About 400,000 people in and around Ohio's fourth-largest city were warned Saturday not to drink or use its water after tests revealed the presence of a toxin possibly from algae on Lake Erie.

The warning left residents across the Toledo area searching for water after they were advised not to to brush their teeth with it or boil the water because that would not kill off the toxin but only increase the toxin's concentration. The city also advised against drinking water filtered by in-home filters. Showers and baths are fine, the mayor said.

People bought carts full of bottled water, bags of ice and flavored water, emptying store shelves within hours of the advisory that was issued overnight.

"It looked like Black Friday," said Aundrea Simmons, who stood in a line of about 50 people at a pharmacy before buying four cases of water. "I have children and elderly parents. They take their medication with water."

Toledo issued the warning just after midnight on the city's Facebook page after tests at one treatment plant showed two sample readings for microsystin above the standard for consumption.

The city's advisory said Lake Erie may have been affected by a bloom of harmful algae that produces the toxin and warned that it could make residents ill.

The advisory covers city residents and those in Lucas County served by the city's water supply. The city said more tests are being run.

Many restaurants were closed because of the water warning and Toledo's public school system canceled all its events Saturday.

The University of Toledo closed its campus for the day and encouraged students who are from outside the Toledo area to return to their homes.

Operators of water plants all along Lake Erie, which supplies drinking water for 11 million people, have been concerned over the last few years about toxins fouling their supplies.

Almost a year ago, one township just east of Toledo told its 2,000 residents not to drink or use the water coming from their taps after tests on drinking water showed the amount of toxins had increased.

That was believed to be the first time a city has banned residents from using the water because of toxins from algae in the lake.

The Toledo Blade reports:

Chemists testing water at Collins Park plant found two sample readings for microcystin that exceeded the recommended “do not drink” standard of one microgram per liter standard. What is a microcystin? Algal blooms, often caused by the addition of nutrients to a water body, are made up of cyanobacteria, commonly called blue-green algae. Some cyanobacteria can produce toxins, called cyanotoxins. Although many blooms contain non-toxic species of cyanobacteria, lab tests are needed to determine whether a bloom is toxic or nontoxic. Some cyanotoxins can be toxic for humans, animals and plants.

Most water treatment plants along the western Lake Erie shoreline treat their water to combat the algae. The city of Toledo spent about $4 million last year on chemicals to treat its water and combat the toxins.

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The annual algae blooms have been concentrated around the western end of Lake Erie. The algae growth is fed by phosphorous mainly from farm fertilizer runoff and sewage treatment plants, leaving behind toxins that can kill animals and sicken humans.

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