The Environmental Protection Agency has been issuing ozone alerts for years, but understanding what it is and how it forms can be a bit tricky.
Ozone is a gas found naturally about ten miles up in the troposphere. It protects the earth from the sun's harmful ultraviolet rays. But when ozone is formed at the surface, it becomes dangerous.
And this week's stagnant weather pattern is the perfect breeding ground for surface ozone.
Dona Bergman, Evansville's EPA director, explains, "They get up in the atmosphere and strong sunlight, high heat, it starts this chain of complex chemical reactions which ends up being ozone."
Ozone sensors around southwest Indiana are continuously monitored during the warm, summer months. If ozone levels exceed 85 parts per billion, an alert is issued.
"If we're in doubt, we'll go ahead and call an alert because our main object is to protect people's health," Bergman says.
But there is some good news when it comes to Tri-State ozone levels. Overall, they're dropping.
Bergman says, "Area power plants were required to install some very significant, very costly pollution controls [which] took out 60- to 80-percent of the nitrogen oxides from their emissions."
Before the new standards were put in place in 2004, ozone levels of 120 parts per billion were common. That level of pollution made the air we breathe unhealthy for everyone - not just sensitive groups.
While progress has been made, much more can be done to put less pollution in the air. Here's how you can avoid adding to ozone pollutants: avoid drive-thrus and don't mow your grass or fill-up your gas tank during the heat of the day.