HOUSTON (AP) — The muddy floodwaters now soaking through drywall, carpeting, mattresses and furniture in Houston will pose a massive cleanup challenge with potential public health consequences.

It's not known yet what kinds or how much sewage, chemicals and waterborne germs are mixed in the water. For now, health officials are more concerned about drownings, carbon monoxide poisoning from generators and hygiene at shelters. In the months and years to come, their worries will turn to the effects of trauma from Hurricane Harvey on mental health.

At a shelter set up inside Houston's George R. Bro...