Aruni Bhatnagar, a professor of medicine at the University of Louisville, said a hydrogen fluoride release would be "very, very dangerous."
The Paducah & Louisville Railway train derailed near Dixie Highway, a main corridor between Louisville and Fort Knox. Nine of the 13 derailed cars were carrying hazardous chemicals.
The train was traveling from the company's headquarters in Paducah to its Louisville switching facility, said spokeswoman Bonnie Hackbarth.
P&L Railway opened an outreach center Thursday where people forced from their homes were being reimbursed for lodging, food, lost wages and other expenses. They received additional payments amounting to $100 a day for adults and $50 for children for each day they are displaced.
Art Smith of the U.S. Environmental Protection Agency said Friday water and air samples continue to show there are no immediate concerns about contamination. The derailment site is near the confluence of the Salt and Ohio rivers.