A lethal torrent of toxic red sludge from a metal refinery engulfed towns in Hungary, burning villagers through their clothes and threatening an ecological disaster Thursday.

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KOLONTAR, Hungary — A lethal torrent of toxic red sludge from a metal refinery engulfed towns in Hungary, burning villagers through their clothes and threatening an ecological disaster Tuesday as it swept toward the Danube River.

The flood of caustic red mud triggered a state of emergency declaration by Hungarian officials. At least four people were killed, six were missing and 120 injured, many with burns.

Hundreds were evacuated in the aftermath of the disaster Monday, when a gigantic sludge reservoir burst its banks at an alumina plant in Ajka, a town 100 miles southwest of Budapest, the capital. The torrent of sludge inundated homes, swept cars off roads and damaged bridges.

Red sludge is a byproduct of the refining of bauxite into alumina, the basic material for manufacturing aluminum. It is common to store treated sludge in ponds where the water eventually evaporates, leaving behind a dried red claylike soil, the officials said.

However, Hungarian environmentalist Gergely Simon said the sludge involved in the disaster had been accumulating in the reservoir for decades and was extremely alkaline, with a pH value of about 13 — nearly equivalent to lye — and that is what caused the burns.

MAL Rt., the Hungarian Aluminum Production and Trade Company that owns the Ajka plant, said that according to European Union standards, red sludge is not considered hazardous waste.

Emergency workers wearing protective gear rushed to pour 1,000 tons of plaster into the Marcal River in an attempt to keep it from flowing on to the Danube some 45 miles away.

This week’s spill threatened to eclipse the environmental damage caused 10 years ago, when cyanide poured from a gold mine reservoir in a Romanian town near the Hungarian border into the Danube and four smaller rivers, destroying plant and animal life.