Bad Air Days in Ellis County, Texas: 2 a Year

2 bad air days a year

Fewer bad air days than most monitored US counties. Ranked 566 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (25% particle days).

Which months to watch

Most bad days here fall in July and September and June and May. Averaged over 2016 to 2025.

Is a purifier worth it here?

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

The record for Ellis

In 2000 to 2009 this county averaged 21 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2018, with 8 bad days on 365 monitored days.

Date	AQI	Main pollutant
2020-06-27	125	PM2.5
2017-06-07	122	Ozone
2018-07-17	120	PM2.5
2018-08-21	119	Ozone
2018-07-16	118	PM2.5

Most days, not the worst ones

Across 3,626 monitored days from 2016 to 2025, the air in Ellis County rated Good on 73% of them, close to the 72% typical of monitored counties, and Moderate on 27%. The median day sat at AQI 42; one day in ten went above 59.

The pollutant setting the AQI on an ordinary day here:

ozone62%2,263 days
fine particles (PM2.5)36%1,305 days
nitrogen dioxide2%58 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	362	323	39	0	0	35	93
2024	362	323	38	1	0	36	101
2023	348	286	60	2	0	36	112
2022	364	311	53	0	0	38	97
2021	363	262	100	1	0	43	105
2020	366	239	126	1	0	45	125
2019	365	250	115	0	0	44	97
2018	365	229	128	8	0	45	120
2017	365	183	179	3	0	50	122
2016	366	234	132	0	0	46	100
2015	365	224	138	3	0	46	136
2014	365	193	172	0	0	48	97
2013	365	187	170	8	0	50	150
2012	366	167	188	11	2	52	201
2011	365	172	178	15	1	52	174
2010	365	193	167	5	0	50	115

*2025 may be a partial year in EPA's file.

Elsewhere in Texas

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day, and on why a purifier does not touch ozone in smoke vs ozone.