 After floodwater goes down, buildings typically stay wet for more than a couple of days. And that's when musty odors and mold can show up in flooring and walls. Dan Tranter is the indoor air supervisor at the Minnesota Department of Health. He worries about potential health hazards in homes that have flooded. The mold doesn't grow immediately and mold doesn't grow under water. So the mold will start to grow after a couple of days.