The clean-up of the sludge continues in Kolontar, Hungary Experts tackling the toxic red sludge from a spill at an industrial plant in Hungary say Friday will be crucial in preventing the spread of pollution.
The alkali sludge reached the Danube on Thursday and emergency workers are pouring huge quantities of clay and acid into streams to neutralise it.
There are now fears toxins from drying sludge could be spread through the air.
A million cubic metres (35m cu ft) of sludge spilled from a reservoir at an alumina plant in Ajka, western Hungary.
Four people were killed and about 100 injured. Three more are still missing.
Countries downstream from Hungary, including Croatia, Serbia and Romania, are drawing up emergency plans.
The chief of Hungary’s disaster relief services, Tibor Dobson, said it should become clear in the first half of Friday whether the alkaline solution and heavy metals, which started to reach the Danube on Thursday, had continued to rise or started to fall.
For five days, clay and acid have been poured into affected rivers around the clock to try to soak up the toxic waste.
Hourly checks
The BBC’s Duncan Kennedy in Hungary says officials are checking the alkali level hour by hour. It is currently at pH8.5 and they want to get it to below pH8 by lunchtime on Friday.
He says if that level can be achieved, the emergency effort will have turned a corner, if not the whole clean-up operation will have to expand.
Water is pH7 when neutral, with the safety levels ranging from pH6.5 to pH8.5.
Our correspondent says officials think that they may have done enough to keep the Danube safe, but they are holding on for a few hours to check further.
But he adds that there is an additional concern – the weather.
Recent days of rain have kept the sludge wet and officials now fear that warmer and sunnier weather will create dust and powder that could spread toxins – and possibly low-level radioactive materials – into the atmosphere.
If that happens, our correspondent says, the authorities will have to decide whether to evacuate more areas to keep people away from the dust.
Environment Minister Zoltan Illes confirmed the sludge, which now covers a 41 sq km (16 sq mile) area, had a “high content of heavy metals”, including carcinogens.
“If that [were to] dry out then the wind can blow that heavy metal contamination through the respiratory system,” he said.
The company responsible for the alumina plant, MAL Hungarian Aluminium Production and Trade Company, has offered its condolences to the families of the bereaved but insisted it had done nothing wrong.
It said it was devoting “all its energies and efforts” to tackling the spill.
The firm says it has released 110,000 euros ($150,000) so far to help the clean-up.
Officials in Croatia, Serbia and Romania are monitoring the Danube but hope the sheer volume of water will dissipate pollutants.
The mud has caused massive damage to Hungarian villages and towns close to the plant, as well as a wide swathe of farmland.
Mr Dobson said all life in the Marcal river, which feeds the Danube, had been “extinguished”.
Those who lost their lives were believed to have drowned, with the depth of the fast-moving flood reaching 2m (6.5ft) in places, but many of those injured suffered chemical burns.
PM Viktor Orban has called the spill an “ecological tragedy”.
On Thursday he visited the village of Kolontar, the worst-affected settlement, and said some areas would have to be abandoned.
“Hungary is strong enough to be able to combat the effects of such a catastrophe. But we’re still open to any expertise which will help us combat the pollution effects,” he added.
Herwit Schuster, a spokesman for Greenpeace International, described the spill as “one of the top three environmental disasters in Europe in the last 20 or 30 years”.
Land had been “polluted and destroyed for a long time”, he told AP.
“If there are substances like arsenic and mercury, that would affect river systems and ground water on long-term basis.”
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