Hawaii's erupting Kilauea volcano has been spouting lava "bombs" and threatening escape routes for the people living on Big Island. "Severe conditions may exist such as choking and inability to breathe", the agency said.
"Be aware of the unpredictable nature of risky levels of SO2 gas because it can be carried far from the fissures with wind speed and direction", civil defence said in a Facebook post. "Motorists advised to drive with caution", said the County of Hawaii Civil Defense text message.
It could force another 2,000 people out of their homes and into shelters.
CBS News correspondent David Begnaud reports from Pahoa that people who live in the area are sleeping at night with gas masks beside the bed just in case the wind shifts direction and toxic fumes start blowing their way.
Unsafe levels of sulphur dioxide gas and other emissions prompted state health officials to urge residents to stay indoors or leave the eastern end of Hawaii's Big Island, which has been ravaged by volcanic activity since May 3.
In the evacuated Leilani Estates neighborhood of about 1,500 people, explosions could be heard on Sunday as steam rose from cracks in the roads.
That long-stored lava would be relatively cool, and would move slowly when it flows out of the fissures near Leilani Estates.
Another fissure spewing lava and unhealthy gas opened up Monday, and a crack in the Earth that emerged a day earlier was sending molten rock on a slow run for the ocean, officials said. The eruptions have opened 20 vents in the ground, while lava has destroyed more than 40 structures, including two dozen homes. No evacuations have been ordered from Ka'u district. "Now that we see the risk extending towards Highway 137, if that gets cut off, then the route has got to go back to Highway 130 again so, we're looking at options to minimize any risk as 130".
The U.S. Geological Survey has warned that pent-up steam could cause a violent explosion at the volcano crater, launching a 20,000-foot (6,100-metre) plume that could spread debris over 12 miles (19 km).
"This could generate risky debris very near the crater and ash falls up to tens of miles downwind".
A large explosion in Hawaii's Kilauea volcano could lead to an explosive eruption and send boulders, rocks and ash into the air in the coming weeks, the US Geological Survey (USGA) said on Wednesday.