The agency declared an air pollution "action day," cautioning residents, particularly children and people with pulmonary or respiratory diseases, to limit outdoor activity.
"Thousands of Chicagoans are at risk today," Dr. Joseph Leija, an allergist at Gottlieb Memorial Hospital, part of the Loyola University Health System, said in a statement.
"A toxic cocktail of extreme heat, humidity causing the heat index to be well over 100 degrees, plus widespread ozone and particulate levels expected to be at or above unhealthy levels, means trouble for those with sensitive respiratory systems,"Leija said.
"Even low concentrations of ozone in air are destructive to many plants, animals and, of course, people," he said.
For people with sensitive respiratory systems, Leija suggests:
-- Stay indoors and avoid outdoor activity.
-- Run air conditioning to lower air humidity.
-- Rinse inner nostrils with saline solution to free trapped debris and moisten membranes.
-- Talk to an allergist about adjusting prescribed medication.