The hazards range from potential liquid fuel and chemical leaks to soot from smoky fires.
No major spills or other ecological crises have been reported.
State officials relaxed environmental regulations to expedite the clean-up. The waivers allow limited burning of woody debris. Landfills can accept some types of waste that are normally off-limits.
The tornado leveled much of the city and killed more than 130 people.
Meantime, residents remembered fallen heroes and tornado victims on Memorial Day during a special moment of silence.
The Associated Press contributed to this report.