The environmental group Greenpeace warned that the dust produced by a toxic red sludge that swept over several villages in western Hungary poses a huge health risk.
A study for Greenpeace by the University of Vienna's physiographic laboratory found that the mud produced very fine dust particles measuring less than two micrometres (two-thousandth of a millimetre).
If the sludge dries out and is converted to dust, this will pose a huge health risk, Greenpeace said in a statement.
Indeed, the finer the particles, the deeper they can enter the air ways, threatening the lungs and entering the blood supply.
"The fine particle time bomb is ticking," Greenpeace spokesman Herwig Schuster noted.
Nine people were killed and over 150 injured after the reservoir of an alumina plant in Ajka, 160 kilometres (100 miles) west of Budapest, burst on October 4, unleashing some 700,000 cubic metres (24 million cubic feet) of toxic sludge.
The village of Kolontar, one of the hardest hit, was evacuated Saturday, following fears of a second spill.
But after protective dykes were erected this week, residents were allowed to return on Friday, a move criticised by Greenpeace.
"The authorities must tell the people on the ground the truth about the dangers they face," said Schuster.
"If it stays dry over the next few days and a strong wind picks up, a lot of dust will swirl around and the amount of fine particles will grow accordingly."
Last week, Greenpeace said it had found high levels of arsenic and mercury in the sludge.