Just before the afternoon service started at New Philadelphia Baptist Church, members of the congregation reported smelling fumes and a gas-like smell.
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The church's deacon said they decided not to take any chances and call the fire department.
CFD responded to the 5400-block of South State Street and when they measured the air, they found the levels of carbon monoxide were high.
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Church representatives said said 12 people were taken to three different hospitals, including the University of Chicago Medical Center, after feeling dizzy and sick.
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Deacon Darryl Person said utility crews told him the cause was faulty furnace parts and says this is a good reminder for everyone to check their heating systems checked.
"Make sure your furnace is checked out good if you smell anything call the fire department, don't be a hero cause the carbon monoxide is what they call a 'silent killer,'" Deacon Darryl Person said.
The 12 hospitalized are said to be in good to serious condition.