Thirty people were treated for sulfur gas poisoning after Mount Ijen in eastern Java belched toxic fumes from its crater, Indonesia's disaster agency said Thursday.
Spokesman Sutopo Purwo Nugroho said more than 170 residents of several villages on the volcano's slopes had to flee.
He said some suffered shortness of breath and vomiting following the steam and gas-based eruption about 7:15 p.m. on Wednesday.
The mountain is known for its stunning sulfur lake and night-time sulfur mining involving dangerous and backbreaking work by low paid laborers.
Sutopo said Ijen's summit is now temporarily off limits to activities but the volcano's overall status remains normal.
Indonesia, which straddles the seismically volatile Pacific "Ring of Fire," has more than 120 active volcanoes.
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