Custer County, Idaho air quality history

Daily Air Quality Index summaries from EPA monitors in Custer, Idaho, 1994–2025.

97%

Good days in 2025

Median daily AQI, 2025

Days above 100 in 2025

Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Custer, Idaho on 96 days. 93 of those days (97%) were in the Good range, and the median daily AQI was 10. That median is lower than the typical monitored county in Idaho (median 35 across 18 counties), and lower than the typical monitored U.S. county (40). The main pollutant was fine particles (PM2.5) on 96 days.

EPA did not report an AQI for every day of 2025 here (96 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 (2020, 2021, 2022, 2023, 2024, 2025…) 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)	96	100%

Year-by-year data

Annual AQI summary by year
Year	Days with AQI	Good	Moderate	Unhealthy for Sensitive Groups	Unhealthy	Median AQI	90th pct.	Max AQI	Main pollutant
2025	96	93	3	0	0	10	31	56	PM2.5
2024	109	94	9	5	1	6	62	152	PM2.5
2023	104	103	1	0	0	8	25	60	PM2.5
2022	105	98	7	0	0	7	42	98	PM2.5
2021	110	93	17	0	0	8	57	99	PM2.5
2020	115	105	7	3	0	7	38	126	PM2.5
2019	96	95	1	0	0	8	23	51	PM2.5
2018	90	85	3	2	0	7	26	128	PM2.5
2017	111	100	8	3	0	7	48	142	PM2.5
2016	116	103	9	3	1	11	57	170	PM2.5
2015	117	113	4	0	0	8	28	93	PM2.5
2014	110	107	3	0	0	9	29	54	PM2.5
2013	100	91	7	2	0	9	50	110	PM2.5
2012	109	90	15	2	2	12	70	170	PM2.5
2011	102	100	2	0	0	9	24	78	PM2.5
2010	110	110	0	0	0	10	23	47	PM2.5
2009	105	104	1	0	0	9	26	51	PM2.5
2008	108	101	7	0	0	11	42	72	PM2.5
2007	115	100	13	2	0	11	57	132	PM2.5
2006	98	86	10	1	1	14	51	161	PM2.5
2005	110	109	1	0	0	11	27	71	PM2.5
2004	117	114	3	0	0	13	25	70	PM2.5
2003	110	109	1	0	0	11	30	57	PM2.5
2002	102	101	1	0	0	12	31	53	PM2.5
2001	107	106	1	0	0	14	33	58	PM2.5
2000	89	88	1	0	0	11	32	81	PM2.5
1999	97	94	3	0	0	14	32	58	PM2.5
1998	93	92	0	1	0	10	27	129	PM2.5
1997	104	102	2	0	0	12	27	58	PM2.5
1996	101	99	2	0	0	12	31	62	PM2.5
1995	103	103	0	0	0	10	23	41	PM2.5
1994	89	84	5	0	0	14	41	78	PM2.5

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 Idaho with a similar median AQI in 2025

All monitored counties in Idaho

Source: U.S. EPA AirData, “Annual AQI by County” files for 1994–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem