Thurston County, Washington air quality history

Daily Air Quality Index summaries from EPA monitors in Thurston, Washington, 1985–2025.

94%

Good days in 2025

Median daily AQI, 2025

Days above 100 in 2025

293

Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Thurston, Washington on 293 days. 274 of those days (94%) were in the Good range, and the median daily AQI was 27. That median is about the same as the typical monitored county in Washington (median 28 across 30 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 208 days. Across the years with fuller monitoring, the share of Good days rose: an average of 86% in 1986–1990 versus 93% in 2023–2025.

EPA did not report an AQI for every day of 2025 here (293 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

Years with fewer than 120 monitored days (1985, 1987, 1988) 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)	208	71%
Ozone	85	29%

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	293	274	19	0	0	0	27	47	65	PM2.5
2024	362	338	24	0	0	0	26	44	85	PM2.5
2023	358	327	31	0	0	0	28	48	82	PM2.5
2022	348	309	36	1	2	0	28	51	157	PM2.5
2021	329	309	20	0	0	0	19	41	63	PM2.5
2020	366	316	43	0	3	4	28	53	242	PM2.5
2019	365	290	75	0	0	0	33	58	74	PM2.5
2018	343	291	46	1	5	0	29	55	181	PM2.5
2017	363	287	72	2	2	0	32	62	156	PM2.5
2016	349	308	41	0	0	0	31	52	91	PM2.5
2015	357	295	62	0	0	0	34	58	99	PM2.5
2014	363	294	69	0	0	0	32	60	95	PM2.5
2013	364	256	105	3	0	0	36	66	132	PM2.5
2012	361	271	86	3	1	0	35	63	153	PM2.5
2011	363	304	58	1	0	0	31	57	104	PM2.5
2010	355	330	25	0	0	0	27	46	83	PM2.5
2009	364	281	81	2	0	0	34	65	121	PM2.5
2008	362	256	102	4	0	0	38	67	140	PM2.5
2007	360	279	76	5	0	0	33	68	115	PM2.5
2006	365	237	123	5	0	0	40	67	147	PM2.5
2005	365	228	135	2	0	0	41	69	130	PM2.5
2004	364	223	139	2	0	0	43	70	116	PM2.5
2003	364	214	142	8	0	0	46	78	120	PM2.5
2002	365	235	123	7	0	0	42	73	135	PM2.5
2001	365	251	107	7	0	0	42	69	126	PM2.5
2000	366	261	97	8	0	0	37	69	128	PM2.5
1999	362	310	51	1	0	0	30	57	145	Ozone
1998	365	346	14	4	1	0	28	46	172	CO
1997	288	256	32	0	0	0	17	51	79	CO
1996	366	344	22	0	0	0	18	39	81	CO
1995	365	348	17	0	0	0	18	37	66	CO
1994	358	356	2	0	0	0	22	35	62	CO
1993	360	345	15	0	0	0	26	43	63	CO
1992	303	292	11	0	0	0	23	42	74	CO
1991	182	170	12	0	0	0	19	40	80	PM10
1990	186	175	11	0	0	0	18	41	94	PM10
1989	181	156	25	0	0	0	19	57	87	PM10
1988	105	90	14	1	0	0	21	62	108	PM10
1987	75	59	15	1	0	0	24	62	112	PM10
1986	146	115	28	3	0	0	25	63	120	PM10
1985	39	11	23	5	0	0	57	121	150	PM10

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 Washington with a similar median AQI in 2025

All monitored counties in Washington

Source: U.S. EPA AirData, “Annual AQI by County” files for 1985–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem