For the first time in 40 years, Mauna Loa, the largest active volcano in the world, is erupting, prompting an ashfall warning for Hawaii‘s Big Island and the nearby waters on Monday until 10 a.m. HT (3 p.m. ET).

According to a tweet from the Hawaii Tourism Authority on Monday morning, the eruption in Hawaii Volcanoes National Park poses no threat to downstream villages or planes to the island of Hawaii.

Although winds may send fine ash and volcanic gas downwind, the National Weather Service in Honolulu warned that a “trace of less than one-quarter inch” of ashfall could settle on some areas of the island.

The Hawaii Emergency Management Agency tweeted late Monday morning that reports of lava overflowing into the southwest corner of the volcano’s caldera, or crater, had come into the Hawaii Volcano Observatory.

No evacuation orders have been issued, and there is no indication that adjacent areas are in danger, the agency tweeted.

Two shelters have opened as a precaution even though, according to a different agency tweet, “approximately half” of documented Mauna Loa eruptions have been in the summit area without endangering populated areas.

“People with respiratory illnesses should remain indoors to avoid inhaling the ash particles and anyone outside should cover their mouth and nose with a mask or cloth,” the Honolulu office warned. “Possible harm to crops and animals. Minor equipment and infrastructure damage. Reduced visibility. Widespread clean-up may be necessary.”

According to the weather service, ashfall can harm buildings and cars, taint water supplies, interfere with sewage and electrical systems, and harm or kill flora. Abrasive volcanic ash can also irritate the eyes and lungs.

Hawaii, or the Big Island, is the largest in the Hawaiian chain in terms of area at almost 4,000 square miles, yet the US Census Bureau estimates that there are just over 200,000 residents there or less than 50 people per square mile. The majority of people live in coastal cities and towns.

The US Geological Survey reports that Mauna Loa, a volcano that makes up almost half of Hawaii’s island, has erupted 33 times since 1843 when it first had a “well-documented historical eruption.”

Since it last erupted in 1984, this protracted spell of silence is the longest volcano has ever experienced.