A Hungarian soldier, outfitted against the toxic sludge, walks through the flooded streets of Devecser, Hungary, on Tuesday.
"It's not a subtle effect," toxicologist Ronald Kendall said of the sludge's potential for harm.
"This stuff is highly alkaline; its pH is estimated to be 13, which means it’s basically like lye," said Kendall, of Texas Tech University.
"It's directly, immediately toxic. It will burn your skin and your eyes, and if you are trapped in it, it will kill you. It can kill plants, it can kill animals, and it can kill things in the water.
"It doesn't take five years to demonstrate a response. If you get it on your skin, it only takes about ten seconds."
(Get more toxic-waste facts.)