Industrial waste, including toxic ash from power plants, could also be spread by flooding from the massive storm, which has caused mandatory evacuations of coastal areas in both Carolinas and Virginia, the AP reports.
As of 8 a.m. ET, Florence was 530 miles southeast of Cape Fear, N.C., moving west-northwest at 17 mph, the National Hurricane Center says.
Florence's winds in the afternoon were down slightly to 125 miles per hour (205 kph), from a high of 140 miles per hour, and the Category 4 storm fell to a Category 3. "This is going to be a Mike Tyson punch to the Carolina coast".
Analytics firm CoreLogic predicts that the damage from Florence could be more than $170 billion - making it the most expensive hurricane to hit the continental U.S.
Landfall is expected late Thursday or early Friday, and the National Hurricane Center fears the storm "will slow considerably or stall, leading to a prolonged and exceptionally heavy and unsafe rainfall event Friday-Sunday".
Tropical storm-force winds extended 175 miles (280 kilometers) from Florence's center, and hurricane-force winds reached out 70 miles (110 kilometers).
The threat has sparked a rush of evacuation efforts in SC and North Carolina, with more than a million people urged to get out of Florence's way.
Outer rain bands approaching N. Carolina coast
The storm, far weaker by early next week, is expected to track north along the Appalachian Mountains from Sunday to Wednesday. Emergency declarations were in force in Georgia, South and North Carolina, Virginia, Maryland and the District of Columbia.
"People do not live and survive to tell the tale about what their experience is like with storm surge", he said.
MSNBC reported the Trump administration had diverted almost $10 billion from FEMA to the Immigration and Customs Enforcement agency, which leads border enforcement. Workers are being brought in from the Midwest and Florida to help in the storm's aftermath, it said.
"I'm not approaching Florence from fear or panic", said Brad Corpening, 35, who planned to ride out the storm in his boarded-up delicatessen in Wilmington. "It's a big one". "If we try to leave, we'll just get stuck in the rain".
The last hurricane rated a Category 4 on the five-step Saffir-Simpson to plow directly into North Carolina was Hazel in 1954, a devastating storm that killed 19 people.
Authorities, including President Trump and the Federal Emergency Management Agency (FEMA), are urging residents in the Carolinas as well as parts of Virginia and Georgia to evacuate as the storm continues its path.