(CBS) — More than a dozen workers were transported to the hospital Saturday morning after they were overcome by heat at a downtown Chicago post office.

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The incident happened around 7:45 a.m. at the post office at 540 N. Dearborn. An air conditioning system malfunctioned, prompting several people to suffer heat-related symptoms such as exhaustion and dehydration.

Thirteen people were transported, all in stable condition, the Chicago Fire Department said.

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Mack Julion, President of the postal workers union, says the building’s interior reached 100 degrees.

“It was extremely hot. We’ve always had problems with that air conditioning for the past few years. It’s been happening every summer. Seems like the air conditioner goes out and you have heat blowing,” postal worker Kenneth Norman tells WBBM.

Because of the incident, postal officials say Chicagoans who live in three zip codes — 60610, 60611 and 60654 – will not get mail today. The post office is expected to reopen on Monday.

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“We regret any inconvenience this suspension may cause for our customers,” Acting Postmaster Tangela Bush said in a prepared statement. “Our first priority is to ensure our employees have a safe working environment. We will resume our Fort Dearborn operations at the earliest appropriate opportunity.”