2021 Ozone Season

What is an "Ozone Season"?

An ozone season is a period of time in which ground-level ozone, which is the most common type of air pollution we have in the Alamo Region, typically reaches its highest concentrations in the air we breathe. In this region, the ozone season stretches from March through November. Ground-level ozone reaches its highest concentrations during these months because ozone forms when nitrogen oxides mix with volatile organic compounds in intense sunlight, and sunlight is strongest from March through November. It is during this time that we are most likely to have exceedances of the National Ambient Air Quality Standard (NAAQS) for ground-level ozone. Due to additional atmospheric conditions at certain times during these months, April through May and August through September tend to have the highest concentrations of ground level ozone.

Air Quality Exceedances and Ozone Action Day Alerts

According to the NAAQS set by the U.S. Environmental Protection Agency (EPA) in 2015, ozone levels are considered to be unhealthful when they rise above 70 parts per billion (ppb). The standard is based on readings taken at regulatory monitors during eight-hour blocks of time. EPA uses the average of the annual fourth highest eight-hour daily maximum concentrations from three years of air quality monitoring data to determine whether the ozone standard has been violated.

High concentrations of ozone can cause shortness of breath, coughing, wheezing, headaches, nausea, eye and throat irritation, and lung damage. People who suffer from lung diseases like bronchitis, pneumonia, emphysema, asthma, and colds have even more trouble breathing when the air is polluted. These effects can be worse in anyone who spends significant periods of time exercising or working outdoors.

When it is anticipated by scientists at the Texas Commission on Environmental Quality (TCEQ) that an exceedance of the 70 ppb standard threshold during an 8-hour period will occur the next day, the TCEQ issues an Ozone Action Day, so that health precautions and preventative measures may be taken. Health precautions may include limiting outdoor activity, while preventative measures may include driving less or avoiding traffic congestion, since the formation of ozone in this region is linked in great part to vehicle exhaust.

2021 Alerts and Exceedances

As of June 17, there have been 4 Ozone Action Days issued during the 2021 Ozone Season for the Alamo Region and 2 days when the standard threshold has been exceeded (that is, when ozone was high enough to be considered unhealthy).  Please see the table below for details:

Date Ozone Action Day? Peak Ozone
04/11/2021 No  76 ppb 
04/25/2021 Yes  70 ppb 
05/03/2021 Yes  57 ppb 
05/06/2021 Yes  67 ppb 
05/07/2021 No  71 ppb 
06/16/2021 Yes  69 ppb 

Date Alert Level
Highest 8-hour reading at Regulatory Site
04/23/18 No Alert Issued 77 ppb at Camp Bullis (C58) and 73 ppb at San Antonio Northwest (C23)
04/24/18 No Alert Issued 73 ppb at Camp Bullis (C58)
04/28/18 Orange 72 ppb at Camp Bullis (C58)
05/06/18 Orange 71 ppb at Calaveras Lake (C59)
05/07/18 Orange 83 ppb at Camp Bullis, 75 ppb at San Antonio NW, and 71 ppb at Calaveras Lake
05/08/18 Orange 61 ppb at Camp Bullis (there was no actual daily exceedance)
05/17/18 Orange 64 ppb at Camp Bullis (there was no actual daily exceedance)
05/28/18 Orange 61 ppb at San Antonio, NW (there was no actual daily exceedance)
07/26/18 Orange 73 ppb at Camp Bullis (C58) and 72 ppb at San Antonio Northwest (C23)
07/27/18 Orange 73 ppb at San Antonio Northwest (C23)
08/01/18 Orange 79 ppb at Calaveras Lake (C59)
08/02/18 Orange 83 ppb at San Antonio NW (C23), 83 ppb at Camp Bullis (C58) and 73 at Calaveras Lake (C59)
08/03/18 Orange 71 ppb at Camp Bullis (C58)
For more information, review 2017 Ozone Season Resources.