Hawaiians have been given face masks and told to take shelter after scientists warned the Kilauea volcano, which has destroyed homes, could become more violent within hours.
The volcano has been violently erupting since the beginning of the month, its deadly lava spreading into residential areas.
Rocks the size of microwaves have been shooting out of the volcano in the biggest eruptions for almost a century.
On Thursday, the volcano spewed ash nearly six miles (9km) into the sky.
Scientists are warning this could be the first in a string of more violent explosive eruptions, with the next possibly occurring within hours.
"We may have additional larger, powerful events," US Geological Survey geologist Michelle Coombs told a news conference in Hilo.
Geologists say boulders the size of cows could be hurled from the summit if the explosions continue to strengthen.
Kilauea's falling lava lake has likely descended to a level at or below the water table, allowing water to run on to the top of its lava column and create steam-driven blasts, they said.
Thousands of masks for protection against rising ash levels have been handed out.
Resident Lindsey Magnani said both of her children, a two-year-old and a three-month-old baby, were doing okay, but she and her fiance had both been sneezing all day.
"This morning it smelled like sulfur so we had to close all the windows," Ms Magnani said.
Residents are being told to take shelter from the ash as toxic gas levels spiked in a small southeast area where lava has burst from the ground during this two-week eruption.
"Protect yourself from ash fallout," the officials warned in an alert.
Earlier, a change in wind direction caused gas spewing from fissures to drift towards Pahoa, prompting national guard troops to don gas masks at a nearby road intersection.
Several schools closed because of the risk of elevated levels of sulfur dioxide, a volcanic gas.
The Federal Aviation Administration extended a restriction on aircraft from entering the airspace up to 30,000 feet above sea level.
About 2,000 people have been evacuated. A number of vehicles and 37 homes have been destroyed.
There have been no deaths or serious injuries reported during the current eruption.
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Kilauea, one of the world's most active volcanoes, has been erupting continuously since 1983.
It is among the five volcanoes that form the Big Island, and it is the only one actively erupting.