Chicago is waiting on water test results from Lake Michigan, after residents in the Toledo area were told their drinking water is too dangerous to use.
Ohio health officials say a toxin showed up in readings at a water treatment plant from harmful algae that's growing in Lake Erie, which is Toledo's main water source.
They say boiling the water will not reduce its toxicity.
So far, no one has gotten sick.
Ohio's governor has declared a state of emergency for the area.
Officials in Chicago test Lake Michigan two to three time a week, but are now retesting the water as a precaution. The results are due back in one to two days.