CHICAGO (CBS) — Sweltering heat and unhealthy ozone levels are on tap for the rest of the day Saturday – a brew that usually is not good news for people with asthma.

As WBBM Newsradio’s Felicia Middlebrooks reports, the National Weather Service says ozone levels are expected to be in the orange, or unhealthy, levels for the sensitive groups range.

LISTEN: WBBM Newsradio’s Felicia Middlebrooks reports

An air quality alert is in effect until midnight Saturday night for Cook, Lake, McHenry, DuPage, Kane, Kendall, Will and Grundy counties in Illinois, and Lake and Porter counties in Indiana.

Ozone (O₃) is a triatomic molecule that is far less stable than the diatomic oxygen we breathe (O₂.) It is created when smoke from vehicle exhaust, factories and other sources reacts in the atmosphere at high temperatures.

Active children and adults, and people with respiratory diseases such as asthma, are advised to limit their time outside, the National Weather Service says.

But if you have asthma, then it’s not just the ozone and pollution you have to worry about. Experts say stress should be kept under control, to avoid an asthma attack or flare up.

Dr. Louis Neipris tells Accu-Weather stress is your body’s alarm system. Events such as traffic jams, family fights or deadlines at work can fire up your nervous system, causing an elevation in your blood pressure and breathing.

Emotional stress has been documented as a trigger for some asthma patients too, increasing their risk of attack by up to 35 percent. So, Neipris advises, as you try to beat the heat, make sure you keep your cool not just physically, but emotionally as well.

CBS 2 Meteorologist Steve Baskerville said Friday night that we can expect the high to hit 94 degrees. As of noon, it was 88.

The record high for Aug. 25 is 95 degrees, set back in 2003.

But on Sunday, look for a cold front to pass through as scattered thunderstorms begin popping up.