Experts say Harvey's filthy floodwaters pose significant dangers to human safety and the environment even after water levels drop far enough that Southeast Texas residents no longer fear for their lives.

Houston already was notorious for sewer overflows following rainstorms. Now its system faces an unprecedented challenge. Authorities say standing water will contain untreated sewage, along with spilled fuel and toxic waste. They're urging residents to stay out of it when possible.

More than 160 drinking water systems in the area advised residents to boil water before drinking it. Fifty were shut down.

When the waters recede, standing puddles will be breeding areas for disease-carrying mosquitoes.

Farther downstream, the waters will carry pollution that causes harmful algae blooms into Galveston Bay and the Gulf of Mexico.

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