100	235	20	2	1	60	90	202	PM2.5
2002	302	96	171	24	11	0	58	105	197	PM2.5
2001	262	103	137	16	6	0	58	96	161	Ozone
2000	265	104	138	22	1	0	58	90	166	Ozone
1999	257	106	105	34	10	2	58	126	206	Ozone
1998	229	132	69	22	5	1	46	115	208	Ozone
1997	238	156	61	19	1	1	44	100	204	Ozone
1996	257	172	55	23	7	0	44	108	195	Ozone
1995	257	145	71	23	17	1	46	122	203	Ozone
1994	313	211	65	25	10	2	41	105	219	Ozone
1993	299	243	46	9	1	0	33	74	192	Ozone
1992	249	191	42	12	4	0	40	87	179	Ozone
1991	303	235	44	20	4	0	36	90	182	Ozone
1990	323	288	26	9	0	0	29	54	150	Ozone
1989	334	262	43	25	4	0	34	97	182	Ozone
1988	347	242	58	34	11	2	37	115	225	Ozone
1987	131	128	3	0	0	0	24	44	74	Ozone
1986	58	57	1	0	0	0	30	44	56	PM10
1985	29	25	4	0	0	0	30	59	60	PM10
1982	63	46	11	6	0	0	33	97	120	Ozone
1981	185	118	38	24	4	1	42	115	238	Ozone

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 Missouri with a similar median AQI in 2025

All monitored counties in Missouri

Source: U.S. EPA AirData, “Annual AQI by County” files for 1981–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem