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High Ozone in Your Metro Area

This table summarizes high one-hour and eight-hour ozone averages in the metropolitan areas where the TCEQ monitors ozone.

Please Note: Data in this table is based on the number of monitors operating in an area, as well as on the severity of ozone.
This data should not be used to perform comparisons between cities or across years without factoring in the number of monitors.

The peak one-hour and eight-hour averages are color-highlighted based on the EPA-defined Air Quality Index colors. (See Interpreting the AQI.)

High Ozone Averages for 2023

Highlight with AQI colors
Metropolitan AreaOne-Hour Averages
>= 125 ppb
Eight-Hour Averages
>= 71 ppb
Peak ValueHigh DaysPeak ValueHigh Days
     Harris County, TX
     Montgomery County, TX
     Collin County, TX
     Dallas County, TX
     Denton County, TX
     Ellis County, TX
     Hunt County, TX
     Johnson County, TX
     Kaufman County, TX
     Navarro County, TX
     Rockwall County, TX
No averages at or above 125 08/17/20239645
Fort Worth-Arlington
     Hood County, TX
     Parker County, TX
     Tarrant County, TX
No averages at or above 125 08/31/20239428
San Antonio
     Bexar County, TX
No averages at or above 125 09/01/20238110
Austin-San Marcos
     Travis County, TX
No averages at or above 125 09/01/20237610
El Paso
     El Paso County, TX
No averages at or above 125 08/03/2023788
     Hidalgo County, TX
No averages at or above 125 No averages at or above 71
Corpus Christi
     Nueces County, TX
No averages at or above 125 05/25/2023712
Beaumont-Port Arthur
     Jefferson County, TX
     Orange County, TX
No averages at or above 125 06/09/20238210
Brownsville-Harlingen-San Benito
     Cameron County, TX
No averages at or above 125 08/30/2023711
     Bell County, TX
No averages at or above 125 09/01/2023806
Galveston-Texas City
     Galveston County, TX
No averages at or above 125 08/28/20238921
     Brazoria County, TX
No averages at or above 125 09/01/20238817
     Gregg County, TX
     Harrison County, TX
No averages at or above 125 09/02/2023862
     Mclennan County, TX
No averages at or above 125 08/30/2023797
     Webb County, TX
No averages at or above 125 08/29/2023803
     Smith County, TX
No averages at or above 125 05/27/2023764
     Victoria County, TX
No averages at or above 125 No averages at or above 71
Ozone averages are reported in parts per billion (ppb).
The High Days columns represent the number of days where one-hour or eight-hour ozone averages were at or above the thresholds of 125 ppb and 71 ppb respectively.

What do the Numbers Mean?

The information in the table is composed of ozone measurements from monitoring sites across Texas. There are two averaging periods that are of interest for ozone: a one-hour average and an eight-hour average. There are national air quality standards that have been established by the EPA for both of these averaging periods.

For the one-hour ozone average, a threshold of 125 ppb has been used to determine high value days. For the eight-hour ozone average, a threshold of 71 ppb has been used. Only averages that are equal to or higher than one of these thresholds appear in this table. The High Days column is a count of the number of days each metropolitan area experienced high ozone measurements (either one-hour or eight-hour) for the selected year. Out of all the averages at or above the thresholds, the peak or highest one-hour and eight-hour averages and the date they occurred are shown.

Ozone formation tends to be highest from March through October. Before May, there will be few if any metropolitan areas with high ozone readings. Some metropolitan areas never experience high ozone. In either of these cases, the table will show "No averages at or above 125", or "No averages at or above 71".

Interpreting the AQI

Air Quality Index

Each NAAQS pollutant has a separate AQI scale, with an AQI rating of 100 corresponding to the concentration of the Federal Standard for that pollutant. Additional information about the AQI and how it can be used is available from the EPA's AirNow web site.

Place your mouse pointer over the scale displayed above to view information about the Air Quality Index, and each of the rating levels.

The actual index calculation is different for each parameter measured and is specified by the EPA. The following table shows the various breakpoints used in calculating the AQI.

AQI Breakpoint Definitions
AQI Range 1hr Ozone
in ppm
8hr Ozone
in ppm
0 - 50 Not Defined 0 - 0.054
51 - 100 Not Defined 0.055 - 0.070
101 - 150 0.125 - 0.164 0.071 - 0.085
151 - 200 0.165 - 0.204 0.086 - 0.105
201 - 300 0.205 - 0.404 0.106 - 0.200
301 - 400 0.405 - 0.504 Not Defined
401 - 500 Not Defined Not Defined
500+ Not Defined Not Defined
  • The AQI for ozone is based on either the peak eight-hour running average since midnight OR the peak one-hour measurement since midnight.

PLEASE NOTE:  This data has not been verified by the TCEQ and may change. This is the most current data, but it is not official until it has been certified by our technical staff. Data is collected from TCEQ ambient monitoring sites and may include data collected by other outside agencies. This data is updated hourly. All times shown are in local standard time unless otherwise indicated.

Following EPA reporting guidelines, negative values may be displayed in our hourly criteria air quality data, down to the negative of the EPA listed Method Detection Limit (MDL) for the particular instrument that made the measurements. The reported concentrations can be negative due to zero drift in the electronic instrument output, data logger channel, or calibration adjustments to the data. Prior to 1/1/2013, slightly negative values were automatically set to zero.