Some 400,000 people living in Toledo and surrounding towns and counties were told not to drink, not even boil, tap water due to the presence of a toxin called microcystis.
The toxin was detected Friday and the region has been under a state of emergency for two days now.
Microcystis developed in the water supply as a result of a harmful algae bloom in Lake Erie, a city spokeswoman said.
The Ohio National Guard, and local fire officials and first responders handed out potable water to residents affected by the water ban.
Officials said there have been no reports of people getting sick due to the toxin.
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