The mighty River Danube appears to be absorbing the toxic red sludge that spilt out from a Hungarian alumina plant reservoir on Monday with little immediate harm.
Scientists now say the amount of caustic slurry spewed over western Hungary was nearly as great as the Gulf of Mexico oil spill.
The risk of pervasive and lasting environmental harm remains nonetheless, with laboratory analyses organised by Greenpeace showing high concentrations of toxic substances in samples taken from the sludge.
Analysis of water in a canal near the spill found arsenic levels 25 times the limit for drinking water.
Hungary's state secretary for the environment, Zoltan Illes, also admitted the henna-coloured sludge covering a 16-square-mile swathe of countryside has 'a high content of heavy metals,' some of which can cause cancer.
With rain giving way to dry, warmer weather over the past two days, the caustic mud is increasingly turning to airborne dust, which can cause respiratory problems, Illes said.
A fifth person - an 81-year-old man - has died from unspecified injuries sustained in the flooding.
The red sludge entered the Danube on Thursday and is moving downstream toward Hungary's immediate neighbours, Croatia, Serbia and Romania, amid fears that it will kill off the river's fish and plant life.
Monitors are taking samples every few hours to measure damage from the spill but there were no reports of major harm to the waterway's ecosystem.
The long-term effects on the agricultural region are likely to be devastating. It is still not known what caused a section of the reservoir to collapse, unleashing a torrent of sludge.