When people think about volcanoes, they often call up images of massive explosions, towns covered in ash and rivers of lava destroying everything in their path.

For the estimated 500 million people who live near the world's 600 active volcanoes, those catastrophic events are an all-too-real possibility. But less discussed (and studied) is a much quieter threat — that is in some cases just as ominous.

Volcanoes around the world spew out thousands of tons of sulfur dioxide and other gases. Yet there are few long-term studies on the effects of that on people who live near volcanoes, researchers say.

Scientists tracking emissions from Kīlauea are looking to change that. A handful of studies are examining the effects of vog on Big Island residents, from how emissions affect asthmatics to whether people in voggy communities experience bronchitis and other respiratory illnesses more often.

Bernadette Longo, a University of Nevada-Reno assistant professor of nursing, is a co-investigator in the Kīlauea Volcano Health Study and says the work is aimed at helping people who live near volcanoes worldwide. "Our team is trying to build that evidence base," she said.

It's not clear why so few long-term studies on volcanic haze are available.

Part of the reason, researchers say, is probably because volcanic emissions are unpredictable — they can last days or years and vary in the volume of gas emitted. They are also less apparent a threat than large eruptions.

Hawai'i scientists stress- ed, though, that people living near active volcanoes can learn a lot from each other. "We are not an anomaly," said Tamar Elias, a chemist at the Hawaiian Volcano Observatory, pointing out that there are a host of communities — from Costa Rica to Chile — dealing with volcanic gases.

Donald Thomas, director of UH-Hilo's Center for the Study of Active Volcanoes, said Kīlauea is unusual because emissions are coming from two separate vents — one at Pu'u 'Ō'ō that started in 1983 and another at Halema'uma'u Crater that opened in March 2008. But, he added, "the amount of gases being discharged at Kīlauea summit, it's not huge by worldwide standards."

The two vents in recent years emitted an average of about 2,000 to 3,000 tons of sulfur dioxide daily, though it can fluctuate month to month.

On Thursday, for example, the Halema'uma[0x08]'u vent emitted about 700 tons of sulfur dioxide. On Wednesday, the Pu'u 'Ō'ō vent emitted 560 tons.

Thomas compared the long-term daily average emission of 2,000 to 3,000 tons to a case in Japan.

In 2000, the Miyakejima island volcano started to erupt, initially emitting several thousand tons of sulfur dioxide daily. Within a few months, the sulfur dioxide emitted grew to tens of thousands of tons, with a peak of 80,000 tons a day. The gas was so thick, Japan evacuated the island until 2005, when gases dropped.

By 2006, the volcano was emitting 1,000 to 3,000 tons of sulfur dioxide daily.

The island, just 5 miles in diameter, is home to about 3,000 people — all required to carry one thing with them at all times: a gas mask.