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Toledo Mayor D. Michael Collins said some sampling showed decreased toxin levels but that results from further tests would not be known until later in the day.
"All I can tell you is that everything is trending in a very postive direction," Collins told reporters. But he also cautioned "this is not over yet" and he could not predict when water would be safe to drink.
About 500,000 people get water from the contaminated source but about 100,000 residents of some commmunities have backup water supply systems, said city of Toledo spokeswoman Lisa Ward.
Health officials sent samples to several laboratories for testing after finding Lake Erie may have been affected by a "harmful algal bloom," Ohio Environmental Protection Agency spokeswoman Heidi Griesmer said.
The lake provides the bulk of the area's drinking water.
Ohio Governor John Kasich declared a state of emergency on Saturday for the state's fourth-largest city and surrounding counties. The Ohio National Guard, various state agencies and the American Red Cross of Northwest Ohio were working to truck safe water to the area.
Many residents drove to other states in search of fresh water as news of the crisis led stores to rapidly sell out of bottled water supplies.
Jeff Hauter of Toledo drove to a Walmart in suburban Detroit, where he bought 18 gallons and four cases of water. He said he ran into others from the Toledo area loading up their trucks and cars.
Algal blooms in Lake Erie are fairly common, typically in the summer, state emergency operations spokesman Chris Abbruzzese said.
Potentially dangerous algal blooms, which are rapid increases in algae levels, are caused by high amounts of nitrogen and phosphorous.
Those nutrients can come from runoff of excessively fertilized fields and lawns or from malfunctioning septic systems or livestock pens, city officials said.
Drinking the contaminated water can affect the liver and cause diarrhea, vomiting, nausea, numbness or dizziness, city officials said, adding that boiling will not destroy the toxins.
The water should not be used for drinking, making infant formula or ice, brushing teeth or preparing food, the governor's office said. It also should not be given to pets, but hand washing is safe and adults can shower in it, officials said.
In response to the Toledo crisis, Chicago began additional testing on Lake Michigan water as a precaution and expects results within a day or two, city spokeswoman Shannon Breymaier said.