(CBS) — Firefighters have secured a chlorine leak at the Four Seasons Hotel near the Mag Mile.

Crews were called to the hotel around 3 a.m. The Chicago Fire Department says workers were mixing chemicals to put chlorine into the pool and hot tub on the seventh floor when a small explosion happened.

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Hazmat was called and guests and residents were notified.

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“We gave announcements throughout the hotel for people to stay in their rooms we took readings throughout the hotel, the chlorine levels. They were very on low on the floor that it happened. There were no readings above or below so all the people stayed in place in their rooms,” said Chicago Fire Chief Michael Fox.

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Chief Fox says the leak was contained to that one room. No one was hurt.