The water supply in two apartment buildings in Carouge was poisoned during a routine boiler clean on Friday afternoon.
The fire service says an agent used to descale pipes leaked into the hot water system. Six people were affected, one had to be checked at hospital.
Symptoms included stomach cramps, diarrhoea and sore throats.
One resident’s hair also reportedly turned green under the shower. She also felt a burning sensation.
Locals were told not to use any water, including for cleaning, the utility company, the SIG set up emergency taps on the street and bottled water was distributed.
The system has been flushed out and checks are still being carried out.
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