Mason County, Washington air quality history

Daily Air Quality Index summaries from EPA monitors in Mason, Washington, 1991–2025.

90%

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 Mason, Washington on 365 days. 330 of those days (90%) were in the Good range, and the median daily AQI was 23. That median is lower than 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 365 days. Across the years with fuller monitoring, the share of Good days rose: an average of 78% in 2002–2004 versus 90% in 2023–2025.

Years with fewer than 120 monitored days (1991, 1992, 2001) 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)	365	100%

Year-by-year data

Annual AQI summary by year
Year	Days with AQI	Good	Moderate	Unhealthy for Sensitive Groups	Unhealthy	Very Unhealthy	Hazardous	Median AQI	90th pct.	Max AQI	Main pollutant
2025	365	330	34	0	1	0	0	23	48	159	PM2.5
2024	366	341	25	0	0	0	0	21	43	73	PM2.5
2023	363	313	49	1	0	0	0	24	53	106	PM2.5
2022	365	302	60	3	0	0	0	23	58	125	PM2.5
2021	361	339	22	0	0	0	0	22	43	70	PM2.5
2020	357	312	36	2	2	1	4	23	52	397	PM2.5
2019	361	305	56	0	0	0	0	28	53	75	PM2.5
2018	365	304	55	4	2	0	0	28	56	168	PM2.5
2017	363	301	60	1	1	0	0	25	57	156	PM2.5
2016	366	327	39	0	0	0	0	27	51	72	PM2.5
2015	362	316	45	1	0	0	0	29	53	102	PM2.5
2014	365	319	46	0	0	0	0	27	53	80	PM2.5
2013	336	233	103	0	0	0	0	33	62	78	PM2.5
2012	366	278	87	1	0	0	0	33	59	116	PM2.5
2011	360	272	88	0	0	0	0	33	58	83	PM2.5
2010	365	316	49	0	0	0	0	28	54	76	PM2.5
2009	360	278	82	0	0	0	0	29	58	90	PM2.5
2008	366	286	80	0	0	0	0	36	55	78	PM2.5
2007	365	286	77	2	0	0	0	31	60	133	PM2.5
2006	351	273	78	0	0	0	0	34	57	85	PM2.5
2005	365	258	107	0	0	0	0	36	61	88	PM2.5
2004	366	253	112	1	0	0	0	39	65	112	PM2.5
2003	297	225	70	2	0	0	0	38	63	103	PM2.5
2002	168	152	15	1	0	0	0	32	50	101	Ozone
2001	41	34	7	0	0	0	0	31	59	84	PM2.5
1992	18	17	1	0	0	0	0	28	48	51	PM10
1991	16	16	0	0	0	0	0	29	41	44	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 1991–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem