Russia has evacuated more than 4,000 children from a town in Crimea hit by noxious sulphur dioxide gas, allegedly from a huge nearby titanium plant.
Armyansk, home to about 22,000 people, lies near the de facto Ukrainian border. Russia annexed Ukraine's Crimea peninsula in March 2014.
The Russian-installed Crimean leader, Sergei Aksyonov, says the plant that produces titanium dioxide - widely used in industry - is now suspending output.
Acid from a reservoir has been blamed.
Experts believe the problem is that the summer heat evaporated a huge quantity of sulphuric acid dumped in the reservoir and noxious sulphur dioxide gas has been blowing from it across Armyansk.
Residents are wearing face masks, which are now reported to be in short supply.
The sulphuric acid reservoir covers 42sq km (16 sq miles) and the "Titan" plant itself is 34 years old.
Titanium dioxide - a white powder - is much in demand, for example in the paint industry and pharmaceuticals. But the Titan plant is reported to be in financial difficulties.
School-age children have been bussed south to health resorts in Crimea, initially for two-week stays, and the transfers are continuing. The region is a popular holiday destination for Russians.
Residents quoted by Russian media say the foul gas has been polluting their town since 23 August, and some complained that the authorities had been slow to act.
Reports say the pollution is even worse in Perekop, a village of about 900 inhabitants closer to the Titan plant.
Valentina Anatolyevna, a worker at Titan interviewed by Russia's Kommersant daily, said a greasy "rust" had coated her apricot trees and vines, ruining the fruit. She likened the rust to old engine oil, and said her house was also coated with it.
Clinics in Armyansk say they are coping, although reports from local people say there has been an influx of patients complaining of chemical burns and breathing problems.
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Mr Aksyonov insisted that it was not an emergency and "there is no threat to citizens' lives and well-being".
A Crimean ecologist, Margarita Litvinenko, blamed a water shortage in Crimea caused by Ukraine's decision to stop the flow from the North Crimean Canal, which is fed by the River Dnieper. Water from the canal had previously topped up Titan's reservoir, diluting the sulphuric acid.
Ukraine's defence ministry alleged that shells fired by the Russian military during exercises near Titan had damaged some storage tanks there. That claim has not been confirmed, however.